Note: Descriptions are shown in the official language in which they were submitted.
SYSTEM FOR MAXIMUM EFFICIENCY TRANSFER OF MODULATED
SIGNALS UTILIZING LINE SYSTEMS
The present invention relates to audio processing
and modulation systems and, m~re particularly/ to an
improved signal processing system intended for maximizing
the efflciency of line system transmission or tran~ferring
esp~cially of audio fxequency signals, as in telephony,
for providing high modulation leYels and or enhancing
signal intelligibility and clarity while avoiding loss
of audio dynamics.
In both long and short distance line signal trans~
mission and reception o modulated signals, such as utilized
in telephone and other transmission line systems utilizing
any combination of various signal transmission modes, such
as twisted pair, RF link, laser, fiber optics, and so
forth, a major problem has always been to obtain a high
level o dynamic amplitude while not only cont~ining the
full spectrum of speech harmonics but also keeping the audio
bandwidth of the transmitted line signals as narrow as
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possible. Many modes of line transmission and types of
audio or othex signal modulation have been used where
these mat~ers are of great concern. Principal forms of
modulation presently in use are AM, SSB, and FM. AM and
5 FM utilize a constant carrier principal while SSB has a
direct audio into power out relationship and is not a
cons~ant carrier mode of transmission when the carrier is
suppressed, as is quite common.
Frequenc~ Modulation (FM)
___ __
In constant carrier FM mode, the dynamic content of
~he signal and the audio bandwidth of the signal are
directly proportional to the amount of deviation allowed
to be imposed on the carrier~ E.g., in narrow band FM
transmission line systems, an audio bandwidth of 3 kcs.
lS may be imposed. As a result, dynamic harmonics of voice
characteristics are restricted and lost.
Am~litude Modulation (AM)
In the constant carrier mode of amplitude modulation,
dynamics are e~pressed in a direct relationship with the
amplitude of the constant carrier and audio frequency band-
width being theorectically limited only ~y the line
frequency bandwidth available. In commercial line communic-
ation systems utilizing the amplitude modulation mode of
~transmission, the audio bandwidth usually is restricted to
eliminate spurious AM modula-ted pulses resulting rom
ignition and other electrical noise being induced on the
line~ An AM mode now commonly used takes the form of pulse
~lplitude modulation (PAM), as in teletype and dake trans-
mission systems.
Single Side Band (SSB)
The more recent mode of SSB modulation is by far the
most efficient form of transmitting an audio modulated high
frequency line signal. The major drawback of this mode of
modulation is the extremely restricted audio bandwidth im-
posed resulting in extreme losses of dynamic .intelligibility.However~ these losses are often traded off against the high
efficiency of single side band line systemsO
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Characteristics of Human S eech with Reference to
P _ _ _ _
Radio Transmission
Relative to radio transmission, attributes of human
speech of concern are dynamic amplitude and harmonic
relationship. The latter is extremely important in
identification intelligibility.
Dynamic amplitude can be defined as the varying level
of audio received by a modulation stage in any mode of
modulation. The h~nan voice is made up by a complex
structure of harmonics, the main bands of harmonics falliny
within a 3 kHz bandwidth. A speech band-pass frequency
range commonly selected is 300 H2 to 3000 Hz, and all
other harmonics are genexally suppressed. However, these
out-of-band harmonics define voice character and, thus,
intelligibility. But the suppressed harmonics fall in
such a wide spectrum that if the entire speech harmonic
make-up were to be transmitted, a line transmission band-
width of some 15 kH2 would be required. With modern narrow
band voice line transmission systems, this would become
impossible.
Consequently, speech processing has been utilized
heretofoxe. For example, the use of band-pass frequency
filters in speech line transmission systems is very ommon.
~lthough speech band~pass frequencies often vary, the pass
band rarely exceeds 3 kHz. This type of processing is
used chiefly with narxow band audio line transmission
systems. It has the adv~ntages of being not only simpl.e
and economical to provide but also offering the expedient
of limiting the powex band of speech to a 3 ~Hz window
that can be xeadily utilized in audio line transmlssion
systems.
When electrically processed, signals representing human
speech inevitably vary greatly in ~nplitude. The audio
content of the transmitted signal thus varies accordingly.
Yet the received audio level can only be taken as an
average between the maximum and minim~ dynamic amplitudes
transmitted. One way of increasing the average received
audio level is to use a speech compression system between
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the audio sound and the modulation system, resulting in a
higher average level of modulation and effectively in~
creasing the transmitted audio power. However, in in-
creasing the lower level sequences to ~fectively increase
the average audio content of the transmi~ted signal, all
speech dynamics have to be sacrificed. Thus, increase in
the transmitted or received audio level, after compression,
must be traded of~ against speech intelligibility. Audio
compression has been best ut.iliæed in teletype and data
transmissions systems in which single ~requency pulses are
transmitted and dynamics axe not of concern.
Circuitry providing gain control for high and low
audio bands and subsequent compression of a mixed audio
signal is described in Bloy U. S. Patent No. 4, 400, 583,
entitled l'Compl~te Audio Processing System9'.
In addition to filtering and compression techniques,
various other types of processing have been used in
commercial line transmisslon syst~ns. Where the speech
input to the line modulator is not considered adequate in
d~namic amplitude, simple pre-amplifier devices are often
used.
Line S~stems
Owing to the fact many present day line systems not
only carry audio information in the form of speech but also
carry digital and pulse amplitude modulated carriers, a system
for interfacing audio processing circuitry with line systems
should be designed to handle these various modes of line
tra~sferred information. In mult-iplex carrier systems,
frequency modulation is also used. To accommodate these
functions, interfacing should be able to prove a system
contains demodulation and/or modulation for AM, FM~ CW
and SSB modes of transmission over a communication link~
.
- s -
An object of the invention is to provide a system for
maximizing the efficiency of transfer of modulated signals
in general and especially audio ~requency energy, and
particularly such a system utilizing transmission line
systems.
A further object o the invention is to provide such a
system useful with modulation systems.
A further object o the invention is to provide such a
system ox providing transfer of audio frequency signals,
especially over transmission lines, in such a way that high
average modulation pow~r is attainable.
A still fllrther object of the invention is to provide
such a system for providing transfer, especially over
transmission lines, of signals particularly those of
audio frequency, to enhance signal intelligibillty and
clarity while preventing loss of dynamics.
Another object of the invention is to provide such a
system which allows the processing of signals demodulated
from received line transmitted signals in order to re-
trieve signals, particularly audio signals, with effectivelyhigh signal~to-noise ratio~ even where there is high noise
level associated with the reeived line transmitted signals.
A further object of the invention is to provide such
a system which can be utili~ed for processing.of various
types of signals, e g~, voice~ tones, data, etc., prior
to their use in mcdulatin~ a line transmitted signal, as
well as processing demodulated audio frequency signals upon
reception of a line transmitted signal.
An additional object of the invention is the provision
of such a system which is useful with various modulation
systems and modulation stages to achieve maximum carrier
utilization, with result in incxease in ef~ective line
transmitted power and clarity.
Another object of the invention is the provision Qf
3s such a system which processes audio frequ~ncy signals in
such a way as to reconstitute wide frequency spectrum dynamics
associated with voice and other audio frequency signals
used, inter alia, for modulation of transmission lines, and
which does so by recovery and amplification to audible levels
of harmonics othe~ise suppressed or filtered.
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A related ~urther object of the invention is the pro-
vision of such a system which permits transmission of audio
signals with high average modulation levels, approaching
100% average modulation, but without customary dramatic
S loss of audio dynamics and intelligibility.
An object of the invention also is ~he provision of
such a system which can be utilized in the fields of line
transmission not only of speech but also high speed data,
teletype, facsimile, and other modes of data communication,
and when so utili2ed in data communication, may be advant-
ageously part of a data communication link for reduciny
data drop-out.
It is also an object of the invention to provide such
a system which not only is relatively simple and utilizes
integrated, compact circuit components but which also is
conducive to simple operation~ having a minimum of useable
controls that once set to the appllcation at hand need no
furt~er adjustment.
A related further object of the invention is the pro-
vision of such a system utilizing various visual indicatorsfor keeping the user constantly informed and aware of the
extent to which signals are properly pxocessed through the
system~
An additional object of the invention is the provision
of such a system which accepts substantially any low level
signal within the audio requency spectxum whlle according
to the user the options of utilizing and emphasizing
various components of the audio spectrum, and of selective-
ly filtering and/or amplifying the input and/or output
signals.
Another object of the invention is the provision of
such a system in which the output of the audio siynals
processed through the system can be preset in amplitude ~o
be fed to any modulation stage currently used in conventional
line communication systems.
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A further object of the in~ention is the provision
of such a system which is especially o advantage in
connection with naxrow band VHF line transmission, making
possi,ble extraordinarily narrow band txansmission containing
full dynamic characteristics of audio frequency signals
so processed while keepiny a very high character level of
modulation.
Among other objec~s of the invention may be noted the
provision of such a system which can be utilized in
connection with AM, FM, SSB, PAM, FSK (frequency shift keying),
PSK (phase shift keying) and tone activated TTY line trans-
missions, which also can be utilized in public address
application and muslc amplification systems.
A further object of the invention is the provision of
such a system which provides interfacing with telephone
lines providing impedance matching and automatic selective
direction control for transmission and receiving functions.
Finally, among still other objects of the invention
may be noted the provision of such a system which allows
individual tailoring of harmonic response curves depending
upon natural fxe~uencies of voice signals; which allows
continuous audio frequency gain control; which achieves
linear tracking during audio processing; which operates to
eliminate or greatly reduce third harmonic distortion; which
operates to reconstitute out-of-band dynamics for allowing
them to be transmitted on a narrow band signal; which makes
use of solid state integrated circuit technology; which is
essentially uncomplica-ted as well as being simple to use
and maintain; whirh can advantageously be operated from a
low voltage or battery power supplies; and which exhibits
low power consumption and inherent high efficiency during
operation.
Other objects and ~eatures will be in part apparent and
in part pointed out hereinbelow.
Briefly, a signal transfer and processing system of
the invention includes interface circ~itry and processing
circuitry. The processing circuitry includes a signal
input fox receiving signals (such as audio frequency,
telephone, digital, encypted t or data signals, etc.) to
be processed. These slgnals are supplied to a bandpass
input filter and, thus filtered, to a primary active
frequency control. The output of the latter drives a
primary voltage compressor which selectively llmits signal
dynamics to a predetermined window, being controllably
preset and driven with different bands of amplitude
determined by the primary frequency control to maximum
compression levels indicated by a visual indlcator. The
output of the compressor is presented to a secondary active
frequency control which drives a secondary dynamic compressor
with different audio bands. A second signal indicator
indicates maximum compression levels of the latter. An
automatic gain control tied to the latter compressor supplies
feedback to the primary voltage compressor and also feeds
a third visual indicator which displays compressed output
voltage as well as average peak dynamic compression. The
processed output of the secondary compressor is provided
to a bandpass output filter and, sharply attenuated by the
latter, is delivered for furthex use. A high level output
stage is also selectively utilized. The AGC feedback signal
to the primary compxessor limits primary compression as a
time-delayed function of increase in the level of the
secondary compressor outputO The primary and secondary
compressors may be regarded as respective first and second
dynamic control means, the primary and secondary active
frequency controls as respective first and second tonal
control means, since both compressors and both actlv
frequency controls are configured for permit-ting selective
controlling of their respective functions. The bandpass
input and output filters each may be selectively switched
in or out of the signal processing path. The third visual
indicator may be a meter having a moving coil type movement
to provide an averaging displayO The interface circuitry
provides interconnection of said processing clrcuitry with
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a transmission line for causing signals transmitted over
the transmission line to be automatically directed through
the processing circuitry thereby to produce processing of
audio frequency signals transmitted over the transmission
line. The interface circuitry includes switching circuits
for providing interconnection with telephone lines, for
example, and for selectively switching into the system
demodulators and modulators for AM, FM, CW and SSB modes of
communication.
