Note: Descriptions are shown in the official language in which they were submitted.
2~
SYSTEM FOR MAXIMUM EFFICIENT TRANSFER OF MODULATED ENERGY
BACKGROUND OF T I~V~TlON
The present invention relates to signal processing and
modulation systems and, more particularly, to an improved
S signal processing system intended for maximizing the afficiency
o transfer signals of various frequencies for enhancing
signal intelligibility and clarity while avoiding loss of
dynamics The invention has applicability for processing of
audio and higher frequency signals transmitted by various
transmission modes, such as by radio, TV, and by line
systems.
With regard to the transmission of intelligence-carrying
signals, including modulating electromagnetic radiation in
accordance with a modulating signal (such as audio modulation
of a radio frequency signal), there has always been a problem
o loss of;attenuation of intelligence which is attempted to
be kransferr0d within a given frequency band where the frequency
band over which intelligence must be txansferred is narrower
than the range of frequencies which normally convey such
intelligence. Thus, when the band width or deviation of a
radio frequency signal is narrow, the full spectrum of speech
and other audio signals has been heretofore limited. It has,
therefore, been general practice merely Jo ~ra~sfer only a
portion of those frequencies of the audio spectrum with
consequent loss of intelligibility, voice character, dynamics,
clarity and fidelity in general.
In the line and radio frequency transmission and reception
of modulated signals a major problem has always been to obtain
a high level of dynamic amplitude while retaining a usable
portion of the full spectrum of speech frequencies, while
keeping thP band width of the transmitted radio signals as
narrow as possible.
Relative to transmission of voice signals by moclulation
ox radio frequency signals, attributes of human speech of
concern are dynamic amplitude and harmonic-relationshipO
The latter is extxemely important in identification intellig-
ibility. Dynamic amplitude can be defined as the varying level
of audio received by a modulation stage in any mode of modul-
ation. the human voice is made up by a complex structure of
harmonics, the main bands of harmonics falling within a 3 kHz
band width. A speech band pass frequency commonly selected
is 300 Hz to 3,000 Hz, and all other harmonics are generally
supprassed. However, these out-of bands harmonics define
voice character and, thus, intelligibilityO But the suppressed
harmonics fall in such a wide spectrum that if the entire speech
harmonic make-up were to be transmitted, a transmission band
width of some 15 XHz would be required. With modern narrow
`band voice transmission systems, this would become impossible.
There are many modes o radio and line transmission using
audio or other signal modulation where these matters are of
great concern. PrinCiple-forms of modulation presently in
use are AM, SSB and FM.
--3--
SUMMA.RY OF TÆ INVENTION
An object of the invention is, in genexal, to transfer,
within a given requen~y band, intelligence-carrying signals
in such a way that in~ellige~ce normally conveyed by signals
- 5 of frequenoies beyond the given fxequency band is instead
conveyed by signals within such frPquency bands, thereby to
transfer intelligence otherwise lost or attenuated
An object of the invention is to provide a system for
maximiæing the efficiency of transfer of signals of various
frequencies, such as modulated audio frequency energy.
A urther object of the invention is to provide such
a system useful with modulation systems.
A further object of the invention is to provide such
a system for providing transfer of modulated signals in such
a way that high average modulation power is attainable
still urther object of the invention is to provide
such a system for providing transfer of signals to enhance
signal intelligibility and clarity while providing loss of
dynamics.
Another object of the invention is to provide such a
system which allows the processing of signals demodulated
from receiYed RF signals in order to retrieve signals with
effectively high signal-to-noise ratios even where thexe is
high noise level associated with the received RF signals.
A furthex object of the invention i5 to provide such a
system which can be utilized for processing of signals either
prior to their use in modulating a.transmitted OF signal,
or for processing demodulated signals upon reception of the
transmitted RF signal, or both.
~30 An addikional object ox 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 resulting increase in effective trans-
mitted power and clarity.
It is also an object of the invention to provide such
a system which has both processing circuitry and method aspects.
~7~
-4
Another object of the invention is the provision of
such a system which processes sig~al.s in such a way as
Jo recons~i~ute wide frequency spect:rum dynamics associated
signals te.g., voice signals used).
Another object of the invention is the provision of
such a system which reconstitutes si.gnals otherwise lost
by recovery and application of hanmonics by recovery and
amplification.to audible.lsvels of certain harmonics
otherwise suppressed or filtered.
A urther object of-the invention is the provision
of such a system.permitting transmission of signals
with high average modulation levels, approaching 100%
average modulation, without customary dramatic loss of
dynamics.and intelligibility.
.. An object of the.invention also.is the provision of
such a system which can be utilized in the fields of
RF transmission of speech, audio signals generally, and
other intelligence-carryins siynals such us high speed
tone encoded dat2, teletype, facsimile, and other.modes
~0 of data communication, and which when utilized as part
of a data communication li~kj reduces data drop-out.
. It-is also an object o the invention to provide
such a system which not;only is relatively sLmple and
utilizes integrated, compact circuit components.but also
uses a minimum-number of control elements setable to
the application at hand without need fox further adjustment.
A~xelated.further object-of the invention is the
provision of such a system utilizing various visual
. indicators.fox keeping the user co~sta~tly informed and
aware of the extent to which signals are properly
processed through the system.
An additional object of the invention is the provision
of such a system which can accept substantially any low
level audio frequency signal.while according to the user
the options of.utiliæing.and.emphasizing vario~ls components
of the audio spectrum, and of selectively utilizing
filtering of the input and/or output signals.
TV
-5-
Another object of the invention is the provision of
such a system in which the output of signals pro~ess~d
through the system can be presPt it amplitude Jo be fed
to any modulation stage currently used in conventional
communication systems.
A further object of the invention is the provision
of such a system which is especially of advantage in
connection with narrow band V~F and VHF transmission,
such as in voice, television, and data communications
and in satelliteorelayed RF transmission systems, and
making possible extraordinary narrow band transmission
containing full dynamic characteristics of signals so
processed while maintaining high modulation levels.
among other objects of the invention may be noted
the pxovision of such a system which can be utilized in
connection with EM, FM, TV, SSB, PAM, FSK (frequency shit
keying) and tone activated TTY transmissions, which also
can be utilized in public address application and music
amplification systems.
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 frequencies of signals to be processed; while
allows continuous gain control; which achieves linear
2S tracking during pxocessing; which operates to eliminate
or greatly reduce third harmonic distortion; which operates
to reconstitute out~o~-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 uncomplicated as well as being simple to use
and maintain; which can advantageously be operated from
a low voltage or battery power supplies; and which
exhibits low proper consumption and inherent high efficiency
during operation.
3s Other objects and features will be in part apparent
and in part pointed out hereinbelow.
Briefly, a signal processing system of the invention
includes circuitxy by which signals Jo be processed 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 limits
signal dynamics to a predetermined window, being controllably
preset and driven with different bands of amplitude of the
processed signal spectrum determined by the primary frequency
control to maximum compression levels. The output of the
compressor is presented to a secondary active frequency control
which drives a secondary dynamic compressor with different
bands of frequencies. An automatic gain control tied to the
latter compressor supplies time-delayed feedback to the primary
voltage compressor to achieve overall dynamic response. The
processed output of the secondary compressor is provided to
a bandpass output filter and, sharply attenuated by the latter,
is delivered for further use.
Thus broadly, the invention contemplates a signal processing
system for processing dynamically varying intelligence-carrying
signals represented by a plurality of different frequencies
by transfer within a pass band narrower than the range of
the frequencies, and the system is characterized by first
compressor means for compressing only signals within a predefined
pass band, the first compressor means including gain cell
means providing gain varying as a function of the magnitude
of signals within the predefined pass band, a means for shaping
the compressed signals into a predetermined power envelope
having fundamental frequency signal components and harmon-
ically related constituents of the fundamental frequencies,
a second compressor means for further compressing signal con-
stituents of the power envelope including further gain cell
means providing gain varying as a function of the magnitude
of signals within the power envelope, a negative feedback
means for dynamically limiting compression of the first compressor
means as a function of increase of compression by the second
compressor means, and a means for recombining the harmonically
'~Z~
-6a-
related constituents delivered by the second compressor means
thereby to retrieve the dynamically varying intelligence-
carrying signals.
A further embodiment of the invention provides a signal
processing system for processing intelligence-carrying signals
representable by a plurality of different frequencies by transfer
within a given pass band narrower than the range of the frequencies,
that system being characterized by a first frequency control
means for defining a range of original signal constituents
to be processed including signal fundamentals and harmonically-
related components of the fundamentals, a primary dynamiccontrol means for dynamically compressing the range of original
signal constituents thereby to provide a frequency controlled
dynamically compressed signal including gain cell means providing
gain varyi.ng as a function of the magnitude of the original
signal constituents within the range, a second frequency control
means for selec-tively controlling the amplitudes of signal
constituents of the dynamically compressed signal to define
a predetermined power envelope of signal constituents, a secondary
dynamic control means for dynamically compressing signal con-
stituents of the power envelope including further gain cellmeans providing gain varying as a function of the magnitude
of signals within the power envelope, a gain control negative
feedback means for limiting compressing by the primary dynamic
control means as a time-delayed function of increase of the
power envelope, thereby to provide the frequency controlled
dynamically compressed signal with dynamic variation, and
a signal output means for providing the power envelope of
processed signal constituents for recombination, thereby to
retrieve the intelligence-carrying signals.