In accordance with the principal object, the invention
contemplates a signal transfer and processing system for use
with a transmission line and comprises interface circuitry
and processing circuitry. The processing circuitry comprises
signal input means for receiving a line transmissible input
signal to be processed, and signal output means for providing
a processed signal for further use, characterized by primary
dynamic control means for dynamically compressing the input
signal thereby to provide a primarily compressed signal. A
frequency control means selectively controls the relative
level of the primarily compressed signal within different
frequency bands, thereby to provide a frequency controlled
dynamically compressed signal. A secondary dynamic control
means dynamically comresses the frequency controlled primarily
compressed signal, thereby supplying a secondarily compressed
signal. A feedback means provides between the primary and
secondary dynamic control means feedback for limiting the
dynamic compressing of the input signal as a function of
increase in the level of the secondarily compressed signal.
The signal output means provides the secondarily compressed
signal as the processed signal, and the interface circuitry
comprises signal means responsive to signals to be processed
for interconnection of the processing circuitry with the
transmission line for causing signals transmitted over the
transmission line to be automatically directed through the
processing circuitry thereby to produce processing of signals
transmitted over the transmission line.
2~ ,
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BRIEF DESCRIPTION OF THE DRAWINGS
FIGURE 1 is a block diagram of a system constructed
in accordance with and embodying the present invention.
FIGURE 2 is a block diagraIn of the syst~m of FIGURE 1
but illustrating certain features thereof in greater
functional detail.
FIGURE 3 is a block diagram of audio processing
circuitry of the system of FIGURE 1.
FIGURE 4 is a schematic circuit diagram of a band-
pass input filter of the circuitry of FIGU~E 1.
FIGURE 5 is a schematic circuit diagram of a primary
active frequency control circuit of FIGURE 1.
FIGURE 6 is a schematic circuit diagram of a primary
voltage- compressor of the system of FIGURE 1.
FIGURE 6A is a block diagram of an integrated circuit
device forming part of the primary compressor of FIGU~E 6.
. FIGURE 7 is a schematic circuit diamgram of a
seco~dary active freguency control fo ~he system of
FIGURE 1.
FIGURE 8 is a schematic circuit diagram of a secondary
dynamic compxessor of the system of FIGURE 1.
FIGURE 9 is a sch~matic circuit diagram of a bandpass
output ~iltex of the system of FIGURE 1.
FIGURE 10 is a schematic circuit diagxam of an L~n
driye cixcuit including an LED indicator utilized in
the system of FIGURE 10
FIGURE 11 is a schematic circuit diagram of circuitry
o~ an automatic gain control as well as meter drive
circuitry and a meter or indicating operation performance
of the syst2m of FIGURE 1.
FIGURE 12 is a schematic circuit diagram of an ac-tive
filter power supply utilized in the system of ~IGURE 1.
FIGURE 13 is a schematic circuit diagram of an audio
frequency amplifier utilized in the system of FIGURE 1.
FIGURE 14 is a yraph of a family of curves illustra-ting
operation of certain compressor circuitry of the invention.
FIGURE 15 is a graph of a family of curves illustrating
the operation of certain amplification circuitry of the in-
vention, appearing with Figures 12 and 13.
FIGURE 16 is a block diagram illustrating a simplex
communications configuration of the invention, appearing
with Figures 1 and 17.
FIGURE 17 is a simplified block diagram illustrating
duplex communications configuration of the invention, appearing
with Figures 1 and 16.
:10 Correspondi.ny reference characters indicate correspondiny
elements throughout the several views of -the drawings.
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DESCRIPTION OF THE PREFERRED E~BODIMENT
General Descrlption of System
Referring now by reference characters to the
drawings, reference character A is used to designate
generally a system of the present invention for maximum
eficient transfer of modulated audio frequency energy
utilizing transmission line systems. In this sense, the
term transmission line is intended to encompass any of
the general class of signal-transmission line systems
which either comprise conductive connections (such ~s
twisted pairs, telephone lines, etc.3 between system
elements which carry signal power or which have other
types of transmission line components (such as coaxial
cable and waveguide 7 etc.~ for carrying signals of various
frequencies but wherein the signals are modulated or
otherwise contain audio frequency information or are pulse
or otherwise encoded ~as by frequency shifting~ with
information at audio frequency rates, whatever form or
mode Qf transmission or propagation is used.
~he invention is especially useful for use with
signals transmitted by telephone systems which have an
assemblage of telephone stations, lines, channels, and
switching arrangeme~ts for their interconnection, together
with all of the accessories for providing telephone
communication. Thus, in telephony, audlo frequency signals
are transmitted by a telephone line consisting o the
conductors and circuit apparatus associated with the
particular communication channel and consisting usually of
a conductor pair between telephone stations and/or be~ween
such stations and a central office or offices. However,
a telephone channel suitable for the transmission of
telephone signals may not merely consis~ of conductors
bu-t comprises frequently also various other transmission
media, including radio frequency links in general~
%~.
-13
Referring to FIGURE 1, 20 designates an audio
facility such as a telephone transmitter and/or telephone
receiver such as may constitute a telephone set, i.e., ins~ent, in
commercial, domestic or governmental use. Facili~ or ins~n~ent 20
receives unprocessed audio signals as an input, such as
provided by a microphone of the telephone transmitter~
Howevex, audio frequency signals provided as an input may
be those supplied by any of myriad possible sources, such
as the output of a conventional microphone, pre-amplifier 7
tape recorder, output of a modulat.ion system or of a
receiver, etc. These signals may consist of tone pulses,
frequency shift keying pulses, tone encoded information,
teletype signals (TTY), facsimile date, date signals,
and various ~oice txansmissions, data transmissions, etc.
and may consist also of: cyphered or decyphered signals
having constituent components of audio frequency. In this
regard, it can be said that any audio waveform that
de~iates from a true sine wave can be said to be constructed
of related harmonics~ A system of the invention provides
modification of these harmonics.
Audio facility 20 also provides an output for audio
signals which have been processed in accordance with the
invention, these signals being derived from those received
by the system and transmitted through the system to the
system of said audio facility~ These signals are of the
same character howe~er processed by circuitry of the
invention, which are provided as àn input to audio facility
20.
Audio facility 20 is inkerconnected with inkerface
and switching circuitry 30. The function of circuitry 30
is more fully developed hereinbelow but it is o~served at
the outset that circuitry 30 is connected to, and provides
interfacing with, a telephone or other communication
transmission line 40. Transmission line 40 may be a line
consisting of one or more conductors, or waveguide; fiber
optics, cable, and any of various other transmission line
components, including those associated with microwave trans~
mission, which are utilized for the transmission of communic-
ation signals, including data, over one or more channels~
-14,
Interconnected with switching circuitry 30 is a
so-called modem (modulator-demodulator) 50 representative
of any of various types of modulators and/or demodulators
which may be utilized for either modulating signals, in-
cluding radio frequency signals, with the audio input
provided to audio facility 20 to be transmitted via trans-
mission line 40, or for demodulating i.ncoming audio frequency
signals to be provided, as processed audio output, from
audio facility 20. Alternatively, modem 50 may represent
L0 circuitry ox recovering data received over transmission
line 40 and ~or suitably modulating a carrier with encoded
data provided as an input to audio faci.lity 20 or trans-
mission by means of transmission line 40. Also indicated
is a voice operated switching system or so called llvoxl~
15 which contxols operation of the system to permit processing
of audio frequency signals dependent upon the direction over
transmission line 40. Thus, incoming signals received
ovex transmission line 40 may be processed but~ upon the
provision o unprocessed audio fre~uency signals to audio
20 facility 20, VOX 60 is operative to cause circuitry 30
to enable the system for providing processing of the
signals to be transmitted over transmission line 40.
Also shown interconnected with circultry 30 are audio
frequency amplifiers 70 and 80 fox respectively amplifying
25 audio signals provided to audio frequency 20 for trans~
mission and fo~ amplifying incoming signals to be provided
by audio facility 20 as an output. Designa-ted at 90 is a
monitor circuit to provide isolation for monitoring signals
before they are processed by processing circuitry de.siynated
30 generally lO0 in accordance with the inven~ion. Circuitry
30 operates to provide routing of unprocessed audio signals
from audio facility 20 for routing through processing
circuitry 100 when the signals are to be provi.ded for
transmission over transmission line 40. Similarly,
35 incoming signals received from the transmission line
are routed by circuitry 30 through the processing circuitry
lO0 to be provided by audio faci].ity 20 as
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2~ t
-15-
processed audio output. In the preferred embodiment of
the invention to be described with regard to FIGURE 2,
VQX circuit 60 causes the interface and ~switching circuitry
30 to route the signals through processing cixcuitry 100
s in accordance with whether they are incoming or outgoing
with respect to transmission line 40O
Referring now to FIGURE 2, circui-try of the invention
is shown in block diagramatic representation. ~udio
facility 20, which may be a telephone set, including a
telephone transmitter 20a and telep'none receiver 20b,
provide or receives signals from a relay RLla wh~ch is
interconnected with a counterpart relay R1lb by a bypass
line Ll. Relay RLlb is operated with relay RLla under
the control of a switch SWl. In this regard, various
power supply connections ~or the operation of relays of
the invention are not shown~ their connection being apparen~
to skilled circuit designers. Relays RLla and RLlb are
thus opexated by switch SWl either to route signals to and
from audio facility 20 or being transmitted or received
by transmission line 40 either via bypass line Ll or
through circuit~y of the invention.
~ eatures of the circuitry of FIGURE 2 are best
demonstrated by considering the flow of signals through
the circuitry, in accordance with the invention. Unless
switch SWl is operated for permitting signals to be
deli~ered through the bypass line, relays RLla and RLlb
route siynals through circui~ry to be processed by
processing ci~cuitry 100 of the invention. Thus, assuming
that signals are received from transmission line 40, the
same ~re routed b~ relay RLlb to a further relay RL2a
controlled by a switch SW2 D Relays RL2a, RL2b, being under
the control of switch SW2, thus, allow selective amplific-
ation of incominy signals but are controllahle to permit
outgoing signals to be provided throuyh line L2 without
amplification. Switch SW2 provides manual override also
to permit incoming signals to be received without amplific-
ation if desired~
Switch SWl may be manually operated or automatically
operated by the initiation of a telephone connection. In
this regard, it is desired that there be no line dialing
pulses to be pxoces~sed by processing circuitry 100. These
pulses, being DC, would be blocked by circuitry 100 and
cause zero loop current on the telephone line, thus causing
the telephone instrument 20 to become inoperatlve. Thus,
relays RLla and ~Llb can provide automatic bypass switching
that connects the processing c.ircuit~y 100 in line after
0 all dialing and line connection has been achieved. Relays
RLla and RLlb may preferably provide time delayed DC
switching action which allows 15 secs. for line dialing and
phone line access upon whlch kime the processing circuitry
becomes an integral part of the telephone circuit. A dotted
.5 line connection of swit~h SWl with audio facility convention-
ally reflects control over relays RLla and RLlb by audio
facility 20 for providing the ~oreyoing functions.
Relay RL2_ is operated by switch SW2 to selectively
route the incoming or outgoing signals through a conven
~0 tional audio frequency amplifier 80 or else through a line
L2a without amplification, to a corresponding relay RL2b
opexated with relay RL2a and also under the control of
switch SW2. Relay RL2h routes the incoming slgnals to an
impedance matching tr~nsformer Tl through a line L2b.
Outgoing signals are routed to relay RL2a, for amplification,
by a line L2c ~rom transformer Tl, The latter provides
impedance matching of the 600 ohm impedance, for example,
o~ transmission line 40 to a higher impedance of nominally
50,000 ohms associated with circuitry 100. O course,
50 ohm or 300 ohm transmission lines may be used for trans-
mission. Thus, trans~ormer Tl and other impedance matching
trans~ormers T2 and T3 are selected (or may have appropriate
windings~ for the desired impedance match.
Transormer Tl supplies the incoming signals to a
~urther relay RL3a which, with a corresponding relay RL3b~
is under the control of a manual switch SW3. Relay RL3a
delivers the incoming signals via line L3 to relay RL3b
-17- ~
and ~lence, via lines L4 and L5 to the input of processins
circuitry 100 of the invention. Signals are processed in
accordance with the operation of the processing circuitry
100 discussed hereinbelow, beiny provided upon an output
S line L6 which also is interconnected with relay RL3a. ~he
latter routes signals provided upon output line L6 through
a further line L7 to a further impedance matching transformer
T2, which again provides a match between the 50,000 ohm
impedance of circuitry 100 and ~he 600 ohm impedance associated
L0 with telephone lines~ Trans~ormer T2 provides the processed
signals by a ~ine L8a to a relay ~L4a which, with a corres-
ponding relay RL4b, both being operated by a switch SW4,
permits signals ~o be amplified by an audio frequency amplifier
70 or else provided directly over a line L8b. A line L9
between relay RL4a and relay RLla routes the processed
received signals to audio facility 20 for being reproduced
by telephone receiver 20b.
The route taken by audio signals picked up by telephone
receiver 20a demonstrates still other features of the
circuitry. Thus, the user at a local facility providing an
audio signal to transmit~er 20a has the result of pxoducing
signals by means of audio facility 20 which are delivered to
relay RLla for beiny routed through line L9 to relay RL4a.
Depending upon the position of switch SW4, relays RL4a and
RL~b permit audio frequency amplifier 70 to amplify these
signals so that they are of a level adequate for being pro-
cessed by circuitry 100. Or, if they are already o high
level, they may be provided directed by line L8 to relay
RL4b. Regardless of whethe.r ampliied or not, signals re-
ceived by RL4b are made available by a llne L8c to a furtherimpedance matching transformer T3, which again may provide
matching between the 600 onm ].ine impedance and the 50,000 ohm
impedance of circuitry 100. The high impedance secondary
OL transformer T3 provides the output signals -to the VOX
circuit 60~ The latter controls a relay RL4 to provide a
bypass route for signals lf the VOX circuit 60 senses the
presence of audio signals being delivered by the secondary
of transformer T3. Circuit 60 is of an entirely conven-
tional nature such as will be understood by those skilled
in the design of audio circuitry and is confiyured for
contxolling the operation o~ relays RL3a and RL3b. ~en
VOX circuit 60 senses signals provided by transEormer T3,
18-
thus indicating that telephone transmitter 20a is the
predominant source of signals, relays RL3a and RL3b are
switched to provide routing of signals as ~ollows:
Relay RL3b supplied signals ~rom bypass relay RL4
via line L4 to the input, via line L5 of processing
circuitry 100. Similarly, line L6, providing the
processed audio output signals, .is connected with relay
RL3a and the latter then routes signals to -transformer
Tl and, thence, via relays RL2a, RL2b, to relay RLlb
for being delivered upon transmission line 40.
Also interconnected with relay RL3b is a line L10
which pro~ides for the delivery of signals, pri.or to
processing, to an RF amplifier 90a, the operation of
which .is contxolled by a switch SW5~ When enabled,
lS amplifier 90a provides high level output via a line Lll
for permitting test monitoring, recording, etc. of
signals prior to being processed through circuitry 100~
Indicated at 50a, 50b and SOc are three modulator-
demodulato.r circuits (so-called modems). These have
respective outputs L12a, L12b and L12c for receiving
un~odulated RF inputs of AM, FM or SSB characterO These
modulato~-demodulators have respective outputs L13a,
L13b and L13c for providing modulated outputs of ~M, FM
or SSB type, respectively, for delivery via t~ansmission
line, microwave, etc. Signals which are received by any
of circuits 50a, 50h or 50c and which are demodulated
are made availa~le for processing in accordance with the
invention. For this purpose, there are shown interconnected
with the circuits respecti~e switches SW6a, SW6b and SW6c
which are each interconnected with th~ respective modulator-
demodulator circuit 50a, 50b or 50c and also with the
input line L5 of processing circuitry 100 and its ou-tput
line L6. Accordingly, the demodulated signals are provided
to processing circuitry 100 to be processed before being
redeli~ered via the respective switches to the correspondinc3
modulator~demodulator for modulation o the appropriate
RF output thereo~
-19-
Therefore, it is understood that the circuitry of
the invention may be used for the processing of audio
frequency signals no matter whether they are transmitted
via telephone or other transmissi.on line or via radio.
Although the system demonstrated in FIGURE 2 is
described as having various relays, pxefera~ly the
system is constructed utili~ing electronic switching
devices or solid state relays in lieu of mechanical
relays~ Therefore, a system of the invention can be
L0 constructed using conventional state-of-art logic
circuitry, including microp~ocessors and the like, for
control of switching and other operatisnal unctions.
Further understanding of the system will be had
by considering the various features of the circuitry
15 shown in block diagrammatic form in FIGURE 1 and in
greater detail in FIGUXE 2, Because an undexstanding
of the audio prscessing provided by the system of the
invention is cxitical to an understanding of the
configuration and operation of the system in its
20 entiret~l, the audio processing circuitry 100 is first
described.
-20
Audio Processing_C ~
Audio frequency signals to be pxocessed by the audio
processing circuitry 100 delivered to an input 113. After
being processed, -the signals are delivexed for urther
use by an ouput 115. Output signals delivered by output
115 are utilized for purposes such as the driving of a
modulator, for delivery to an audio amplification stage,
further audio processing or various purposes such as
amplification, recording, decoding, retransmission, and
so forth. An auxiliary output 117 is provided for
presenting output signals at high levels such as for
driving various recording devices, e.g., oscilloscopes,
spectrum analyzers, and the like, without limitation.
Although signals to be processed before, or after,
transmissisn over a line system may most typically be
constituted of audio frequency information containing
voice content or speech, it is emphasized that tones and
-tonal data of various types of an audio frequency, as
alluded to hereinabove, may be processed advantageously
by processing circuitry 100, such as to improve signal
to-noise ratio (S/N) and/sr achieve other objects of the
invention~ Thus, it is to be understood that audio
processing circuitry 100 is utilized for transferring audio
frequency signals before or after live transmision in such
a way that the efficiency o transfer is maximized or
optimized by processing the signals for various purposes
such as, for example, to provide high average modulation
power, to enhance signal intelligibility, to provide high
clarity fo signal transmission, and to avoid loss of audio
dynamics, among numerous other objects hereinabove stated.
Input 113 delivers audio signals to a bandpass input
filter 119 for providing filtering of frequencies to
achieve a pass band o between 300 Hz and 3 kHz. Filter
119 effectively limits all other frequencies. The pass
band thus achieved is merely that preferred and utilized
to advantage in the operation of the presen-t system and
may be varied in accordance with the purpose intended Eor
-21-
the present system, e.g~, in haviny different widths and
different lower and upper frequency limi~s. A switch
SW7 connected by a circuit lead 120 between input 113 and
the output 121 of filter 118 permits selective disablement
of filter 119 for purposes stated hereinbelow.
The output of the bandpass input filter 119 is
delivered by output connection 121 to a primary active
frequency control 1230 Control 123 effectively splits
the audio spectrum of the signals delivered to it into
two separate frequency bands. Pxeferably, although not
necessarily, the lower band is from 250 Hz to 1.2 kHz
and the upper frequency band is from 1.4 kHz to 3.5 kHz.
Control 123 includes means for selectively controlling
the amplitude of the audio content of each of these audio
bands allowing the user either to attenuate or to provide
gain in each of said bands over a preferable xange of
+ 10 db. Also, the primary active frequency control 123
prererably incorporates an input gain control for purposes
later appearing. The control components of circuit 123
also allowing individual tailoring of o~t-of-band harmonics
which are reconstituted by processing of audio signals
by circuitry 100 of the system.
Signals from control 123 axe then provided by a
connection 125 to a primary voltage compressor 127. The
latter provides a relatively high compression ranget i.e.,
preferably 135 db, as well as pre-emphasis of high frequency
audio components to compensate for high frequency losses
which otherwise could occux during processing. Compressor
127 is preferably selected to lLmit all audio dynamics
to a 27 db window with a tracking error of not greatex
than about ~ 3 db. Included within compressor 127 is
a variable gain cell which is indirectly controlled,
via a lead 128, by an automatic yain control (AGC) circui~
129 described hereinbelow.
-2~-
Also connected with compressor 127 by a lead 130 is
an LED drive circuit 131 for driving an LED indicator 133
in accordance with the operation of compressor 127 to provide
the system operator with an indication of the extent to
which maximum useable compression is being provided by
compressor 127.
The outpu~ of compressor 127 is delivered to a secondary
active frequency control 135. This circuitxy is adapted
to split the now dynamically compressed audio signals into
10 two frequency ~ands; there being a lower frequency band of
preerably from about 300 Hz to 1. 5 kHz and the upper
frequency band of preferably from about 1.5 kHz to 3 kHz.
Control 135 is adapted for providing gain and attenuation
control within these two frequency bands variable over a
15 range of preferably ~ 12 db. For this purpose, manual con-
trol means are provided fox the user to selectively deter-
mine the gain or attenuation within each frequency bandO
U~like the pri~ary active frequency control 123, control
135 is adapted for providing gain peaking and attenuation
20 occurring about cent~r frequencies with the respective lower
and upper bands at preferably 1 kHz and 2.4 kHz. This
feature ~llows control 13S to selectively disregard inter-
acting, and possibly distortion-productive audio harmonics
occurriny within the input pass band establis~ed by input
25 filtex 119. The frequency controls within control circuit
135 may be set to provide neither attenuation nor gain so
that not only would no gain or attenuation but also no
distortion will be provided ox the signal passing through
control 135.