The invention also contemplates the method of signal
processing for transfer within a given pass band of intelligence-
carrying signals represented by a plurality of different original
frequencies having a range greater than the pass band which is
characterized by defining a range of original signal constituents
~7Z~L
,,
-6b-
to be processed including signal fundamentals and harmonically-
related components of said fundamentals, primarily dynamically
compressing the range of original signal constituents, selectively
controlling the amplitudes of signal components of the dynamically
compressed signal to define a predetermined power envelope
of signal constituents, further dynamically compressing the
signal constituents of the power envelope to provide processed
signal constituents while providing time-delayed limiting
of compressing of the original signal constituents to permit
dynamic variation of the processed signal constituents, and
recombining the processed signal constituents to reproduce
the intelligence-carrying signals and recover original
frequencies beyond the pass band.
BRIEF DESCRIPTION OF THE DRAWINGS
FIGURE 1 is a block diagram of a system constructed
in accordanGe with and embodying the present invention.
FIGURÆ 2 is a schematic circuit diagram o a band-
5 pass input filter of the circuitry of FIGURE 1.
FIGURE 3 is a schematic circuit diagram of a prLmary
active frequency control circuit of F:rGURE 1.
- FIGURE 4 is a schematic circuit diagram of a.prima~y
voltage compressor of the system o FIGURE.l.
FIGURE 4A is a block diagram of an integrated
circuit device worming part o the primary compressor of
FIGURE 4.
FIGURE 5 is a schematic circuit diagram of a
secondary active frequency control of the system of
lS FIGURE 1.
FIGURE 6 is a schematic cixcuit diagram of a
secondary dynamic compressor of thy system ox FIGURE 1.
FIGURE 7 is a schematic circuit diagram of bandpass
output filter of the system of FIGURE 1.
FIGURE 8 is a schematic circuit diagram o an LED
drive circuit including an LED indicator utilized in the
system ox FIGURE 1.
FIGURE 9 is a schematic circuit diagram of circuitry
of an automatic gain control as well as meter drive
circuitry and a meter for indicating operation performance
of the system of FIGURE 1.
FIGURE 10 is a schematic circuit diagram of an active
filter power supply utilized in the system of FIGURE 1.
FIGURE 11 is a circuit schematic diagram of an audio
. 30 frequency amplifier utilized in the system of FIGURE 1.
FIGURE 12 is a graph of a family of curves illustrating
operation of certain compressor circuitry of the invention.
FIGURE 13 is a graph on which are plotted the
amplitude versus frequency of certain-signals of re-
presentative fundamental frequencies provided as input
signals for the system of FIGURE 1.
2~
FIGURE 14 is a graph on which are plotted the
amplitude versus frequency of the fundamental requencies
as well us harmonic inputs which result from these re-
presentative fundamental frequencies.
FIGURE 15 is a graph on which ,are plotted the
amplitude versus frequency of the fundamental input
frequencies and harmonically related signals resulting
in various output signals after processing by the system,
and demonstrating the rej eC~ion of certain out-of-band
frequencies and transmission of in-band frequencies.
FIGURE 16 is a graph on which are plotted the amplitude
versus frequency of certain output signals harmonically
related to the input signals which are regenerated from
the transmitted in-band frequencies.
FIGURE 17 is an operational summary in the form o a
graph on which are continuously plotted the amplitude
versus frequency of system input signals, output signals
and regenerated signal output resulting from transmitted
in-band`frequencies.
Jo Corresponding reference characters indicate
corresponding parts throughout the several views of the
drawings.
.
- 9 -
DESGRIRTION OF THE PREFERRED ÆMBODIMENT
,
Genera1 Description of the System
Referring now to the drawings and particulaxly to
. FIGURE 1, the overall relationship of various circuits S of a system of the invention are shown in bloc diagram-
matic form.
Signals to be processed my the system, such s speech
Qr other audio frPquency signals, are de~iverPd to an
input a 13 from any of myriad possible sources 9 such as
l the output of a microphone, pr amplifier, an intermediate
amplification stage in a modulation system, or tap
recorders, the output of an intermediate frequency stage
of an RF transmitter, receiver, etc. Although the input
signals typically may con5i5t of tone pulses, frPquency
shit keying pulses, tone encoded information, teletype
signals (TTY3, facsimile, data signals, and voice trans-
missions, and other tone-related or audio signals. They
may also be of radio frequencies, eOg., above audio
frequencies and including wavelengths to and beyond the
submillimeter range Thy signals, after being processed
through circuitxy o the system, art delivered for further
use by an output 15.
n auxiliary output 17 is provided for presenting
output signals of high levels. Output signals delivered
by output 15 are utilized for purposes such as the driving
of a modulator, or delivery to an audio amplification
stage, further audio or radio frequency processing- or
various purposes such as amplification, recording, decoding,
retransmission, frequency conversion, and so forth. The
high level output provided by output 17 may be utilized for
driving various recording devices, such as oscilloscopes,
spectrum analyzers, and the like, without limitation
Although the system is advantageous for processing
audio frequency information containing voice content or
speech, tones and tonal data of various types of an audio
frequency, as alluded to above, signals carrying other
types of intelligence may be processed advantageously by
Y
-10- i
the invention. Thus, it is to be understood that the
system ox the present invention is u~ilized,for trans-
ferring signals of whateYex frequency in such a way
that the efficiency of 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 int~lligibili~y, to provide high clarity
of signal transmission, to avoid loss of dynamics, and
to improve signal-to-nsise (S/N~ ratios, to achieve
transmission of information within a gî~en frequency
window and otherwise to attain the stated objects. Such
processing is carriea out in such a way that intelligence
normally carried on signals having frequencies outside a
given pass band is nevertheless transferred with:in the
pass band by being carried on other in-band frequencies
which are subsequently recombined to provide the original
frequencies and recover the otherwise lost or attenuated
intelligence.
Input 13 delivers signals to a bandpass input filter (BIF)
19 for providing filtering of frequencies to achieve a
pass band of, for example, between 30 ~z and 3 kHz, being
thus substantially less thaw the lower and upper limits of
frequency components, including harmonics, characteristic
of human speech,.which may extend sPveral kilohertz above
the 3 kHz.band limit. Filter 19 effectively limits all
. other Frequencies. The.substantiall~ 3 kHz pass band
thus achieve it merely illustrative and thaw preferred
and utilized for voice signal processing by the new
system. The pass band may be varied in accordance with
the purpose intended for the pres2nt system, e.g., in
having different widths.and dif,ferent lower and upper
frequency limits, including audio through RF frequencies.
A switch SWl connected by a circuit lead Z0 between input
13 and the output 21 of filter 18 permits selective dis-
ablement of ilter 19 for purposes noted hereinbelow.
The output of the ba~dpass is delivered by output
co~neçtion 21 to a primary active frequency control(PAFC) 23.
Control 23 effectively splits the audio spectrum of the
signals delivered to it into two separate frequency tub-
bands. Preferably, although not necessarily, the lower
band is from 250 ~z to 1.2 kHz and the upper frequency
band is from 1.4 kHz to 3.5 kHz. PAFC' 23 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
the sub-bands over a preferred range of 10 db. Also,
the primary active frequency control 23 preferably in-
corporates an input gain control fox purposes later
appearing. The control components of PAFC 23:also
allow individual tailoring of out-of-band harmonics which
are to be reconstituted by processing of audio signals
by the system. Although two sub-bands are provided and
controlled by control 23 for speech processing, a greater
number may be used for processing other types of signals.
Signals from control 23 are then provided by a
convection 25 to a primary voltage compressor (PVC~ 27. The
latter provides a relatively high compression range, e.g.,
pxeferably 135 db, as well as pre-emphasis of high
frequency audio components to compensate for high frequency
losses which otherwise could occur during processing.
Compressor 27 is preferably selected to limit all signal
dynamics to a 27 db window with a tracking error of not
greater than about + 3 db. Included within compressor 27
is a variable gain cell which is indirectly controlled,
via a lead 28, by an automatic gain control (AGC) circuit
29 described hereinbelow.
Also connected with compressor 27 by a lead 30 is an
LED drive circuit 31 for driving an LED indicator 33 in
accordance with the operation of compressor Z7 to provide
the system operator with an indication of the extent to
which maximum usable compression is being provided by
compressor 27.
--12--
The output o compressor 27 is d.olivered to a secondary
active frequency control (SAFC) 35. It splits the now
! dynamically compressed audio signals into two further frequency
sub-bands, a lower frequency sub-band of preferably from about
5 300 ~z to lo 5 kHz and an upper frequency sub-band of preferably
from about 1.5 kHz to 3 k~z. Like the first set of sub-bands
determined by PAFC 23, the second set of sub-bands determined
by SAFC 35 each involve frequency ranges substantially less
Han the principal pass band.
SAFC 35 is adapted for providing gain and attenuation
; control within these two sub-bands variable over a rangP
of preferably + 12 db. For this purpose, manual control
means may be used to selectively determine the gain or
attenuation within each frequency band, or may be preset for
dedicated applications. If desired, more than two frequency
sub-bands may be provided by control 35 dependent upon the type
of signals to be processed, whereas two are sufficient for
processing of speech fxequencies.
Unlike PAFC 23, SAFC 35 is adapted for providing gain
peaking and attenuation occurring about sub-band center frequencies
with the respective lower and upper bands at preferably 1 kHz
and 2.4 kEz~ This feature allows SAFC 35 to selectively dis-
regard interacting, and possibly distortion-productive harmonics
occurring within the input pass band established by BIF 19.
The output of frequency control 3.5 is fed to a secondary
dynamic compressor (SDC) 37~ The circuitry of SDC 37 is
intended to provide an extremely fast.tracking system with
j extremely low tracking distortion (preferably less than about
0.1%) and to accept and provide compression of signals having
a dynamic amplitude range of up to about 120 db while achieving
- a compression window of preferably only 50 db, yet to provide
a third harmonic distortion (THD) figure of less than preferably
l AGC circuit 29 is interconnected with circuit components
of SDC 37 by a connection 40 to provide an input for AGC circuit
29 which in turn controls PVC 27 by circuit connection 28, there-
by providing a negative AGC feedback loop for limiting the
degree of primary voltage compression provided by PVC 27 as a
time-delayed function of compression by $DC 37 as explained
-13-
below. However, the connection of AGC circuit 29 to SDC
37 provides Jo PVC 27 a d.c. reference signal derived from
compression stages of SDC 37.
In effect, AGC circuit 29 by interconnection with SDC
~5 37 amplifies a tracking voltage output of SDC 37 and delivers
a voltage varying within preset parameters to a variable gain
cell of PVC 27 as a function of this tracking vol~ageO This
is carried out for the purpose of achieving extremely. high
tracking stability and for limiting third harmonic distortion
while reconstituting through voltagè yain via SAFC 35 the
original signal dynamics fed to SDC 37. However, reconstituting
of otherwise lost signal dy~ami~s occurs as well in other
portions of the system circuitry.
Interconnected as indicated at 41 with AGC circuit 29
is a meter drive circuit 43 or driving, as indicated at 44,
a meter 45 preferably of a moving coil type to provide
averaging and serving as a visual indicator for displaying not
only the compressed output voltage provided by SDC 37 but also
average peak dynamic compression, and thus indicating the
extent to which the overall capability of the system is being
utilized. Other types of indicators may be used for processing
of signals other than of audio frequencies.
Interconnected as indicated at 47 with he output 48 of
secondary dynamic compressor 37 is an LED drive circuit 49
for driving an LED indicator 50 to indicate the degree of
compression being achieved by SDC 37. Its output is delivered
to a bandpass output filter (BOF) 52 for providing sharply
attenuated bandpass speech signals, BOF 52 has preferably a
pass band of between 300 Hz and 3 kHz with very sharp roll-off
~30 or corners at the edges of the pass band to limit the processed
audio between these upper and lower limits. The pass band
limits here noted are those suitable for narrow band OF, VHF and
VHF transmission by FM, EM and SSB modes, and narrower or
wider limits may be selected for other purposes.
~z~
-14- -
. BOY 52 preferably provides unity gain and has an
extxemely low noise igure to avoid introducing further
noise into the now processed signals. If desired, the
filtered signals provided at output 15 may be attenuated
S to provide signal levels suitable for other systems being
driven by the present system, but whether dixe~tly, or
: indirectly are delivered Jo a speaker 57 or other
xecombining device. In a circuit 53 connected around BOF
3 52 is a switch SW2 for selectively disabling the operation
of BOF 52 for puxposes noted hereinbelow. The output o
BOF 52 is also provided, at 55, to an audio amplifier 56
constituting a high level output stage and providing output
17.
The following detailed description of circuitry illustrates
; 15 discrete analog design of the system or processing of human
speech signals.
~7Z4~
--15--
Detailed Desertion of Circuitry of the Sys em
Now that the general circuitry of the system has been
described, the specific circuitry of each of the blocXs
designated in FIGURE 1 is described hereinbelow.
In the interest of clarity not all of the various
conventional power supply or similar connections are
necessarily illustrated. In some cases, discrete integrated
circuits as well as discrete components are utilized. But,
it should be understood that some or all of the circuit
devices of discrete commercial types such as those described
may bP replaced by circuitry involving large scale integration
(LSI~ or very large scale integration (VLSI). Also, certain
analog processing circuitry may be supplemented, augmented,
etch by digital processing circuitry.
Additionally, it is noted that in describing a preferred
embodiment, specific terminology is utilized for the sake of
claxity. 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 'Iconnected" 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 con-
strued as describing only the generalized attribute of a circuit
path t branch, or network, rather than to mean that the same
current necessarily must pass through each element so described.
~30 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.
-16-
In the ~mplemen~tion of the invention, the art
blocks may individually or in oar~bina'ciorl be assernbled on
printed cîrcuit boards (PCB's) of standard size, i do
sired, and 7 asse~led in cabine~y. Ihe van
5 n~ules aft bloc:}cs of FI~UEæ 1 IsEly be provided separately
or in a ~inatio~ Ox 's of the having an arrange-
ment of contacts clang an edge for being mated to a so-
c:alled 'other" boæd ox mainfrane.
5eneral~y spea3ci~, i~lte~ lrleCtiOn.5 betwe~ the in-
Referring to FIGURE 2, the circ~:i~ of BIF 19 providescascaded high and l i~requen~y sections. An o1?erational
arplifier Q~l has its norl-irn7e input interconnected
lS with input terminal 13 through Cl, C2 and biased to ground
through Rl. Feedback between the output of the ampl;fier
and the input is provided ky C3 ketween Cl and C2 for pro,
.viding capacitive feedback for tailoring frequency response.
Its node is also biased to ground thrzugh R3. Ihe non-
inverting and inverting inputs æ e oonnected to circuit groundthIDugh respec*ive capacitors C4, C5. C6 conventionally by-
pasts power supply terminal 58 bo ;~ t ground.
lhe output of operational amplifier Cal is series-oonnected
gh C6, R4 and R5 to C~2. Capacitive feedback between its
output and its non-inverting input is p~DVided my C7 for frequency
response ccntrvl with d.c. feedback Qr gain aontrol be mg pro-
vid3d ky R6. It is need that R7 and C8 are connected in
parallel be n the n~-inverting input and circult ground,
the nD~e common Jo R4, R5 and R6 is tied similarly bo ground
th muyh C9. The averting input of Q~2 is tied direc*ly to
ground. The oLput of C~2 is provided through C10 to lead 21.
Capacitor Cll kypasses operational amplifier o~2 to circuit
grcund. Switch SW1, oonnected my lead 20 between input terminal
13 and lead 21, when closed renders BDF 19 m cperative, as
when unneoessary to limit audio signals being processed to
the narrow pass band ordinarily determined by BIF 19.
~zo~a
-17-
The selection of various components utilized in connection
with OAl and OA2 is a matter of design choice to achieve the
bandpass upper and lower frequenoy limits referred to pre-
viously. Each of these operational amplifiers may be part
of a single commercially available integrated circuit tIC)
type such as LM387 exhibiting electrical characteristics and
perLmeters compatible with the node of intended usage of the
present.sys~m and requiring power supply voltages was delivered
by terminal 58) of, for example, + 15 v.d.c. As will be fully
understood Jo whose skilled in the art, toe circuit values such
as that of R6 may be varied to control.the gain of the bandpass
input filter
It is preferred that BIF 19 provide upper and lower
corner frequencies of 2.7 kHz and 300 Hz, respectively, and
lS a roll-of~ characteristic of -40 db. per decade, as well as
very low.third harmonic distortion (THD). BIF l9, preferably
providing unity gain, can handle an input signal of preferably
between -35:and +lO db. without clipping or distortion.
PAFC 23.utilizes opexational amplifiers OA3, OA4 for
control cf the amplification of the gain of individual upper
and lower.frequency bands. Circuits 60a, 60b establish, with
the respective operational,amplifier, means for providing
selective amplification and control of audio frequeney com-
ponents within the respective audio bands.
More specifically, the input signal provided by lead 21
is delivere~acrass R9 having wiper 59 selectively variable
for controlling thy overall gain of stage 23. wince the two
individual active fr.equency control circuits 60a, 60b have
certain corresponding components which are connected in
~30 identical manner, corresponding elements are referred to by
corresponding reference numerals with each numeral being
.followed by a subscript "a" or "b!', as appropriate.
Circuit 60a is described exemplarily. Cl3a couples the
signal at wiper 59 through R10 to Rlla having wiper 61 providing
. high frequency gain con~ol. Connected from opposite sides of
Rll to its wiper are C14, C15. The signal at.wiper 61 is
provided through R13 and C17a to OA3, having its inverting input
referenced to ground through resistor Rl4a, which is shunted
by Cl8a and Rl5a.
The QppOsite end of Rlla is grounded through C20a and
R16, A frequenGy compensating circui.t for OA3 comprises
R17a and R18a and C19a. R20a references the node between
R17a and Rl~a to the power supply potent:ial provided to
terminal 52 for offset error compensation. the power supply
potential is also provided by lead 63 to OA3.