The output of frequency control 135 is fed to a
secondary dynamic compressor 137 providing an extxemely
fast tracking system with extremely low tracking distortion
(preferably less than about 0~ and accepting and providing
compression of signals with a dynamic amplitude range of
35 up to abou~ 120 db while achieving a compression window of
preferably only 50 db, yet providing a third harmonic
distortion (THD) figure of less than preferably 1~. AGC
circuit 129 is interconnected with circuit components of
compressor 137 by a connection 140 to provide an input for
~L~'~..~V
~23~
AGC 129 which in turn controls primary voltage compressor
127 by circuit connection 123, thereby providing a negative
AGC feedback loop for limiting the degree of primary voltage
compression provided by primary compressox 127 as a time-
S delayed function of compression by secondary compressor 137,
all as more fully explained hereinbelow. Xowever, briefly
it may be noted that the connection of AGC circuit 129
to compressor 137 provides to primary compressor 127 a
d.c. reference signal derived from compression stages of
compressor 137~
In effect, AGC circuit 129 by interconnection with
secondary compressox 137 ampliies a tracking voltage
output of the secondary compressor and delivers a voltage
varying withi~ preset paxameters to a variable gain cell
of primaxy voltage compressor 127 as a fu~ction of this
tracking Yoltage. This is carried out for the purpose of
achieving extremely high tracking stability and for limiting
the third harmonic distortion while reconstituting through
voltage gain via ~he secondary active frequency control
Z0 135 of the original audio dynamics fed to secondary dynamic
compressor 137. Howevex, reconstituting of otherwise lost
audio dynamics occurs as well in oth~r portions of the
system circui~ry.
Interconnected as i~dicated at 141 with automatic gain
control 129 is a meter drive circuit 143 for driving, as
indicated at 144, a meter 145 preferably of a moving coil
movement type to provide averagingO The meter serves as
a visual indicatox for displaying not only the compressed
output voltage provided by secondary dynamic compressor
137 but also average peak dynamic compression, and thus
indicates the extent to which the overall capability of
the audio processing circuitry 100 is being utilized.
Interconnected as indicated at 147 with the output
148 of secondary dynamic compressor 137 is an LED drive
circuit 149 for driving an LED indicator 150 to indicate
the degree of compression being achieved by secondary
dynamic compressor 127. The output of compressor 137 is
delivered to a bandpass output filter 152 for providing
sharply attenuated filtering of the now processed audio
2~i
~ 24-
signals within a preferable pass band o~ between 300 Hz and
3 kHz with very sharp roll-off or corners at the edges of
the pass band to limit the processed audio between these
upper and lower limits. Fil~er 152 preferably provides
unity gain and has an extremely low noise Eigure so as to
avoid introducing ~urther noise in~o the now processed
audio. If desired, ~he filtered audio signals pxesented
at output llS may be at~enuated to provide signal levels
suitable for other systems bein~ driven by the presen-t
system. Also interconnected in a circuit 153 around filter
152 is a switch SW8 for selectively disabling the operation
of filter 152 for purposes noted hereinbelow. The output
of filter 152 is also provided, as indicated at 15S, to
an audio amplifier 156 constituting a high level output
stage and providing the high level output 117 noted pre-
viousl~.
Now that the general circuitry of ~he audio processing
circuitry 100 has been described, the spe~ific circuitry
Qf each of the blocks designated in FIGURE 3 i~ described
hereinbelow.
Generally speaking, interconnection between the in-
dividual circuitry described in FIGURES 4-13 are indicated
by the aligNment of leads.
Referring to FIGU~E 4, the circuitry of bandpass
input filter 119 is seen to comprise circuit elements
providing cascaded high and low frequency seasons. An
operational ~nplifier OAl has its non-inverting input
intPrconnected with input terminal 113 through a pair of
capacitors Cl, C2 and biased to g.round through a resistor
~1. Feedback between the output of the amplifier and said
input is provided by a circuit including a capacitor C3
and resistor R2, with capacltor C3 being connected to a
node between capacitors Cl and C2 to provide capacitive
feedback for tailoring ~requency response. This node is
also biased to ground through a reslstor R3. The non-
inverting and inverting inputs are connected to circuit
ground through respective capacitors C4, C5. A bypass
capacitor C6 conventionally connects a power supply te~ninal
158 to circuit ground.
-25-
The output of operational ~mplifier OAl is series-connected
thxough a capacitor C6 and resistor R4 and a further r~sistor
R5 to the non-inverting input of a further operational
ampli~ier OA2. Capacitive feedback between its output and
its non-inverting input is provided bty a capacitor C7 for
frequency response contxol with d.c. feedback for gain
control being provided by a resistor R6. It is noted that
a resistor R7 and capacitor C8 are connected in parallel
between the non-inverting input and c:ircuit ground, the
node common to resistoxs R4, R5, and R6 being ited similarly
to ground through a capacitor C9, the inverting input of
operational amplifier OA2 is tied directly to ground.
The output of operational amplifier OA2 is provided through
a capacitor C10 to output lead 121.
A conventional bypass capacitor Cll connects operation
amplifier OA2 to the circuit gro~nd. Switch SW7, which
is connected by lead 120 between said input terminal 113
and lead 121 is provided so that, when it is closed, the
bandpass input filter is effectively rendered inoperative,
i.e., taken out of the circuit. This allows selective
eli~ination of the bandpass input filtering under certain
possible conditions of use in which it i9 not desired or
not necessary to limit the audio signals being processed
to the narrow pass band ordinarily determined by the
lnput filter circuitry 119.
The selection of various components utilized in
connection with operational amplifiers OAl, OA2 is a
matter of design choice to achieve the bandpass upper and
lower frequency limits re~erred to previously. Each of
the operational amplifiers may be part of a single
commercially available integrated circuit (IC) type such
as LM387 exhibiting electrical characteristics and peri-
meters compatible with the node of intended usage of the
present system and requiring power supply voltages (as
delivered by terminal 158) of, e.g., ~ 15 v.d.c. As will
be fully understood to those skilled ln the art, the
circuit values such as that of resistor R6 may be varied
to control the gain of the bandpass input filter. But
26-
preferably filter 119 is configured to provide unity gain.
It is preferred that filter 119 provide upper and lower
corner frequencies o 2.7 kHz and 300 Hz, respectively,
and a roll-of characteristic of -40 db. per decade, as
well as very low third harmonic distortion (T~ID). Filter
119, providing uni-ty gain, preferably can handle an input
signal of between -35 and ~10 db, without clipping or
distortion.
The primary active frequency control 133 similarly
utilizes two operational amplifiers OA3, OA4 ~or control
of the amplification of ~he gain o individual upper and
lower frequency bands. Interconnected with each of these
operational amplifiers are respective xesistive-capacitive
circuits 160a, 160b for establishing, with respective
-
operational amplifiers, individual circuits for providing
seleckive amplification and control of audio frequency
components within the respective audio bands.
More specifically, the input signal provided by
lead 121 is delivered across a potentiometer R9 having a
wiper 159 selectively variable for controlling the overall
gain of stage 123~ Since the two individual active
frequency control circuits 160a, 160b have certain
corresponding components which are connected ln identical
manner~ coxresponding elements are referred to by corxesponding
reference numerals with each numeral being followed by a
subscript "a" or "b", as appropriate. Circuit 160a is
~escribed exemplarily. It comprises a capacitor C13a
coupliny the signal present on wiper 159 through a resistor
R10 to a further potentiometer Rlla having a wiper 161
which is ~electively controllable by the user to serve as
a high frequency gain control connected from opposite sides
of potentiometer R11 to its wiper are capacitors C14, ClS.
The signal present on 161 is provided through a resistor
R13 and thence through a capacitor C17a to the non-inverting
input of operational amplifier OA3, which has its inverting
input referenced to ground through a circuit including a
resistor R14a which is in turn shunted by a series-connected
capacitor C18a and resis~or R15a~
-27~
The opposite end of potentiometer Rlla is connected
through a capacitor C20a and resistor R16 to the circuit
ground. Also interconnected with the non-invexting input
of operational amplifier OA3 is a frequency compensating
circuit comprlsing resistors R17a and R18a and a capacitor
C19a connected across the latter and wit~ one end thereof
belng connected to circuit ground. A resistor R20a re
ferences the node between resistors R17a and R18a to the
power supply potential provided to a terminal 162 for
offset ~rror compensation, said power supply potential also
being provided by a lead 163 to the operational amplifier
for powering same~
Negative feed~ack for operational amplifier OA3 is
established by a voltage di~ider including resistors ~21a
a~d R22a and a further resistor R23a interconnecting the
node between the former two resistors and the inverting
input of the operational amplifier. Conventional compen-
sating capacitors C21a and C22a are connected in thP
typical fashion to the operational amplifier.
As thus configured, circuit 160a together with
operational amplifier OA3 constitutes an active high
frequensy con~rol circuit providing gain control over
~requencies determined by the setting of potentiometer
wiper 161, and with gain in the re~uency band beiltg variable
over the range of ~ 12 db. within the frequency range of
preferably about 1.5 kHz to 3 kHz~ Gain peaking and
attenuation peaking occurs oreferably about 2.4 kHz. The
output of operational amplifier OA3 is provided through a
capaci~or C25a to output l~ad 125.
5Lmilarly, circuit 150b together with operational
ampliier OA4 constitutes an active low frequency contxol
circuit providing gain control frequency over frequencies
determined by the setting of a wiper 165 o~ potentiometer
C20a, and thus determining the level of signals provided
to the operational amplifier OA4 through capacitor CL7b.
The frequency band preerably is rom 300 Hz to 1.5 kH7
and with gain peaking and at'cenuation peaking occurring
at preferably about 1 kHz. The low frequency components
-28-
are provided from the output of operational amplifier OA4
through a capacitor C25b and thus delivered to output lead
125.
Preerably, although not necessarily, operational
amplifiers OA3 and OA4 may both be available as a single
integrated circuit (IC) such as of the commercially avail
able type UA739, thus providing high loop gain without any
substantial distorkion.
The mixed high a~d low frequency audio components
provided to lead 125 are deliverea to the primary voltage
compressor 127. Referring to FIGURE 6, compressor 127
comprises an integrated circuit ICl constituting a compressor-
expandor or so-called compandor including a full wave
rectifier, a variable gain cell and an operational amplifier
including a biasîng system within it, all as shown in
FIGURE 6A. Although a general depiction of elemen-ts thereof
is represented by FIGURE 6A, no specific description is
necessary in view of the fact that circuit ICl may be of
commercially available form such as that designated NE570/571.
But it is noted that there is an internal summing node
within the operational amplifier (not shown) which is biased
at a voltage preference, and signals supplied to the inte-
grated circuit are averaged by circuitry interconnected
therewith, as shown in FIGURE 5. The averaged value of the,
input signal establishes the ~ain of the variable gain
cell (see FIGURE 6A) which is, accordingly, proportional
to the average variance of the input signal cap,acltiv~ly
coupled thereto. In any event, the operation of such
circuitry is described and will be understood by reference
to the above-noted U.S. Pa~er~t No. 4,400,583 of
Graham P. Bloy, entitled "Cornplete~iAudio Processing
System", dated August 23, 1983.
Although the gain cell of compandor circuit IC1 func-tions
as an expandor, by providing negative feedback to the
operational amplifier therein, compression is realized.
Similar cir'cuitry is utilized in secondaxy dynamic compressor
137 to which attention is directed, as hereinbelow described,
for further understanding of the operation of primary voltage
-29-
compressor 127. For present purposes, it is su~ficient to
observe that ICl is in-terconnected as shown by pinout
numexals, with the following resistive and capacitive
circuit components which may have the nominal values
indicated:
~ABLE 1
R25100 kohm C260.47 mfd
R2620 kohm . C27 10 mfd
R2720 kohm C28 1 m~d
R28120 kohm C292700 mfd
R29100 kohm C30 1 mfd
R3047 kohm C312200 pfd
R3147 kohm C32 5 pfd
R32100 kohm C33 10 md
R3350 kohm C34 10 mfd
R3470 kohm
R3550 kohm
In a~cordance with the inven~ion, interconnected with
the gain cell of the compandor circuit ICl is lead 128
which provides a feedback signal of a limiting character
from the automatic gain control circuit 129 as a function
o~ the overall gain signalled by gain contxol circuitry
129~ Here, it ls observed that said lead 128 provides said
AGC signal to the gain cell o compandor ICl through
resistor R26. The overall purpose and function of thi~
negative feedback is discussed below but it is noted pre-
liminarily that the gain cell within circuit ICl is controlled
in a negative sense in response to increase ln the AGC
signal supplied by lead 128, thereby to reduce the amount
of primary compression achieved by stage 127 with increasing
gain feedback. Thus, the eedback is essentially negative
or limiting.