Negative feedback for operational c~plifier OA3 is
established by R21a, R22a and R23a. Compensating compacitors
C21a and C22a are connected conventionally to the operational
amplifier.
As thus configured, circuit 60a (w:ith OA3) is,an active
high frequency control circuit providing gain control over
frequencies determined by the setting of wiper 61 t with gain
variable + 12 db within the frequency range of preferably
about 1.5 kHz to 3 kHz. Gain peaXing and attenuation peaking
occurs preferably about 2.4 kHz. The output of OA3 is provided
through C25a to lead 25. Similarly, circuit 60b with OA4)
is an.active low frequency control circuit providing gain control
, frequency over frequencies determined by the setting of potentio-
meter wiper 65, determining signal levels provided to OA4 through
C17b~ he frequency band preferably is from 300 Hz to 1~5 kHz
and with gain peaking and attenuation peaking occurrîng at
preerably about 1 ~Hz. The low frequency components at the
output of OA4 are delivered through C25b to output leaa 25.
0~3 and OA4 may both be of the commercially available IC
circuit type UA739, which provides high loop gain without any
substantial distortion.
The mixed high and low frequency audio components are
delivered to PVC 27 (FIGURE 4) which comprises an integrated
circuit Ill of the type used as a compressor-expandor and
including a fullwave rectifier, a variable gain cell and an
operational amplifier including a biasing system within it,
all as shown in FIGURE 4A. No specific description is necessary,
since ICl may be of commercially available type designated
NE570/571. There is an internal summing node within the
operational amplifier (nok shown) which is biased at a voltage
preference, and signals supplied to the integratecl circuit are
-19 -
averaged my interoonnected circuitry (FIGURE 4). me averaged
YalUe 0~ the input signal establishes the gain of the variable
gain oe 11 OFIGURE 4A) which is then proportional to the average
varian oe of the capacitively ooupled input signal.
Although ale gaLn cell of compand3r circuit IC1 junctions
as an ox dor, ky providing negative feedback to the operational
amplifier therein,. oompression is realized. Similar circuitry
is utilized in SDC 370 ICl is in~Proonn0~ted with ~ppropria~e
xesistive z~ld capacitive circuit oomponents.
In acoordan oe with the invention, interconnected with the
gain cell of the compandor circuit ICl is lead 28 for providing
a feeaback signal of a limiting character frGm AGC ~LrcUit 29
as a function of the overall gain signalled by gain oolltrol
circuit1y 29. The AGC sisn21 is provided to the gain cell of
candor ICl through rEsistor R26. The overall purpose and
function of this negative fee~ack is discussed belcw but it is
noted preliminari}y that the gain oe ll within ICl is controlled
in a negative sense in response to increase in the AGC signal
supplied ky lead 28, thereby to redu oe the runt of primary oom-
pression achieved by PVC 27 with increasing gain feedback. Thus,
the feedback is essentially negative or limiting.
A terminal 67 supplies low d.c. voltage for ICl. m e
dynamic211y compressed ouLput signal is provided by C34 across
R32, and is appropriately attenuated by R35 for delivery by
lead 27 to SAFC (FIGURE 5). Hcwever, prior to being ooupled
through C34, lead 30 provides the ccmpressed audio output from
PVC 27 to IED drive circuit 31 (FIGURE 8). m e output of PVC
27, a primarily ~umpressed signal, includes both fundamental
and harmonically related oonstituents of the original intelligence-
carrying input signal. Although these haLmonics are within
the nau~x~w 3 kHz pass band, they represent also, as sums or
differen oe s, frequencies outside the pass band.
',~
~;~7~1
-20-
In FIGURE 8, circuitry is shown for constituting the LED
drive 31 and TED 33, as well as the LED drive 49 and LID 50.
The LED indicator lamp, whether what designated at 33 or that
designated at 50, is driven by circuitry including transistors
Ql, Q2. The base of Q2 is driven by a ~mipolar signal provided
throuyh diode Dl and delivered through R37. The base of Ql is
biased to ground through R38. Its collector is provided with
a supply o potential by terminal 71 through R39, while R40
provides voltage supply to Ql's collector. The base of Ql
~0 is connected through R41 to Q2' 5 collector. Coupling is pro-
vided between the collector of Ql and ~2's base by C36 and
R42. This provides a two-transistor switching circuit wherein
the LED will be driven on when there is sufficient base drive
provided to Q2, as by the output signal of PVC 27, or SDC 37,
1`5 respectively.
Referring to FIGURE 5, SAFC 3S reconstitutes harmonic
dyn~nics lost in processing of the signal through PVC 27.
SAFC 35 comprises operational amplifiers OA5, OA6 connected
to provide high and low pass sections of a Butterworth
Z~ ilter. OA5, OA6 may each be of commercially available type
LM349, each part of a single integrated circuit powered my
+ 15 v.d.c., supplied via terminals 73, 74. Capacitors C38,
C39 bypass these power supply inputs. The input signal is
provided by lead 27 through C40 and R44 to OA5. R45 provides
2~ feedback for OA5, the input being biased to ground through R46.
The output of OA5 is provided through C42 to a frequency
gain control circuit 75 having two parallel branches providing
an input node 76 and an output node 77. A first branch com
prises R47, R48 and resistor R49, with C430 C44 connected
3~` between opposite ends of R48 and its wiper 78. The wiper is
interconnected through R50 and C46 to wiper 78 of R51 forming
with R52 and R53 a second parallel branch. A node 79 between
R50 and C46 provides input for OA6, and this input is biased
to ground through R54. These component values are selected
to provide a low pass Butterworth filter defined by the
following equations:
--21--
sK'
Tts) = ~s~~ B
s2K,
T(s) -- 2
s 2BS B
~3K' 3
~8) a 3 2 2 3
s + ~Bs = 2B s B
s4K'4
T(s) r 2 2 3 4
s 2.613Bs 3.414B s 2.613B s B
For equations relating to the high pass section of the
8utterworth filter, the transfer functions are as fol:Lows:
,
T s ) = - s
- 5. wl
1' (s) = s2
s * W2 s + w~;2wl
~0 3
T~s) = s _
s3 w s2 + w3w25 + W3w2
T(s) = _ s _.
s w4s + w4w3s w4w3w2S + w4w3 2 1
In accordance with these formulas, the SAFC 35 provides
tailoring o the response curve between 300 ~2 and 3 k~z. The
low frequency section of SAFC 35 controls response between
250 Hz and 1200 Hz, the high frequency section controlling
response between 1400 Hæ and 3500 ~z. These figures are those
preferred for various purposes contemplated for the invention
but are not necessarily rigidly absolute or certain other
applications, to which extent they may be subject to variation
within the scope of the invention.
-22-
To provide such operation, OAS is a buffer to ensure
low driving impedance to circuits 75. C40 and R44 provide
d.c. blocking and impedance matching for OA5's inputO R45
provides feedback between the output and the non-inverting
input of OA5. C42 provides d.c. blocking of OA5's output.
High frequency control elements are R50, R52 and R53. RSl
allows the high frequency section to establish between 0
and 22 db gain, preferably. The low frequency elements are
R47, R49, R48, C43 and C44. R48 allows low frequency
l control preferably between 0 and 22 db gain. A feedback
path is provided through node 77 by R49 and R53. C40 and
C42 provide d.c. blocking and establish low frequency roll-
of.
Devices other than type LM 349 noted above may be used but
it is preferred that any integrated device substituted have a
slew rate allowing undistorted full swing performance up to
a frequency of 25 kHz. Also, the total harmonic distortion
is preferably kept low being typically not greater than 0.05
[ O dbm (i.G.) 0O77 v.] across the complete audio spectrum.
Preferably, the low frequency section of SAEC 35 has a
gain of 22 db with a low frequency lower 3 db corner (i.e.,
representing 3 db insertion loss) at 30 Hz and with a high
frequency upper 3 db corner at 10 kHz.
In effect, SAFC 35 operates to provide, at any given
instant of time, a power envelope vf predetermined shape within
the processed frequency band, and in accordance with the
` overall gain of SAFC 35 and the amplitudes of the lower and
upper requency sub-bands processed by it. This power
envelope is provided to SDC 37, and thus determines via
AGC 29 the control by PVC 27 over signal dynamics, whereby
the system may reconstitute harmonics while responding to
and preserving signal dynamics, quite unlike compressor or
compandor techniques which diminish or destroy signal
dynamics and produce characteristic "flatness".
~2~7~9~
23~
FIGVRE 10 shows circuitry for providing regulated potentials
suitable for the operational amplifiers of SAFC 35O Terminals
81, 82 provide a.c. line voltage across the primary winding
83 of a transformer having its secondary winding 84 connected
- S acrsss a full wave bridge rectifier 85. Acros$ the latter is
a first filtering of a.c. decoupling capacitor C48. Across
it are R56, R57 which in turn each have connected across them
capacitors C50, C51 to provide a floating ground node 86.
The potentials on the opposite sides C53, C51 are provided to
respective integrated circuits IC2, IC3, such as each of
commercially available type UA7815 integrated circuit regulators.