At 167 is indicated a terminal for supplying low
voltage d.c. potential for operation of circuit IClo As
will be apparent, the dynamically compressed output slgnal
is provided by capacitor C34 across a load resistor R32,
and is appropriately attenuated by R35 for delivery through
a resistor by lead 127 ko the secondary active frequency
control circuitry, shown in FIGURE ~. However, prior -to
A ~I T~ P~ ,pl~ ~
-30~
being coupled through capacitor C34, circuit lead 130
provides tha compressed audio output from stage 127 to
LED drive circuit 131, as disclosed in FIGURE 10.
Referring to FIGU~E 10, circuitry is shown for con~
5 stituting the LED drive 131 and LED 133, as well as the
I.E~ drive 149 and LED 150. As shown therein, the LED
indicator lamp, whether that designated at 133 or that
designated at 150, is suitably mounted for being observed
by the user, being driven by circuitry including a pair
10 of NPN txansistors Ql, Q2~ The hase of transistor Q2 is
drivan by a unipolar signal provided thxough a diode Dl
and d~livered through a resistor R3~o The base of transistor
Ql is biased to ground through a resistor R38. Its collector
is provided with a supply potential by means of a terminal
15 171 through a current limiting resistor R39, while a similar
resistor R40 provides a supply of voltage to the collector
of txansistor Ql. ~he base of the latter is connected
through a xesistor R~l to the collector of transistor Q2.
Coupling is provided between the collector of transistor
Ql and the base o~ transistor Q2 by a series connected
capacitor C36 and resistor R42. Thus, there is provided
a two-transistor switching circuit whexein the LED will be
dxiven on when there i5 sufficient base drive provided to
transistor Q2, as by the output signal of primary compressor
127 or secondary compressor.
Referring now to FIGURE 7, the secondary active
frequency control 135 fùnctions to reconstitut~ harmonic
dynamics lost in processing o~ the signal through primary
comp.ressor 127. Control ~35 comprises a pair of operational
amplifiers OA5, OA6 which are connected to provide high
and low pass sections o~ a Butte~70rth filter. The
operational ampll~iers may each be of the commercially
available type designated LM349 with both operatlonal
amplifiers being part of a single integrated circuit which
is adapted for being powered by positive and negative
potentials, as at 15 v.d.c., supplied to respective terminals
173, 174. Bypass capacitors C38, C39 are connected across
-31~
these power supply inputs. Each of the operational
amplifiers has its inverti.ng input connected to the
circuit ground. The input signal is provided by lead
127 through a capacitor C~0 and resistor R44 to the
non-inverting input of opexational amplifier OA5. A
resistor R45 interconnects the output and latter input
of the operational amplifier for fee~back purposes, the
input being biased to ground through a further resistor
R46.
The output of operational amplifier OA5 is provided
through a capacitor C42 to a frequency gain control circuit
175 having two parallel branches providing at one end a
node 176 constituting an input of the circuit and an
opposite node 177 interconnected with the output lead
136 of frequency control circuitry. A first one of the
branches comprises a resistox R47, a potentiometer R48t
and further resistor R49, there being capacitors C43, C44
connected between opposite ends of potentiometer R43 and
its wiper 178. The wiper is interconnected through a
resistor R50 and capacitor C46 to ~h~ wiper 173 of ano~her
potentiometer R51 forming wit~ resistors ~52 and R53 in
series with it a second parallel branch of circuit 175.
A node 179 between resistor and capacitor C46 is connected
to the non-inverting input of operational amplifier OA6,
which input is biased to ground through a resistor R54~
These component values are pre~erably selected to provide
a low pass Butterworth filter defined by the following
equations:
sK
T(s) - s + B
2K'
T(s) =
s + 2Bs + B
T (5~ ~ s3K 3
3 2 2 3
s + 2Bs + 2B s -~ B
~ '
T(s) - s K 4
A ~ '~ J 4
~0 s~ + 2.613Bs + 3.414B s + 2.613B s ~ B
-32-
For equatio~s relating t.o the high pass sec~ion of the
Butterworth filtex, the transfer functions are as follows:
T (s~ = s
T (s) = 2- 5
5 ~ w2s t w2w
T(S) = s
1 0
T~s)
s ~ w4s * w4w3s + w4w3w2S ~ W4w3w2wl
In acçordance with the above design formulas, the
secondary active frequency control is designed to provide
tailoring of the response curve between 300 Hz and 3 kHzo
The low frequency section of the circuitry of frequency
control 35 is designed to control response botween 250 ~z
and 1200 Hz~ the high frequency section being designed ~o
control response between 1400 Hz and 3500 Hz. These ~igures
are those preferred for various purposes contemplated for
the invention but are not necessarily rigidly absolute for
certain other applications, to which extent they may be
subject to variation within the scope of the invention.
To provide such operation, operational amplifier OA5
acts as a buffer to ensure low drivlng impedance to the low
and high frequency control circuits 175. The input impedance
of this stage o the buffer amplifier is about 0.1 megohms.
Capacitor C40 and resistor R44 provide d9c. blocking and
impedanc~ matching for the input of operational amplifier
OA5. Resistor R45 provides a feedback path between the
o~tput and the non lnverting input of thls operational
~lplifier. Capacitor C42 provides d.c. blocking of the
output of the operational ~nplifier~ High frequenc~ con-trol
elements comprise resistors R50, RS2, and R53. Potentiometer
R51 allows ~he high frequency section of the filter to be
set be-tween preerably 0 and 22 db gain. The low frequency
~33-
section comprises resistors R47, R49 and potentiometer
R48, as well as capacitoxs C43, C44~ Potentiometer R48
allows low frequency control preferably between 0 and 22
db gain~
In effect, node 179 represents the output of the
frequency control circuitry 175, belng this supplied to
the non-inverting input of operational æmplifier OA6, which
acts as the active element within this active frequency
control circuitry. A feedback path is provided by the
interconnection with node 177 because of the interconnection
with resistors R49, R53. Capacitoxs C40, C4~ provide not
only d.c. blocking but establish low frequency roll-off
of the circuitry.
Although integrated devices other than the type LM349
noted above may be utilized, it is preferred tha~ any
inteyr~ted operational amplifier substituted have a fast
slew xate such ~s better than 2~5 v. per micro second
allowing undistorted full swing performance up to a frequency
of 25 kHz. Also, the total harmonic distortion is preferably
kept low being typically not greatex than 0.05~ ~0 dbm
(i.G.) O.77 v.3 across the complete audio spectrum. In this
regard, resistor R45 ensures stability at unity gain as the
integrated circuit operational amplifiers are internally
compensated for positive gains of five or gre~ter, in the
case of the LM349 integrated circuit.
Chosen design specifications are preferably such that
the low fre~uency section o~ the seconda~y active filter
circuitry has a gain of 22 db with a low frequency lower
3 db corner at 30 Hz and with a high frequency upper 3 db
corner at 10 kHz. As referred to hereinabove, the term
corner or corner frequency re~ers to cut~of frequency
conventionally representing 3 db insertlon loss of the
xespective circuit.
-34-
Circuitry for providing regulated potentials suitable
for the operational amplifiexs fo the secondary active
frequency control 135 is illustrated in FIGURE 12. In the
circuitry shown therein, a pair of terminals 181, 182
provide a.c. line voltage across the prlmary winding 183
of a transformer having its secondary winding 184 connected
across a full wave bridge rectifiex 185. Connected across
the latter is a first filte.ring or aOc. decoupling capacitor
C48O Across this is connected a pair of resistors R56,
RS7 which in turn each have connected across them capacitors
C50~ C51 whereby there is provided a floating ground node
186 intermediate these capacitors. Node 186 is connected
to the circuit ground. The potentials on the opposite
sides of each of capacitors C50, C51 are provided to
respective integrated circuits IC2, IC3, such as each o~
commercially available type UA7815 integrated circuit
regulators. Each such integrated circuit is connected
also to the circuit ground and each provides a respective
output 187, 188 to provide highly regulated voltages,
preferably, +15 v.d.c~ and ~15 vod~c~ for powering operational
amplifiers OA5, OA6 of the secondary active frequency
control and others wi~h a voltage stability o~ within ~
Similar circuitry may be utilized for developing other
supply potentials for operation of other components of this
system. Alternatively, battery power supplies may be
utilized.
Re~erring to FIGURE 8, which demonstrates the circuitry
of che secondary dynamic compressor 137~ inpuk.lead 134
provides the signal processed by secondary active frequency
control 135 across a resistor RS9 and through a coupling
capacitor C53. The signals are then provided through a
high frequency emphasis network comprising a capacitor C54
and resistor R60 to a resistor R61. Said resistor R61 then
couples the inpu'c signal directly to the inverting input
of operational amplifier OA7. The inverting and non-inverting
inputs of the operat~onal amplifier are tied together by a
35-
series circuit comprising a capacitor C55 and resistor
R6~ The inputs of operational ampliier OA7 are inter-
connected with a gain cell 185 and a full-wave rectifier
186, bo~h of which preferably are portions of an integra-ted
circuit compressor expandor i.e., compandor, device such as
that commercially available under type designa~ion NE570/571,
all as described below.
Such a device may also include an internal or self-
contained integrated circuit operational amplifier but, in
the case of the preferred compandor circuit NE570/571
employed in the present system, the internal operational
amplifier is not utiliæed~ Conneckions are instead made,
as indicated in FIGURE 8, to the gain cell 185 and
rectifier 186 components of such compandor circuik. Also,
there are internal resistive components of compandor device
including a resistcr R63 which is connected to the output
: 187 o the gain cell and connected at the opposite end to
the circuit ground. A node 188 of the cixcuit repxesents
the input to the gain cell, there being an intexnal resistor
R64. Gain cell 185 is adapted to be controlled by rectifier
186 iIl accordance with a capacitor interconnected by a
terminal 189 of rectifier 1860
Brie.~ly, rectifier 186 provides full~wave rectification
of the audio signal supplied to the input of compressox
137, as reflected at the output of operational amplifier
OA7, which output signal, as fed back to the rectifier, is
averaged by the capacitance seen at termlnal 189. The
averaged signal is then provided to the gain cell 185 which
in turn provides a gain control signal to the inverting in-
put of operational amplifier OA7.
Normally, a capacitance as provided by a discrete
circuit capacitor would be in~erconnected with said
terminal 189. But, in accordance with the present in-
vention, a capacitance expandor circuit 190 comprising
operational amplifiexs OA8, OA9 is interconnected with
rectifiex 186. Circuit 190 includes a capacitox C56 which
is interconnec'ed between the output of operational amplifier
OA9 and the non~inverting input of operational amplifier OA6,
which point is a~so interconnected with the capacitance
input 189 of rectifier 1860 The inverting input o
2~ , .
-36-
of operational amplifiex is tied directly to its output,
which is coupled by a resistor R65 to the non-inverting
input of operational amplifier OA9, which input is biased
to ground through a pair of diodes D2, D3O A load resistor
R66 is connected across the output of operational amplifier
OA9.