Each of ICl, IC2 provides a respective output 87, 88 to provide
highly regulated voltages, preferably, + 15 v.d.c. and - 15
v.d.c. for powering operational amplifiers OA5, OA6. Similar
circuitry may be utilized for developing other supply potentials
or operation of'othe~ components of this system. Alternatively,
battery power supplies may be utilized.
Referring to FIGURE 6, showing circuitry of SDC 37, lead
34 provides the signal from SAFC 35 across R59 and through CS3,
thence to a high frequency emphasis network of C54 and R60.
R61 then couples the signal directly to OR7. Inputs of OA7
are tied together through C55 and R62, nd axe interconnected
with gain cell 85 and full-w ve rectifier 86, both portions of
an integrated circuit compressor-expandor, i.e., compandor,
device such as of commercia}ly available type NE570/571, des-
cribed below.
Such a device may also include an internal or self-
contained integrated circuit operational amplifier but, in
the case of the preferred circuit NE570/571, the internal
operational amplifier is not utilized. Connections are instead
made, as indicated in FIGURE 6, to grin cell 85 and rectifier
86 components of such compandor circuit. Also, there are
internal resistive components of compandor device including R63
connected to gain cell output 87. Node 88 represents the
gain cell input,, there being an internal resistor R640 Gain
cell 85 is adapted to be controlled by rectifier B6 in accordance
with a capacitance at terminal 89 o rectifier 86.
~L~72~
. . .
. . -2~-
Briefly, rectiier 86 provides ~ull-wave rectification
of the signal input to compressor 37, as reflected at the
output of operational amplifier OA7. Its output signal,
as fed back to the rectifier, is averaged by the capacitance
seen at terminal 8g. The averaged signal is then provided
to the gain cell 85 which in turn provides a gain control
signal to the inverting input of OA7.
The capacitance provided at terminal 89 is established
by capacitance expandor circuit 90 comprising OA8, OA9 inter-
connected with rectifier 86~ Circuit 90 includes C56 which is
interconnected between the output of OA9 and the non~invert~ng
input of OA6, which is also interconnected with the capacitance
input terminal 89. R65 couples the output of OA8 to the non-
inverting input of OA9, which is biased to ground through
D2 and D3. R66 provides a load across ~A9's output.
R67 is internal to the preferred integrated circuit device
NE570/571 whereby node 91 is the input to rectifier 86. R68
and C58 provide high fxe~uency pre-emphasis for the input
signal to rectifier 86 r which is delivered by C5~ from OA7.
Similarly, C60 interconnects OA7's output with gain cell
input 88.
Accordingly, there are interconnected with the output
of OA7 two feedback paths, a first being provided to the gain
cell 85 through C60 and R64. A second feedback path is
provided through C59, the pre~emphasis network noted above,
and R67 to rectifier 86. Rectifier 86 contrsls the gain
provided by gain cell 85, which is an integrated circuit-
realized current in current out device with the ratio IoUt/
Iin providing an ovexall gain which is an expone~t.ial function
of the input signal. Since the gain cell output 87 is inter-
connected with the inverting.input of OA7, the gain cell provides,
in effect, negative feedback for causing compression of the
input signal.
~20729Ll
-25-
Furthexmore, since circuit 90 operates to expand the
apparent magnitude of C56 as a function of the relative
dynamic amplitude range of the signal at the output of OA7,
the amount of current provided by rectifier 86 to gain cell 85
is varied as a further function of the magnitude, i.e., dynamic
range, of the signal being processed.
In addition to the foregoing feedback circuits, R70, R71
provide feedback between the output and inverting input of
OA7. C62 provides a.cO bypass to ground. R70 and R71 provide
d.c. feedback for OA7 since there is no d.c. feedback path
through gain cell 85. Bias potential for the inverting
input of OA7 through a resistor R72 by connection of a terminal
92 to a suitable potential. Terminal 93 supplies the gain
cell with a THY trim potential or bias through R73 connected
to wiper 94 of R74. Acxoss R74 is a 3.6 volt d.c. potential
lS supplied by terminal 95. D;c. shift trim for the gain cell
input 88 is provided through R75 according to setting of
wiper.96 of R7~ across which is supplied a 3.6 volts d.c.
To limit the output of OA7, zener diodes D4, DS are
selected for threshold potentials of, e.g., 3 v., precluding
the output of OA7 from exceeding predetermined levels, limiting
its swing to avoid overloading any succeeding circuit stage,
e.g., if excessive signals are caused by ignition or electrical
noise, etc. C63 couples the output to lead 48, across which
is load resistor R77.
Accordingly, SDC 37 comprises an operational amplifier
(OA7) and feedback circuitry for providing a nonlinearly in-
creasing negative feedback signal to the input in response to
increase in the level of the tonally controlled audio signal
on lead 36. Ignoring the change in attack time or response
produced by capacitance expandor circuit 90, the signal gain
is 1/2
Gcomp=
where Ib is the current flowing into an effective internal
summing node of operational amplifier OA7, vin is the average
input voltage of the audio signal input to compressor 37 and
K is merely a gain constant. Gain cell 85 provides an
-26-
exponential response in gain in response to step changes
in amplitude, said exponential response being effected
by the time constant resulting from the capacitance re-
presented by circuit 30 a terminal 89 of rectifier ~6.
In FIGURE 12, a family of curves represents typical
input-output tracking of compressox 37. The ordinate
repxesents output level and the abscissa represents the
input level.
Operational amplifier OA7 may be of cor~mercially
available type TDA1034 and OA8, 0~9 may each be of
corr~ercially available type LM387.
Concerning SDC 37, operation in accordance with
the invention involved reconstituting harmonics typically
characteristic of human speech, among other aspects of
operation, which otherwise would be of inaudible and of
essentially inefective amplitude owing to constraints imposed
by normal compression or narrow-band filtering normally en-
countered in modulation and RF transmission systems. Accordingly r
it is desired that OA7 have a high slew rate and be capable
O of providing transmission of audio frequencies up to at least
15-16 kHz. Also, in the case of commercial types LM387
utilized for OA8, OA9~ a minimum upper bandwidth is typically
75 k~z~ Thus, the circuitry utilized exhibits high gain and
wide bandwidth. For increased stabil ! ty, in view of these
characteristics, an input compensation network constituted
by C53, C54 and R60 is utilized.
Circuit 90 effectively increases the capacitance at
terminal 89, being the apparent magnitude of capacitor C56,
in accordance with decrease in signal level, i.e., the dynamic
3~ input of SDC 37. Thus, in effect, the response time of SDC
37 is considerably longer at low signal levels, since circuit
90 operates effèctively 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
i f 10 7 C X 103 to 32 6C X 103 where C
is the effective capacitance interconnected with rectifier 86.
This in turn effects the gain cell response because gain cell
85 is controlled by rectifier 86. This avoids or greatly
reduces any mistracking of low signal dynamics. Here it is
noted that in cornpressor-expandor (compandor) systems in w
~2~2gl~
-2~-
the overall gain is unity such change in attack time would not
produce any overall gain erroxl and the resultant gain or loss
would appear to be manifested as such mistracking of low
signal dynamics. But it the present system, such problem is
largely averted, since unity gain is not necessarily provided.
Accordinyly, SDC 37 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 34, the ga:in is reduced . Over-
loading is, however, precluded by the clamping action of D3,
D4 and D5~
Furthermore, with regard to transient response, the time
taken for recovery of SDC 37 from an overload condition is
determined by the capacitance provided by circuit 90 at
terminal 89. If there were a smaller capacitance, faster
response to transients would be permitted but such would
produce more low frequency THD because of gain modulation.
The use of a relatively small capacitance, i.e., one micro-
farad or capacitor C56 and circuit 90 avoids such a difficulty
in the present system.
Compressor-expandor systems are subject to a problem known
as breathing. This comes about since, as a system is changing
its gain, the change in background noise level sometimes can
be heard. In order to avoid any comparable breathing, C54,
R60, C58 amd R68 provide high ~reguency pre-emphasis by
altering compression yain accordingly. It is preferred that
SDC 37 provide up to 135 db of dynamic compression for a 0 db
input signal, but the system does not unction in an overall
sense as a compressor or compandor since it responds to and
rètains signal dynamics in sharp constrast with compression
systems.
AGC circuit 29 is shown in FIGURE 9. Lead 41 provides an
input for signals processed by SDC 37. The output of SDC 37
effectively provides a tracking voltage provided by feedback
circui* to the d.c. rectification stage, i.e., rectifier 86
and expandor circuit 90, of the secondary compressor 37, while
serving also to deliver the processed audio signals to BIF 52.
This AGC input is supplied to D6 for rectification and thence
to the gat-e of FET 99. Connected between its gate and drain
-28-
electrodes i5 a time constant delay circuit providing delayed
AGC action, having C65 in parallel with R80, and C56 connected
ts the drain electrode of FET 99.
FET effectively isolates the input by its high impedanse~
changing its conductivity to change the signal level applied
to the non-inverting input of OA10, which is biased to ground
through R81 and interconnected to its output for feedback
through R82 to establish yain control.
The inverting input is biased to ground through R83
and is connected thxough R84 an R85, which has wiper 101
vied to a lead 102 supplied by terminal 103 with positive
voltage, whereby gain of OA10 may be controlled by setting
wiper 101. C68 bypasses to ground any AC component present
on lead 102. Lead 102 is interconnected with lead 103 for
supplying OA10 with its operating voltage.