Also shown interconnected with rectifier 186 is a
resistor R67 which is internal to the integrated circuit
device type NE570/571 preferred whereby a ~erminal or node
191 constitutes the input to the rectifier stage. Inter-
connected with node 191 is a parallel combination of a
resistor R68 and capacitor C58 wilich provide high frequency
pre-emphasis for the signal provided to the inpu~ of the
rectifier. Such input is delivered by a capacitor C59 whlch
is interconnected with the output of operational amplifier
OA7. Similarly, a capacitor C60 interconnects this output
with the input 188 of the gain cell.
Accordingly, there are interconnected with the output
of the operational ampliier two feedback paths, a first
bei.ng provided to the gain cell 185 through capacitor C60
and resistor R64. A second feedback path is provided
through capacitor C59, the pre~emphasis network noted above,
and resis~or R67 to rectifier 186. The rectifier controls
the gairl provided by gain cell 185, which is an integrated
circuit-reali2ed curxent in, current out device with the
ratio IoUt/Iin contxolled by rectifier 186 providing an
overall gain which is an exponen-tial function of the input
signal. Gain cell 185 equivalently functions as an expandor
but, since its output 187 is lnterconnected with ~he in-
verting input of operational amplifier OA7, will be seen to
provide, in effect, negative feedback for causing compression
of the input signal delivered by input lead 136 to be
realized.
Furthermore, since circuit 190 operates to expand the
apparent magnitude of capacitor C56 as a function of the
relative amplitude range of the signal provided at the out-
put of operational anplifier OA7, the amount of current
provided by rectifier 186 to gain cell 185 is varied as a
further function of the magnitude, i.e., dynamic range, of
the signal bein~ pxocessed.
~ lt~
-37-
In addition to the foregoing feedback circuits, a
fuxther feedback circuit comprising a pair of resistors
R70, R71 is interconnected between the output and inverting
input of the operational amplifier, there being a capacitor
C62 providing a.c. bypass to circuit ground. Resistors
R70, R71 provide d.c. feedback path for operational amplifier
OA7 since there is no d.c. feedback path th.rough gain cell
1~5. A bias potential as provided also to the inverting
input of the operational ~mplifier through a resistor R72
by connection of a terminaL 192 to a suitable potential.
In addition, the gain cell is provided with a terminal
193 to be supplied with a THD trim potential or bias which
is provided through a resistor R73 connected to the wiper
194 of a potentiometer R74 across which is provided a 3.6
volt d.c. potential by means of a terminal 195. Wiper 194
may be positioned to adjust the THD trim. Terminal 193 is
also interconnected by resistor R75 with the non-inverting
input of operational amplifier OA7.
D.c. shift trim for the gain cell input 188 is provided
similarly through a resistor R75 in accordance with the
position of wipex 196 of a potentiometer R76 across whi.ch
a 3.6 volt doc~ potential is supplied. Upon calibration of
the circuitry to provide appropriate THD trim.and d.c. shift
potentials by positioning of wipers 194, 195, their further
adjustment is not required.
In order to limit the ou~put potential of operational
amplifier OA7, a pair of back-to-back zener diodes D4, D5
are connected between the output and circuit ground, being
selected for threshold potential of, fo.r example, preferably,
3 v. to lLmit the output of the amplifier and preclude it
from e~ceeding predetermined levels. This limits the output
swing to avoid overloading any succeeding circuit stage to
which the output of the secondary dynamic compressor circuitry
is supplied, such as otherwise may occur if excessive
signals caused by ignition or electrical no.ise, e-tcO are
present in the signals being processed. Thus limited, the
output of the operational amplifier is provided ~hrough a
capac.itor C63 to output lead 148, across which there is a
load resistor R77.
-3~-
Accordingly, it is seen that compressor 137 comprises
an opera~ional amplifier (OA7) as well as feedback circuitry
interconnecting the output and non-inverting input of this
ampLifier for provlding a nonlinearly increasing negative
feedback signal to the input in response to increase in
the level of the tonally controlled audio signal on lnput
lead 136. Ignoring the change ln attack time or response
produced by capacitance expandor circult 190, ~he signal
gain is:
K I 1/2
G = b __
comp
in
where Ib is -the current flowing into an effective internal
summing node of ope.rational amplifier OA7, Vin is the
average input voltage of the audio signal input to compressor
137 and K i5 a gain constant, Gain cell 185 provides an
exponential response in gain in response to step changes
in amplitude, said exponential response being effected by
the time constant resulting from the capac.itance represented
by circuit 190 at terminal 189 of rectifier 1860
In FIGU~E 14~ family of curves represents typical
input-output tracking o~ compressor 137. The ordinate
represents output level and the abscissa represents the
input levelO
The following table indicatPs circuit components and
nominal values thereof which preferably may be utilized
in the construction of the secondary dynamic control
circuitry 137 wherein operational amplifier OA7 may be of
commercially available type TDA1034 and opertional amplifi~rs
OA8, OA9 may each be of commercially available type LM387:
TABLE II
R59100 kohm R67 10 kohm
R6062 kohm R68 30 kohm
R6120 kohm R70 47 kohm
R6222 kohm R71 47 kohm
R6330 kohm R72 68 ko~
R6420 kohm R73 220 kohm
R65 1 kohm R74 100 kohm
R66 1 kohm R75 1 kohm
%~ l
-39~
R76 220 kohm C56 1 mfd
R77 100 kohm C580.01 mfd
R78 100 kohm CS9 2 mfd
C53 1 mfd C60 5 mfd
C54 0.005 mfd C62 10 mfd
C55 270 pfd C63 5 mfd
With xespect to the operation of the secondary dynamic
compressor 137, it may be obsexved that operation in
accordance with the invention involves reconstituting
harmonics typically characteristic of himan speech, among
other aspects of operation, which ha~nonics o~herwise would
be of inaudible and of essentially ineffective amplitude if
the audio signal were not processed in accordance with the
invention, owing to constraints imposed by noxmal compression
or narrow band filtering normally encountered in modulation
and RF transmission systems. Accordingly, i~ is desired
that operational amplifier OA7 b~ of a relatively high
quality character, providing typically a high slew rate and
capable o~ providing transmission o~ audio frequenci.es up
to at least 15 - 16 kHzo
Also, in the case of commercial types LM387 ut.ilized
for realizing operational amplifiers OA8, OA9, a minimum
uppe~ band width is typically 75 kHæ although lessex values
may be utilized. Thus, the circuitry utilized exhlbi~s high
gain and wide bandwidth~ For increased stability, in view
of these characteristics~ an input compensation netwoxk
constituted by capacitors C53, C54 and resistor R60 is
utili2ed~
Circuit 190 is a gyra~or which effectively increases
~he capacitance at te~ninal 139~ being the apparent
magnitu~e of capacitor C56, in accordance with decxease in
signal level, i.e., the dynamic input of compressor 137~
Thus, in effect, the response time of the compressor becomes
considerably longex at low signal levels, since circuit 190
operates effectively to speed up the compressor attack time
at such low signal levels. For example, when the rectifier
input level drops from 30 dbm to -30 dbm, the time constant
%~ ~
-40-
increases from 10.7 Crect X 103 to 32.6 Crect X 103, where
Crect is the Pffective capacitance interconnected with
rectifler 186. This in kurn effects the gain cell response
because gain cell 185 is controlled by rectifier 186. This
avoids or greatly reduces any mistxaking of low signal
dynamics. Here it is noted that in compressor-expandor
(compandor) system in which the overall gain is unity such
change in attack time would not produce any overall gain
erxor, and the resultant gain or 105s would appear to be
manifested as such mistracking of low signal dynamics.
But in the present system wherei~ compression techniques
are primarily used, such problem is largely aver~ed, since
unity gain is not necessarily provided.
A~cordingly, the compressor is operating at high gain
when there is a small input signal, but when a higher level
input signal is provided to the circuit lead 134, the
circuitry operates to effectively reduce the gain. Over-
loading is~ however, precluded even in the case of a large
signal supplied to input 134 ~as for example, i~nition noise)
because o the clamping action of diodes D3, D4, D5.
Furthermoxe, with regard to transient respo~se, it will
be observed that the time taken for the compressor to
recover from an overload conditio~ is determined by the
capacitance in~erconnected at terminal 189 of the rectifier
186. If there were a smaller capacitor, faster response
to transients would be parmitted but such would produce
more low frequency THD becuase of gain modulation. The use
of a relatively small capacitance, i.e., one mlcrofarad
for capacitor C56 and circuit Cl90, avoids such difficulty
in the present system. In effect, THD which otherwise
would be genexa~ed by the compressor is effec~ively cancelled
by circuit 190.
~-* ~
-41-
Moreover, compressor-expandor systems heretofore
utilized are subject to a problem known as breathing.
This comes about since, as a system is changing its gain,
the change in background nolse level someti.mes can be
heard. In order to avoid this problem, capacitor C54
and reslstor R60 as well as capacitor C58 and resistor
R68 provide high :Erequency pre-emphasis by alteriny com-
pression gain accordingly.
In calibrating the circuitry of FIGURE 8 for operation,
wiper 194 o~ potentiometer R74 is adjusted ~or proper THD
trim by utili~ing a 0 db signal at 10 kHz input audio in-
put signal to the system and will be referenced by meter
145. Also the d.c. trim potentiometer R77 may have lts
wiper 196 adjusted to provide minimum en~elope bounce when
inserted tone bursts are applied to the input of the
system.
It is prefexxed that the secondary dynamic cOmpressor
circuitry will provide up to 135 db of dynamic compression
fox a 0 db input signal.
Referring now to FIGURE 11, the automatic gain control
(AGC) circui~ 129 is shown. Lead 141 provides a~ input fox
the audio frequency signals processed by the secondary
dynamic compressor 137 whose OUtptlt effectively provides
a tracking voltage provided by feedback circuit to the
d.c. rectification stage, i.e., rectifier 186 and expandor
circuit 190, o~ the secondary compressor 187, while serving
also to deliver the processed audio signals to bandpass
output filter 152. Thls AGC input is supplied,to a diode
D6 for rectification and thence to ~he gate of a field
effect transistor (FET) 199. Connected between the gate
and drain electrodes thereof is a time constant delay
circuit for providing a delayed action of the AGC circuitry
and comprising a capacitor C65 in parallel with a resistor
R80, both connected a-~ one end to the circu.it ground, and
a capacitor C66 having one end connected to the drain
electrode o~ the FET.
-42-
The FET effectively isolates the input by its high
impedance, being adapted by change in its conductivlty
to change the level of a signal applied to the non-inverting
input of an operational amplifier OA10/ which input is
biased to ground through a resistor R81 and interconnected
to the output o the operational amplifier for feedback
purposes through a resistor R82 thereby to establish control
over ~he gain of the latter.
The inverting input is biased to ground through a
resistor and is connected through a resistor R84 and
potentiome~er R85 having its opposite end and a wiper 201
tied to a lead 202 which is supplied by a terminal 203
with positive voltage, i.e., 12 v~d~co~ whereby the gain of
the operational amplifier may be controlled by the setting
of wiper 201. Further, a bypass capacitor C58 couples to
ground any AC component present on lead 202. Lead 202 is
shown interconnec~ed with the lead 203 for supplying the
operational amplifier with its operating voltage.