Connected between lead 102 and the output 104 of OA10
is a circuit comprising R86, R~7 and meter 45, preferably o
the moving coil type. Output 104 is tied directly to lead
28 for providing an AGC feedback connection to PVC 27. One
side of meter 45 is connected to ground through R88. Wipers
105, 106 of R86~ R87 provide meter calibration.
The meter indicates overall signal compression gain level
and thus displays an indication of the extent to which the
system is heing utilized to capacity. Since the meter is of
the moving coil type, it provides averaging of the instant-
aneous variations in the output of OA10 to avoid rapid changes
in the indicated output which would be difficult to observe.
For proper operation, wiper 101 is adjusted to permit
variation in the level of voltage provided to the inverting
input of OA10 within a normal swing or variation in the potential
at output 104 of from about ~2 to about ~9 v.d.c., which are
of proper magnitude for PVC 27.
There is thus provided a feedback signal to PVC 27 rom
AGC 29 in accordance with the extent of compression by SDC 37.
As a consequence, a negative or limiting feedback occurs which
operates to provide the function of controlling the amount of
THD by limiting the primary voltage compression as a function
of the compression provided by 5DC 37. Not only does the
~2C~72~
-29-
feedback serve to control overall dynamic compression but
there is also a control of the bandwidth of the system by
limiting out-of-band harmonics which would otherwise pass
through the system as a result of THD.
For certain applications of the invention; one might
permit rapid secondary reaction, so what the feedback to PVC
27 would operate to prevent an overloaa result.ing from ex-
cesses in the ou~pu~ of SDC 37, avoiding output overdriving.
E.g., digital signals of the type used for data transfer
require such r pid reaction. Also in the case of data
transmisslon rather than audio, narrow bandwidths are in-
volved so that owe need not provide capacity for handling
harmonics normally present in speech or other audio, but only
those typically characteristic. However, squaxe and other
pulse-form signals may be represented, as by a Fourier series
of sinusoidal components spanning an indefinitely ~idP
requency range. Thus, such frequencies are of significance
and may be transferred effectively by the system.
The amount of delay provided in the AGC delay path is
preselected in accordance with applications of the system and
the type and`character of signals to be processed. Variation
of the AGC time constant is prov?ded by C65 and R80 which may
be varied, as by selection of different components. Examples
of the AGC delay which may be provided by C65 and R80 are from
0.010 seconds to 3 seconds, as a broadly preerred range,
whereas 0.3 - 0.5 seconds delay may be typically adequate for
various aùdio signals. In general, the delay is to be shorter,
i.e., for fastex response, for transmission of signals of the
data character and slower such as 0.3 - 0.5 seconds or even
~30 longer for signals of audio or voice character. In this way,
the amount of AGC feedback can be established in direct
- relevance to the amount of out-of-~and harmonics expected to
be present in the signals being transmitted.
As noted above, LED drive 49 and LED 50 are also
connected to the output 48 of SDC 37 to indicate the amount
of compression attained by SDC 37. LED 50 signals an overload
condition xesulting from excessive compression. Thus, for
a normal 0 db input, i.e., 120 db compression, the control
2~
. -30-
of the system may be adjusted to provide operation under
conditions such that peak compression can be handled, as
indicated by LED 50 duriny processing of signals, the
controls being adjusted to prevent LED 50 from normally
remaining iliuminated.
The output of the secondary dynamic compressor 37, as
thus monitored, is provided to BOF 52 (E'IGURE 7).
Referring to FIGURE 7, OAll xeceives processed signals
through C70 and C71. The non-inverting input is biased to
ground through C72. Node 108 is biased to ground through
R89 and also is provided through C73 with a feedback.signal
from the output of OAll. A further feedback path is provided
through R90 dixectly to the non-inverting input, which is
biased to ground through R91. The inverting input is connected
to circuit ground through C74. Terminal 109 provides d.c.
operating voltage for OAll and OA12, the two being both pre-
~erably part of the same integrated circuit such as that
commercially available under type LM387.
OAll's output is coupled through C75, R92 and R93 to
OA12, the input of OA12 being biased to ground through R94
and C76. Feedback for operational amplifier OA12 is provided
by R95 connected between the output and node 110 between
R92 and R93 which node is bypassed to ground throuyh C77.
Feedback is also provided directly to the non-inverting input
thxough C78. The inverting input of OA12 is bypassed to ground
through C80.
Thus, BOF 52 has high and low frequency sections, pre-
ferably providing low frequency gain of 22 db with the low
frequency upper 3 db corner a 30 Hz and with the high fre-
quency upper 3 db corner at 10 kHz. BOF 52 provides a much
greater pass band than BIF 19.
Switch SW2 is connected in a lead 53 from input 48 to
output 15 and, when closed, bypasses BOF 52 as may be desired
for testing or specialized purposes of the invention, or when
processing narrow band signals which do not require further
filtering after processing. Lead 55 supplies the output of
BOF 52 to the HLO stage 56 (FIGURE 11).
~2~
-31-
Loudspeaker 57 or other suitable transducer may be
connected directly or indirectly to output 15. Such a
device is a recombiner for recombining
requency components of the various in-band electrical signals
-5 present at output 15 and providing a rPcombined audio signal
in which are present reconstituted harmonics characteristic
of the original signals provided to the system. Other
recombinexs, including solid state devices, may also be used.
Thereore, the processed signal present on output 15 may be
provided to the modulator of a transmitter, transrnitted via
carriex signal, recovered by demodulation and then provi~e~
to a recombiner. In this way, frequencies normally
outside the passband will have been transmitted and, by
recombina~ion at the recombiner, are reproducedO
Referring to FIGURE 11, lead 55 provides the processed
audio frequency signal C81 to the input terminal of IC4.
Terminal 111 provides operating voltage, through R99, for
powering IC4. Amplifier circuit 56 operates in effect to
provide a buffering of output 15 and for providing high level,
e.g., with 40 db gain, useful for driving various auxiliary
apparatus, such as oscilloscopes, monitor ds splays, frequency
counters, spectrum analyzers, and the like.
The AGc feedback signal to PVC 27 limits primary com-
pression as a time-delayed function of increase in the level
of the output of SDC 37. PVC 27 and SDC 37 may be regarded
as respective first and second dynamic control means, PAFC
23 and 5AFC 35 as respective first and second sub-band or
tonal control means, since both compressors and both active
frequency contxols are configured for permitting selective
controlling of their respective functions. BIF 19 and BOF
52 each may be selectively switched in or out ox the signal
processing path.
Since system circuitry requires no inductive elements,
is transformerless and utilizes integrated circuits and
associated discrete components, the system can be configured
in many ways. One especially preferred configuration is that
having the circuitry divided between two integrated circuit
devices, e.g., of DIP type, one having requisite processing
-32-
circuits, all of which may be provided by single VSLI chip,
the other being a programming device having all other circuit
elements necessary or establishing, i.e., progxamming, para-
meters necessary for operation by the ~SLI chip device or a
particular dedicated use of the system. ThusO the same VSLI
chip device may be used for many different systems applications
by changing only the programming device.
~2~2~
-33-
Operation of the Sy~_m
The system shown in FIGURE 1 is connected to a suitable
source of audio signals, such as.the output of a microphone,
pre-amplifier, tape recorder, or the output of various inter-
s5 mediate stages present in transmitters, receivers or the likewhere it is desired to utilize the system for pro essing audio
signals provided by such a source. The source may be at either
end of an RF transmission link. If the source is the inter-
mediate amplification stage of a modulation system providing
an audio signal to modulate an RF signal, as in AM. FM, SSB,
en., the system may be utilized for improving intelligibility
and clarity of such audio signals utilized for.modulation,
thereby improving the character of the txansmitted rad:io
frequency inormation, even though such may be in a relatively
narrow band such as 3 kHz. Also, the source may be an inter-
mediate amplification stage of a receiver which amplifies a
demodulated audio signal, which similarly may be of a very
limited bandwidth as received, and will retrieve from the
received modulated audio signals the available clarity, in-
telligibility and general characteristics of normal human voiceor other tonal signals. .In such uses, the system enhances the
character of the signals transmitted or received to achieve
the various objects of the invention.
But such use of the system is not necessarily limited
to only one end of an RF transmission link. Instead, a
system of the invention may be;utilized at each end of such
a link. And, when so utilized, the two systems or units of
the invention effectively~multiply.the phenomena produced by
the single system on either end of such a link. When utilized
3~ on both ends, the system processes both the transmitted and
- received audio information to add substantial dynamic range
as well as improve the S/N ratio. In the strict sense, the
system is not a compressor-expandor (compandor), since a
compandor is a combination of a compressor at one point in a
~5 communication path for reducing the amplitude range of signals
followed by an expandor at another point for complementary
increase in the amplitude range for increasing the S/N ratio.
7~
. -34-
Regardless of its use in one of the types of situations
referred to above, and as more fully contemplated in accordance
with the objects of the invention and as described hexe and
above/ the new system provides overall performance which goes
well beyond that of a compandor in the customary sense, since
it not only improves S/N ratios but also reconstitutes lost
harmonics and improves clarity, intelligibility and generally
enhances the character of signals processed there~hrough.