Connected betw~en ~02 and the output 204 of the
operational amplifier is a circuit comprising a pair of
potentiometers R86, R87 and meter 14,5, which is preferably
of the movlng coil type. The output 204 is tied directly
to lead 128 for providing an AGC feedback connection to
the primary voltage compressor 127. One side of the meter
145 is connected to ground through resistor R88. Wipers
205, 206 of potentiometexs R86, R87 provide calibration of
the meter.
The meter indicates the overall signal co~pression
gain level and thus displays an indication of the extent
to which the system is being utilized to capacity. The
meter is o~ the moving coil type, providing averaging of
the instantaneous variations in the output of operàtional
amplifier OA10 to avoid wildly fluctuating changes in the
indicated output which would be difficult to observe.
-43-
For proper operation, wiper 201 is adjusted to p~rmit
variation in the level of voltage provided to the inverting
input of the operational amplifier within a no~mal swing
or variation in the potentlal at output 204 of f.rom a~out
+2 to about ~9 v.d.c., which potentials are of the proper
magnitude Eor the primary voltage compressor 127.
There is -thus provided a feedback signal to the primary
voltage compxessor from the automatic gain control 129 in
accordance with the extent o~ compression by secondary
dynamic compressor 137. As a consequence, a negative or
limiting feedback occurs which opPrat s to provide the
function of controlling the amount of ~HD to provide the
function of controlling the amount of THD by limiting the
primary voltage compression as a function of the compression
provided by the secondary compressor 137. Thus, not only
does the ~eedback serve to control overall dynamic compression
but there is.also a con~rol of the bandwid~h of ~he system
by limiting out~of-band harmonics which would otherwise
pass through the system a~ a result of TH~.
For certain applications of the inven~ion, one might
permit rapid seconda~y reaction~ so that the feedba~k to the
primary compressor 127 would operate to prevent an overload
resulting from excesses in the output o~ the secondary
dynamic compre-sor 137, thus avoiding overdriving of the
output. For example, diyital signals of the type involved
in data txansfer mi.ght require such rapid reaction. Also
in the case of data transmission, rather than volce trans-
mission, narrow bandwidths are involved so ~hat one need
not provide capacity for handling haxmonics normally
present in speech or other audio.
The amount of delay which results in the AGC delay
path is preselected in accordance with applications of the
system and the type a.nd character of siynals to be processed.
-
2~
44-
Variation of the AGC time constant are provided by
capacitox C65 and resistor R80 which may be varied, as by
selection of differen-t components. Of course, conventional
multiple switches may be used to permit capacitances and
resistances of various values -to be subs-tituted for
capacitor C65 and resistor R80. Examples of the AGC
delay which may be provided by capacitor C65 and R80 are
from 0.010 seconds to 3 seconds, as a broadly preferred
range, whereas typically a delay of 0.3 - 0.5 seconds may
be adequate for various types of audio signals. In
general, the delay is to be shorted, iGe., for faster
response or transmission of signals of the date character
and slower such as 0.3 - O.S seconds or even longer for
signals of audio or voice character. In this way, the
amount o~ AGC feedback can be established in direct
relevance to the amount of out-of-band haxmonics expected
to be present in the signals being transmitted.
As noted above, an 1ED device circuit 149 and LED 150
of the circuit configuration shown in FIGURE lO is also
connected to the output 148 of secondary dynamic compressor
137 thereby to indicate the amount of compression attained
by compressor 137. In this regard, the LED 150 is utilized
to signal an overload condition resulting from excessive
compression. Thus, for a normal 0 db input, i.e., 120 db
compression, the control of the system may be adjusted to
provide operation under conditions such that pea}c
compression can be handled, as lndicated by elimination of
LED 150 during processing of audio signals, the controls
being adjusted to prevent LED 150 from normally remaining
illuminated.
The output of the secondary dynamic compressor 148~
as thus monitored, is provided to bandpass output fllter
152, -the circuitry of which is shown i~ FIGURE 9~
-45-
Referring to FIGURE 9, an operational amplifier OAll
receives thus processed audio frequency signals through a
pair of capacitors G70, C71. The non-inverting lnput to
which the signal is supplied is also biased to ground
through capacitor C72. A node 108 between capacitors C70,
C71 is biased to ground through a resistor R89 and also is
provided through a capacitor C73 with a feedback signal
provided by the output of the operational amplifier. A
further feedback path ls provided through a resistor R90
directly to the non-inverting input, which ls biased to
ground through a resistor R9lo ~he inverting input is
connected to cixcuit ground through capacitor C74. A
terminal 209 is provided for providing d.c. operating
voltage for the operational amplifier OAll as well as a
further operational amplifier OAl~, the two being both
preferably part of the same integrated circuit such as that
commercially available under type LM387. The power supply
terminals connected to ground tArough a conventional by-
pass capacitor C74.
The output of operational amplifier OAll is coupled
through a capacitor C75 and re~istors R92, R93 to the
non-inverting input of an operational amplifier OA12. The
latter input is biased to ground through a resistor R94
and also thxough a capacitor C76~ Fe~dback for operational
amplifier OA12 is provided first by resistor R95 which is
connected between the output and a node 210 between
resistors R92 t R93. This node is coupled to ground through
a capacitor C77~ ~eedback is also provided directly to
the non-inver~ing input ~hrough a capaci~or C78. The
in~erting input o~ operational amplifier 0~12 is connected
to ground through a capacitor C80~ The output of operational
amplifier OA12 constitutes the output of the bandpass
output filter.
-46-
Thus, there is provlded a bandpass outpu~ filter
having high frequency and low frequency sections and
preferably providing low frequency gain of 22 db with the
low frequency upper 3 db corner at 30 Hz and with the
high frequency upper 3 db corner at 10 kHz. It is
emphasized that bandpass output filter 152 is adapted
for providing a considerably greater pass band than that
of bandpass input filter 119. Selection of component
values to achieve these corner frequencies and pass band
characteristics as described above will be apparent to
those ski~lled in the art.
As previously observed, a switch SW8 is connected in
a lead 153 which extends from the input 148 to the
output llS of the bandpass outpu~ ~ilter and, when closed,
switch SW8 thus directly connects the input to the output
for bypassing the bandpass filter, just as input bandpass
may be desired for testing ox specialized purposes of the
invention 7 or when processing narrow band signals which
do not require filtering before or after processing. Lead
155 supplies the output of bandpass output filter 152 to
the high level output stage 156, which is shown in FIGURE 13.
Referring to FIGURE 13, lead 155 provides the processed
audio ~requency signal through a coupling capacitor C81
to the input terminal o~ an integxated circuit amplifier
IC4 is interconnected with the ~ollowing components having
nominal values as indicated~
TABLE III
R961 kohm R1021 kohm
R97820 kohm C81Ool mfd
R98100 kohm C8215 mfd
R99560 koh~ C8325 mfd
R10047 kohm C840.01 mfd
R10147 koh~ C85 1 mfd
-47-
A terminal 211 is provided for supplying operating
voltage, e.g., 12 v.d.c., through resistor R99 for powering
the integrated circui~ IC4. Amplifier circuit 156 operates
in effect to provide a bufering of the output 115 and for
providing high level, e.g., with 4Q db gain, useful for
driving various auxiliary apparatus, such as oscilloscopes,
monitor displays, frequency counters, spectrum analyzers,
and the like.
Audio amplifier 70 provides relatively low level
ampl.ification to provid~ adequate slgnal levels for audio
processing and to ensure th t pxocessecl signals are provided
to instrument 20 at levels typical of telephone systems.
FIGURE 15 is plot o~ the volkage gain of circuitry of
audio amplifier 70 as a function of frequency showing two
different repr sentation gain curves, and thus serving to
illustrate the frequency response of audio ampli~ier 70.
Audio mpliier 80 similarly exhibits broad requency
response It has a capability o deli~ering substantial
power gain, e.g., several watts to enable the system to
present to transmission line 40 a signal of level adequate
or telephone transmission over a balanced 600 o~ line
pair, e.g~, to present a zero db 0.775 vrms signal even
at low levels of input.
~:.b~
VOX circuit 60 may take any of various forms which
axe within the design skill of ordinarily skilled circuit
designers or may be of a circuit arranyement known in the
art, it being re~uired that such circuit procude switching
action upon the initiation and continuation of voice
signal input to instrument 20. In one preferred form o
VOX circuit, a speech~compression preamplifier realization
of a commercially available type 1S370 integrated circuit
diffexential operational amplifier may provlde a volta~e
output for electronic switching of the system from a
receiving to a transmitting mode. Such circuitry includes
an audio output of the operational amplifier which is
gain controlled to provlde a level for proper audio pro~
cessing and for proper modulation~
-49
Modulation-Demodulation
.
AM modulator-demodulator 50a is preferably of
conventional design such as known within the art,
employing a peak detector for demodulation o the audio
from conventional IF signals, e.g., at 455 kHz, as may
result from down-conversion of VHF AM signals.
FM modulator-demodula~or 50b may prefexably be a
narrow band type employing a quadrature demodulator in
a balanced-mixer circuit of conventional design useful
for limiting bandwidth operatlo~ to ~ 5 kHz duration~
For SSB modulator-demodulator 50c (which pref~rably
also ~onstitu~es a CW demodulator), a balanced mixer may
be used as a productor detector.
If desired, a carrier generator may be part of the
system for supplying RF signals to the modulator portions
of circuits 50a, 50b, 50c.
Input and output ports pexmit the coupling of
signals to and from lines L12a - L12c and L13a - L13c
fox interconnection with coaxial transmission lines.
Commercially available lntegxated circuit devices
of type families LM273 and LM274 may b~ employed for
the realization of AM, FM, SSB, CW detection circuit
associated with the modulation-demodulation portions
of the system as descri~ed above.
Genera~ Construction and Interpretatlon
In the interest of clarity, not all of the various
conventional power supply or similar connections of the
present system have been necessarily illustrated. In
some cases J discrete integrated circuits as well as dis-
crete components are utilized. But it should be under-
stood that some or all of the circuit devices of discrete
commercial types such as those described may be replaced
by circuitry i~volving large scale integration (LSI) or
very large scale integration (VSLX). Also, cextain analog
3s processing circuitry may be supplemented t augmented, etc.
by digital processing circuitry.
-50-
Additionally, it is noted that in describing the
preferred embodiment, specific terminology has been
utilized for the sake of clarity. However, it is not
intended to limit to the specific terms so selected
and hence it is to be understood that each specific
term includes all technical equivalents which operate
in a similar or functionally homologous or synonymous
manner to accomplish a similar or equivalent purpose.
For example, the term "interconnected" or "connected"
is not limited to a meaning of directly connected but
rather includes indirect connection or connection through
intervening components. Also, the term "series" or
various expressions such as "series circuit", "series
connected", and the like are all to be construed as
describing only the generalized attribute of a circuit
path, branch, or network, rather than to mean that the
same current necessarily must pass through each element
so described. Accordingly, if terminals of the plurality
of the elements of the present circuitry form a path
between two circuit nodes, they may for convenience of
reference be said to be in series even though other
elements may have a connection to intermediate nodes of
such path.
In the implementation of the invention, the circuit
blocks may individually or in combination be assembled
on printed circuit boards (PCB's) of standard size, if
desired, and may be assembled in cabinetry. The various
modules and blocks of FIGURES 2 and 3 may be provided
separately or in a combination of PCB's of the type having
an arrangement of contacts along an edge for being mated
to a so-called "mother" board or main frame.
Although references have been made herein to cycles
per second, or Hertz, data may be processed by circuitry
of the invention. Thus, bit or pulse rates, as measured
conventionally in baud or as bits per second, etc., are
to be understood as included within such references.