Without use of the new system, the usua:Lly audible characteristics
of human speech, involving various harmonics particularly of
the higher order which gives life-liXe, normal character as
well as richness and'quality to human speech are typically so
suppressed or reduced in magnitude in narrow band transmission
as to 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 of the audio signal,
particularly speech.
Effectively, one may think of the operation of the system
as first involving the pulling out or limiting certain out-
of-band harmonics which normally would be present in signals,
amplified, and then subsequently returning such harmonics
'' to the signal in such a way that they are emphasized to
advantage. The system may be likened to a crystal oscillator
circuit which, through ringing, may be utilized to create
harmonics. The new system returns harmonics of human speech
to audible levels of undamental frequency. This manifestation
is best appreciated only upon objective aural comparison
between signals from various sources processed, and those not
processed, through use of the invention.
The circuit values of the LED drive circuits 31, 49 are each
chosen to provide for energization of the respective LED 33,
50 when the output of PVC 27 and SDC 37 respectively inter-
connected with each drive circuit is providiny maximum
compression. Thus, referring to LED 33, adjustment of gain
~l;~72~
-35-
potentiometer Rs allows the output of the primary voltage
compressor 27 to reach its full maximum va.lue, thus being
indicated by illumination of TED 33. This LED will remain
unlit when PVC 27 is operating at less than full compression.
.5 The overall system gain may be varied to allow normal
operation to proceed so that LED 33 would only light upon
maximum peak magnitude of signals being processed. Similarly,
potentiometer wiper 84 of SDC 37 is adjusted so that LED 50
normally will remain unlit but will flash momentarily only
at maximum peak signal levels. Meter 45 provides an
indication of the extent to which the system is operating
to the extent of its capabilities thus displaying the overall
performance of the system.
The indication of meter 45 when speech signals are being
lS processed gives visual indication of the averaged output
voltage of SDC 37, but at the same time the meter will indicate
.~luctuationin dynamic audio content. This indication is
particularly useful when speech is being processed through the
system. Howevex, it is noted that when data signals are
processed through the system, such as in the case of facsimile
or teletype signals, meter 45 produces only an average reading
from any given dynamic fluxation pattern, since dynamic voltage
peaks will appear fax too fast to be tracked by the moving
coil type movement. On the other hand, the indication provided
US by LID 50 is such that any rapid increase in signal which may
be in excess of the maximum compression utilizable will be
instantaneously displayed by flashing of the LED.
-36-
General Design and Methodology
The genexal design of the processing system of this
invention is explained and modelled mathematically as
follows:
An input signal to the system yoes into BIF 19. The
operation of this filter is described by a transfer function
developed in the following manner:
Define: n = the number of poles in the bandpass
input filter.
~JL- the low cutoff frequency of the filter.
OH= the high cutof frequency of the filter.
7~(2k n - 1)/2n, for k = 1, 2, . . ., n-
when the transfer function is defined by:
HBIF(s) = - ko
n
s 2 S k]
Z~ k= lLs(C~
where ko is a normalizing constant.
The output signal amplitude spectrum o BIF 19 is
determined by evaluating aiH(j6~ for each component in the
input signal with amplitude ai and frequency I' This
amplitude spectrl~ may be computed as it passes through the
system my using the following equations in the same manner as
for BIF 19.
The signal is next passed to PAFC 23. There it is acted
30~ upon in two separate paths: a low frequency path and a high
frequency path The output of the low frequency path is deter-
mined by the transfer function:
HL~s) = apAFcHl (s) H2 ( )
The output of the high frequency path is determined by the
transfer function:
HH(s) ~PAFCH3(S) H4(s)
where: apAFC = circuit gain setting
-37-
Hl(s), H2(s), H3(s)~ H4(s) = four bandpass filter
transfex unctions. P~FC 23 for any sub-band a transfer
function which is:
H (s) =
ni
r2+~i~i Six
k--l (b)Hi Li
with si = ei (2k + ni 1)/2n
k
ni = number of poles in the ith sub-band
l OH = low and high cutoff frequencies for the ith sub~band
the output of PAFC 23 is obtained by summing the signals from the
low and high frequency (plus any additional) paths, thus giving
an overall PAEC transfer function as:
HpAFC(s~ = HL(S) + HH(S~
The signal next passes into PVC 27. Its relative output
voltage is determined by the equation (note: dB = 20 log v):
out apVC bpvc * dBin
where dBin i5 the relative input voltage from PAFC 23, the co-
efficients, apVC and bpVC, are determined by the level of the
AGC fèedback voltage from SDC 37.
SAFC 3~ receives to signal from PVC 27. As in the PAEC,
SAEC acts on ~he-signal in both low and high frequency paths.
The low frequency path has a transfer function of the form:
) = asAFcbsAFcHl( )
and the high frequency path:
Ho = aSAFC(b~SAFC H2( )
where aSAFC and bSAFC are two variable gain setting constants,
which can be set to balance the signal level in the two paths
~2~7~D~ -
-38-
Hl(s), H2(s) = two bandpass filter transfer functions.
For any sub-band, the transfer function is:
Hi (s) = - ko
ni
r2+~ Xi 5i~
k=l ( phi i
ik = ei~(2k + ni 1)/2ni
ni = number of poles in the ith sub-bana
~JI OH low and high cutoff frequencies for the ith
sub-band
The output signal of SAFC 35 i5 obtained by summing the signals
from the low and high frequency (and any additional) paths,
resulting in an overall transfer function of:
HSAFC(s) = HL(S~ HH( )
The signal is next passed into SDC 37. Its output voltage
is given by the equation: .
V t = aSDC + bSDC * dBin
The output voltage also serves as the source signal for the
AGC feedback via AGC 29 to control the operation of PVC 27.
In order to maintain the-dynamics of the original input signal,
however, the AGC signal i5 delayed by an appropriate time
Thus, the PVC control at time comes from:
VAGC ( ) Vout( l
The effect of this control voltage is to adjust the gain and
compression levels at which the PVC module operates.
.q ~o~-~s ,4
owl
_39_
rom SDC 37, the signal is passed to BOF 52. It has a
transfer function of the form:
HBOF (S) = ko _ _ _ _
n
2 + L OH
k~ 3L~
where Sk = ej ~2~ + n 1)/2n
n = the number of poles in the filter
) = the low and high cutoff frequencies of the filtex
L .
The output of BOF 52 is essentially the electrical output
of the new system prior to linear recombination. It consists
of a number of frequency components concentrated within the
pass band of the system. These components all have essentially
the same amplitude. When this complex signal is passed through
a recombiner (such as a loudspeaker, othex transducer or
solid state device), the generated harmonics will tend to re-
inforce each other at the frequencies corresponding to the
original input frequencies. This reinforcement will even
reproduce the input frequencies that lie completely outside of
the pass band of the system.
As a simplified example, consider a case in which there
are two undamental frequencies in the original input signal,
with l in the bandpass ahd f2 outside. These two frequencies
will produce a beat frequency f2 fl within the bandpass.
The output signal will consist of fl and f2 l plus other
harmonics). When these two are xecombined in an external
speaker, the beat frequency l - fl) + fl will be produced.
Thus, the original frequency f2 will be regenerated e~ren
though it was not present in the system output signal during
transmission.
~2~
-40-
For the genèral case, fll . . fn be a set o original
fundamental frequencies. The system input signal will
contain all of the beat frequencies of these fundamentals,
up to the third harmonics. That is, the input frequency
' 5 spectrum will consist of:
all original fundamentals: fly fn
all second harmonics: 2fl, . . , 2fn~ +fl +f2 +f3' .
all third harmonics: 3f~ 3fn~ +fl +f2 -~3' '
and so forth.
The system output signal spectrum will contain those
frequencies from the above list that are within the band pass
of the system, say within the range from fL to fHO With a
reasonably random distribution of original fundamentals, there
will be a large number of these harmonic frequencies within
the pass band. Frequencies will be available that contain
all ox the information on the original frequency spectrum,
and in fat the original spectrum will be essentially recreated
by the beat frequencies generated by the system output signal.
The foregoing mathematical model of the system can be
validated by use of a large general purpose digital computer.
For this purpose, one may validly assume that a voice signal
can be represented at any yiven moment by a discrete number
of separate components of differing frequencies which may be
termed key frequencies. The following example illustrates
the operation of the new processing system by use of such a
computer modeling technique.
EXAMPLE: It is assumed that a 3 k~z pass band is
established by processing circuitry of the system. Frequency
components representative of a voice signal to be processed
by the system are randomly selected, providing frequencies
of 350, 801.3, 1834.52 and 4200 Hz, thus xepresenting input
fundamental frequencies. Table I shows the amplitude and
frequency of the fundamental input signals and other signals
at various points of the system, and resulting from processing
by the system. Referring to FIGURE 13, the amplitudes of
these input fundamentals, which vary, are plotted as a function
of frequency. The amplitudes, like the frequencies, of these
fundamentals are arbitrarily representative of voice con-
on stltuents at a aiven moment.