--51
Operational Configuration and O~ration
Referring to FI~URE 16, a system of the invention
constructed as hexeinabove described, and generally
as shown in FIGURE 1, is connected to provide a
processing and transmitting facility A having an audio
transmitting facility a, audio processiny circuitry b
and an appropriate terminal facility c. The latter may
be part of a telephone system, or part of an .~F trans~
mitting system. Facility A is shown representatively
connected by a communication transmission line (which
may or may not include an RF link or links) to a
corresponding processing and receivlng facility B having
a receiving terminal facility c', audio processing
circuitry b and an audio receiving facility a'. Thus,
lS identical audio processing circuitry is used in both
facilities A and B~ Simplex transmission of signals
occurs, and both the transmitted and received audio
signals axe preferably processed in accordance with
the invention. However, audio processing according to
the invention may, under some circumstances, not be
able to be provided ak both ends of such a simplex
transmission system, even though otherwise desirable.
FIGURE 17 illustrates use of systems employiny the
invention fsr duplex txansmission. Thus, at one end
of the transmission line, or lines, axe a processing
and transmitting facility A and processing and receivins
facility B. Corresponding facilities A' and B' at the
opposite end permit simultaneous two-way communications.
The facilities are constructed as in FIGURE 16 and
specifically as previously described.
-52-
Regardless of whether simplex or duplex communication
is to be carried out, the system shown in FIGURE 3 is
connected as a part of the system where it is desired to
utillze the system for processing audio signals prov1ded
by an audio source. In this regard, the source may be
at either end of the line transmission link. Thus, if
the source may be the intermediate amplification stage of
a modulation syste~n providing an audio signal to modulate
a line signal such as AM, FM, SSB , etc., the system may
be utilized for improving intelligibility and clarity of
such audio signals utilized for modulation thereby to
improve the character of the line transmitted information
even though such may be in a relatively narrow band, such
as 3 kHz or less or the source may be the intermediate
amplification stage of a line transmission receiver which
amplifies a demodulated audio signal, which similarly
may be of a very limited bandwidt~ as received, so as to
retrieve from the received modulat.ed audio signals clarity
and intelligibility and general characteristics of no~nal
human voice or other tonal signals. In such uses t the
system enhances the character of the signals transmitted
or received to achieve the various objects of ~he in-
vention.
When audio processing o~ the invention is utilized
at each end of a line transmission, the two audio pro-
cessin~ facilities o~ the invention e~fectively multiply
the phenomena produced by a single processing facili-ty on
either end of such a link. When utilized on both ends,
the system in effect provides what is most closely
analogous to a compressor-expandor phenomena in view of
the fact that the system as thus u-tilized to process both
the transmitted and received audio info~nation adds
substantial dynamic range as well as improves the signal
to noise (S/N) ratio. But i.n the stric-t sense, the
system is not merely a compxessor expandox (compandor)
-53-
since a compandor is a combination of a compressor at one
point in a communication path for reduciny the amplitude
range of signals followed by an expandor at another point
for complementary increase in the amplikude range for
increasing S/N ratio~
Regardless of its use in one of the type of situations
xeferred above, and as more fully contemplated in accordance
with ~he objects of the invention and as described herein-
above, the new system provides overall performance which
goes well beyond tha~ of a compandor in a customary sense
since it not only improves S/N ratios but also reconstitutes
losk harmonics and improves clarity, intelligibility and
generally enhances the character of signals processed
therethrough. Without use of the new system, the usually
audible characteristics of human speech involving various
harmonics particularly of the higher order which give
life~like, normal character as well as richness and quality
to human speech are typically so suppressed or xeduced in
magnitude in narrow band ~ransmission as ~o deprive the
received voice signals of character and intelligibility.
The processing of audio signals by the invention involves
effective reconstitution of these various harmonically
related attributes of speech so that the processed audio
has a richness characterized by the presence of out-of-band
harmonics which have been returned to the processed audio,
producing normal life-like intelligibility o~ the audio
signal, particularly speech.
Effectively, one may think of the operation of the
system as first invol~ing the pulling out or limiting
certain out-of-band harmonlcs which normally would be
present in signals amplified, and then subsequently
returning such harmonics to the signal in such a way that
they are emphasi~ed to advantage. One may liken the
system to the use of a crystal oscillator circuit which,
through ringing, may be utilized to create harmonics.
In this analogous way, the syskem returns harmonics of
human speech to audible levels of fundamental Erequency.
-54-
This manifestation is not well understood, but i5 an
attribute of such significance that it i5 best appreciated
only upon objective aural comparison between signals rom
various sources processed, and those not processed,
through use of the invention.
In a practical embodiment of the invention, the user
can be provided with ready panel front access to gain
control potentiometer R9 for variation of the wiper 159
thereof to control the level of signals provided to the
primary active frequency control circuit 123. Also, the
user can be provided with controls for varyiny the
position of potentiometer wipers 161 and 165 to preselect
the amount of gain within the low and high frequency
portions of primary active frequency control 123 in
accordanc~ with the type of signal to be processed. In
additionl the wipers for to~e contxol potentiometer R48,
R51 can be controllable by the user to preselect ~he
- response of secondary active frequency control circuit
135. Front panel accessibility can further be provided
for controlling the wipex of potentiometex of R61 to allow
selective response not only of the overall gain provided
by the secondary dynamic compressor operational amplifier
OA7 but also ~or controlling THD duriny operation of the
secondary dynamic compressor 137. Of course, as noted
above, switches SW7, SW8 are also located for operator
usage to permit selective use of the bandpass lnput
and output filters 119, 152~ Additionally, meter 145
is located for being observable by the user.
The circuit values of the LED drive circuits 131, 149
are each chosen to provide for energization o the
respective LED 133, 150 when the output o~ the compressor
127, 137 respectively interconnected with each drive
circuit is providing maximum compression. Thus, by
referring to LED 133, the operator may provide adjustment
of primary active frequency control gain poten-tiometer R9
to allow the output of the primary voltage compressor
127 to reach its full maximum value, as indicated by
-5~-
illumination of L~D 133. This LED will remain unli
when primary compressor 127 is operating at less than
full compression. ~he user may thus appropriately adjust
the overall system gain to allow normal operation to
proceed so that LED 133 would only light upon maximum
peak magnitude of signals being processed. Similarly,
potentiometer wipex 184 of the secondary dynamic com
pressor 137 is adjusted so that LED 150 normally will
remain unlit but will ~lash momentarily only at maximum
peak signal levels.
The user is continuously provided by meter 145 with
an indication of the extent to which the system is
operating to the extent of its capabilities thus dis-
playing the overall performance of the system.
In conjunction with the u~e of meter 145, it is
noted that its indication when speech signals are being
processed by the system is such as to give a visual
indication o the averaged output voltage of secondary
compressor 137, but at the same time the meter will
indicate ~luxakion in dyn~lic audio conte~t~ This
indication is particularly use~ul when speech is being
processed through the system. However, it is noted
that when data signals are processed through the system
such as in the case of facsi~ile or teletype signals,
meter 145 produces only an average reading from any
given dynamic fluxation pattern, since dynamic voltage
peaks will appear far too fast to be tracked by the
moving coil type of movement of meker 145. On the other
hand, the indication provided by LED 150 is such ~hat
any xapid increase in signal which may be in excess of
the maximum compression utilizable will be instantaneously
displayed by flashing of the LED.
-56-
It is, therefore, seen that the system is provided
with the minim~ of control as well as relatively simple,
easily understood indications for properly monitorlng
its operation and determining performance. Addikionally~
by virtue of its IC circuitry, the system is easily housed
in a compact, small lightweight enclosure or cabinet and
may, if desired, be battery powered to provide complete
portability as well as long periods of opexating time.
Telephone Volce ~lications
Wi~h regard to the previously mentioned practical
applications of the invention with telephone line systems,
it should be realized that circuitry of the invention will
work equallv as well with antiquated PBX systems as with
the most highly sophisticated computerized trunk call
systems. Actually, audio processing phenomena of the
invention would be f~r better realized on an antiquated
system than with a modern telephone system because the
dynamic increase in intelligibility and sustinction of
speech will be far more noticeable on these aforesaid
systems as no type of processing is normally applied.
The improvements over conventional or standard compandor
systems presently utiliæed are self-evident if the
operation of audio processing circuitry of the invention
is understood as all the enhancement qualities of a
system of the invention while working in an R~ communic-
ations mode would be present and transferred to a land-
line when the system is interfaced with a telephone line
system. Due to the new system's effectivenessj the
invention can be used to update antiquated exchanges for
long distance communicakions without the expense of
using conventional multiple inline compandor systems and
filtering techniques; since the inherent qualities of the
new system would provide the benefits of the latter
conventional approach in gen~ral operating conditlons.
..
Data Communica~ions
Utilizing the new sys~em, digitized information can be
passed over a standard narrow band telephone line to
an extent previously regarded as ei~her impractical or
impossible. Such information may typically be high speed
data such as characteristic of computer links. Wi-th the
bandwidth available, this information can be sent at
higher rates than previously practicalO Other applications
include mixed communications wherein voice characteri~-
ation as well as quality are required, such as livebroadcast reporting where distinction as well as good
definition in voice character are required. In the
field of data co~munications where multiple repeater
and landline operation are used, the new system can be
utilized for both radio transmission and landline trans-
mission to achieve extremely high levels of intelligibility.
Fre~uencles of Signals Processed and Texminology
Because a system of the invention is primarily intended
to provide processing of any of various signals transmitted
by line systems (which may, as noted, include RF links)
wherein the intelligence is conveyed by voice or data
at frequencies corresponding to a normally audible sound
wave, and typically within the audible spectrum of 15-~0,000
Hz, the terms "audio frequency" or "audio signals" have
been employed. However, it is to be understood that
principles of the invention may be employed in the design
and operation of circuitry for processing signals greater
than 20,000 Hz, including those of frequencies or bit
rates typically regarded as radio frequencies. Therefore,
such terms as "audio frequency signals", etcO, are not
to be construed as limiting.
Since, regardless of frequency, "audio processing"
may occur either before or after transmission, with
respect to 2 s~stem of the invention configured only for
processing of signals received a~ter transmission, the
term "unprocessed signals" is used to denote signals
received but rot yet further processed by processing
circuitry of the invention, even though such signals may
already have been processed before transmission
~5~-
Therefore, circuitry of the invention may be used
to process "unprocessed" audio frequencies before trans-
mission of the now-"processed" signals. The received
signals, while improved, may be regarded as "unprocessed"
at the receiving end, where processing circuitry of the
invention may again provide "processed" signals which
are, thus, doubly processed without contradiction in
terms.
Modificati
Al~hough the foregoing includes a description of the
best mode contemplated for carrying out the invention,
various modifications are contemplated.
For example, it is within the purview of the invention
to allow the compression levels of the various circuits
to be varied as well as to allow the compression circuitry
of the invention to be operated in an expansion mode for
incxeasing, rather than decreasing, dynamic range, as
may be desira~le. When circuitry of the system is utilized
in such mode, high frequency de-emphasis may be utilized
in the system wherein expansion occurs to compensate for
high frequency pre-emphasis which is normally performed
by secondary compressor 137 in the system presently
configuredO
As varlous modifications could be made in the
constructions herein described and illustrated without
departing from the scope of the invention, it is intended
that all matter contained in the foregoing description
or shown in the accompanying drawings shall be interpreted
as illustrative rather than limiting.
, . ..