~41-
Recause of these input fundamentals, a variety of beat
fxequencies are produced. Their magnitudes and frequencies
are shown in Table I under the heading "Input Haxmonics
Generated By Fundamental Frequencies". It will be understood
S t~a~ the input fundamental frequencies produce beat frequencies
according to the sums and differences of t:he input fundamental
frequencies. FIGURE 14 represents a depiction of the amplitude
of the resultant input harmonics as a function of their fre-
quencies. As will be apparent, there are input harmonics which
extend well beyond the uppex limit, 3000 Hz, of the pass band.
In FIGURES 13-17, the illustration of frequencies does not
extend beyond 5000 Hz for purposes of facilitating illustration
but it will be understood that higher frequencies are involved.
Similarly, Table I arbitrarily is limited to an uppermost
frequency Qf slightly gxeater than 10 kHz although higher
frequencies are involved. From FIGURE 14, it will be seen
that the four input fundamentals provide numerous harmonic
input frequencies of varying amplitudes. Many of these are
beyond the upper pass band limit of 3000 Hz.
FIGURE 15 illustrates the output signals present at
output 15. Thus, in FIGURE 15 are plotted, as a function of
frequency, the amplitude of signal components provided after
processing by the system. These include harmonics as well
as input fundamentals. The transmitted in-band frequencies
are seen to be each at substantially 0 dB level, or slightly
less, and with a roll-off beginning at about 2750 ~z and with
sharp attenuation being thus provided at all frequencies above
3000 Hz. Therefore, signal components above 3000 Hz may be
termed rejected out-of-band frequencies, the levels of which
are of negligible value, being less than -60 dB above 4250 Hz.
Similarly, the lower limit of the pass band, being
approximately 300 Hz, provides sharp attenuation of all signal
components having less than this frequency. Only one harmonic,
namely, 231.92 Hz, is shown and its magnitude is -36.9 dB.
The levels of the various components at the output are shown in
Table I in the column designated "Amplitude of Harmonic and
Fundamental Frequencies from System", which may be compared to
the adjacent column designated "Amplitude of Input Frequencies
to System".
~7~
-42- ,
When the signals at the output 15 are provided Jo a
recombiner, such as a loudspeaker or other transducer,
the .~ignal components shown in FIGURE 15 regenerate a series Of
signal components shown in FIGURE 16. Such components, as
explained previously, extend well beyond the 5000 Hz upper
limit of the graph and there is substantial variation in the
amplitudes of these various output signa:Ls, which are har-
monically relat.ed to the input signalsO
,
TABLE I
INPUT INPUT AMPLITUDE AMPLITUDE AMPLITUDE
FUNDAM~NTA~ HARMONIC5 OF INPUT OF. HARMONIC OF SIGNAL
FREQUENCIES GENERATED BY FREQUENCIES FUNDAMENTAL OUTPUT AFTER
~z) FUNDAMENTAL TO SYSTEM FREQUENCIES LINEAR RE-
FREQUENCIES ~Hz) FROM SYSTEM COMBINATION
. (HZ) ~dB) (dB)
~31;.92. ~24.74 -36.90 -7.71
350~00 -1.95 1.19 -4.67
4~.. 30 27 -1.17 -5.58
530.96 -19.40 -1.18 -11.84
. 683.22 -19.36 -1.18 -6.09
700.00 -5.90 -1.15 -5.88
801.30 -7.32 -1.16 -2.22
1033.22 -15.41 -1.17 -3.59
1050.00 -9.84 -1.16 -8.25
1134.52 -13.98 -1.17 -5.76
1151.30 -11.27 -1.17 -1.92
1252.60 -20.60 -1.18 -5.67
1383.22 -19.36 -1.15 -2.93
1484.52 -10 04 -0.93 -1.74
1501.30 -15.22 -0.90 -3.09
1564.1~ .-20.64 -0.92 -6.20
1602.60 -16.65 -0.97 -3.17
1834.52 . -6.09 -1.12 -0.00
1952.60 -20.60 -1.17 -3.05
2015.48 -15.26 -1.16 -6.12
2184.52 -10.04 -1.15 -1.04
2285.82 -19.36 -1.18 -2.41
2365.48 -11.31 -1.16 -4.94
2403.90 -25.97 -1.19 -7.37
- 43 -
INPUT INPVT ~MPL~IUDE AMPIITUDE AMPLITUDE
F~ rut En nG~L H~RMDNICS OF INPUT OF Ho NIC OF SIGNAL
FRÆQUENCI~S Gæ2E~rED By FREQUENCIES & E1~ 13YTlL OUTPUT AFTER
FRE~UENC~ES (Hz) FRoM SYSTEMCoMBIN~TION
(~Z) -- (~3) (dB)
2534.52 13.98 -:1.19 -3058
2597.40 -21.88 -:~ .24 -8.63
2635.8~ -15 41 -:1.25 -1.39
2715.4 g -15.26 -1.38 -5.66
2867.74 -23.50 -2.21 -6.50
2~8S .82 -19.36 -4.11 -1.67
3048.70 -16.50 -5.90 -6.37
3166.78 -20.64 -10.39 -5.01
3319.04 -18.12 -17.03 -I .90
3398.70 -12.55 -20.57 -4.85
3437.1~ -24.74 -22.32 -3.27
3500.00 -11.12 -25.07 -8.56
3669.04 -14.17 -32.55 -4.82
3748.70 -16.50 -36.05 -6.05
3850.00 -7.17 -40 O 43-6.51
4019.04 -18.1 -48.00 -5.45
4200.00 -3.23 -56.28 -5.24
4470.34 -23.50 -72.43 -6.69
4550.00 -7.17 -79.03 -6.88
4651.30 -16.50 -400.00* -9.65
4900.00 -11.12 -400.00 -9.63
5001.30 -12.55 -400.00 -g .77
5233.22 -20.64 -400.00 -10.83
5351.30 -16.50 -~00.00 -9.27
5503.56 -22.26 -400.00 -14.95
5684.52 -15.26 -400.0~ -12.20
5802.60 -21.88 -400.00 -12.11
6034.52 -11.31 -400.00 -11.75
6384.52 -15.26 -400.00 -12.54
6565.4~ -16.54 -400.00 -18.21
*In the table, -400 dB is used in the real time ccmputer analysis,
ky Rich the table is derived, to represent any magnitude less than
40 -80 dB.
-44-
INPVT DNPUT AMPLITUDE ~MæLITuDE AMPLITUDE
ECWDhMEN~aL H~RM~NICS OF INPUT OF HARMCNIC OF SIGNAL
FREQUENCIES GENERPIED BY FRE~LENCIES & FUNDAMENTAL OUIPUT AFTER
~Hz3 FI~Da~EW~aL IO 5YSTEM FREQUENCIES LINEAR RE
E~EQ~ENCIES (Hz) FF~M SYSTEM CQMBINATION
(Hz~ (dB) ~dB)
6835.82 -20.6~ -400~00 ~15.53
7598~70 -17.78 -400.00 -23.43
7869.04 -19.40 -~Q0.00 -25.07
8~50.00 -12.40 -400.00
8400.00 -8.45. -400.00
~750.00 -12.40 -400.00
9201.30 -17.78 -400.00
10234.52 -16.54 -400O~0
Ihe resultant signals are audio ccmponents shown ln the extreme
xight-hand column of Tale I, having the heading "amplitude of
Signal Output A~H~r Linear Reoombination".
Referrlng to FIGURE 17, the input sigr~Lls of FIGURE 14 and
output signals of FIGURE 15 are plotted continuously and designated
as "ME~G~E Input" and "ME~E~E Output", respectively. The arbitrary
teLm kED3~E derives from an acronym for "M~ih~Im Efficiency Transfer
of Modulated Energy"l being thus symkolic of the processing effect
of the new system. FIGURE 17 also shows, oo~tinuously plotted,
the audio signal output regenerated from the transmitted in-band
~5 frequencies. As will be ~nifestly apparent, the resultant
audio signal output curve substantially follows the input signal
curve but has relatively greater amplitude throughout the extent
of the graph which, as noted, does not extend beyond 5000 Hz alp
thnugh resultant audio oomponents do exist above 5000 Hz, as
shown in Table I. m erefore, audio signals are provided after
processing having frequencies well above and below the pass band
limits. These signals result from the regeneration of o~mponents
transmitted within the pass band. m e system accordingly re-
generates original frequencies even though the pass band severely
limits the actual frequencies of signal oomponents which may,
for example, be transmitted by narrcw kand radio frequency
techniques.
The data it Table I are those calculated by use of a
large general purpose digital computer in accordance with
the mathematical constraints represented in the foregoing
description harmonics higher than the third, being normally
'S ox very low amplitude, may be neglected in such computer
simulations. The amplitudes of all assumedly existent
harmonics, i.e., sums and differences, are calculated
according to the amplitudes of signal constituents, such as
input fundamentals for convenience. The second and third
harmonics may be established as 2 dB and 4 dB, respect:ively,
lower than the fundamental. Also, the amplitudes of higher
frequency signal constituents are reduced in accordance with
the normal roll-off of human hearing and speech characteristics.
Although the foregoing includes a description of the
best mode contemplated for carrying out the invention,
various modifications are contemplated.
As various 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.
