Note: Descriptions are shown in the official language in which they were submitted.
BackcJround of the Invention
The present invention pertains to the communication
art and, more particularly, to a means ~or controlling khe
level of signals carried over a duplexed line.
Controlling the level of signals carried over a duplexed
transmission line has been a long established problem in the
communication art. The problem arises as a resul-~ of
insertion loss of long transmission lines. For example,
if two stations are separated by twenty miles of conventional
telephone line, the signal received by one station rom the
other may be attenuated 20db from its original level. In
many applications~ it is imperative Eor successeul operation
of the system that compensation be made ~or transmission line
losses. For example, at the rec~iving station wherein an
operator handles a headset, i~ the operator's signal is some
20db above that of the signal from the remote station, feed-
back within the operator's headset may obliterate the o~her
stations signals. F'urther, in applications wherein either
the remote stati~n or base station signals are fed to a
transmitter, it is important that the level o~ signals
applied to the transmitter be constant.
One prior art approach to compensating for transmission
line losses has been the use of two independent transmission
lines between the base and remote stations. Fixed gain
amplifiers are provided in each line ~hus exactly compen-
sating or line losses. This system, while e~fective, ties
up two transmission lin~s and thus, is quite expensive.
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A second approach has been the use of complicated
hybrid circuitry which provides precise impedance matching
and transormer couplin~ -to the transmission line such that
the independent signals carried by the line may ~e separated
and individually amplified. Such hybrids are extremely
costly to manufacture and may re~uire periodic adjustment
for optimum opera~ion.
The prior art has developed numerous automatic gain
circuits which help assure that signal levels are maintained
at a desired level. Such systems have exhibited two
fundamental problems. ~'irstly, the yain control circuit
must establish the relative level of a processed signal.
This invariably takes a fixed peri.od o~ time, du:ring which
noise bursts or periods of very low volume might occur,
dependent on the initial state of the gain control circuit.
In addition, the prior art gain control circuits have worked
of~ of the average level of the signal being processed on a
continuous basis. Thus, for transmissions, such as pauses
between words in a message, prior art gain control circuits
sense a very low siynal level thereby increasing gain and
amplifying background noise. Now when the next word is
processed the initial portion thereof is amplified by a
high gain factor thereby resulting in an annoying burst.
Summary of the Invention
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It is an object of this invention, therefore, to provide
in two way communication systems, improved level controlling
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circuitry which provides a precise means to correc-t for
transmission line lossesO
It is a further object of the invention to provide the
above described improved level controlling circuitry which
includes means to prevent system malfunctions due to pauses
in received transmissions.
Briefly, according to the invention, improved circuitry
is provided in a two way communi.cation system of the type
wherein fixst and seco:nd stations generate first and second
signals, A and B/ respectively. These signals are duplexed
on a transmission medium which couples between t:he stations.
The transmission medium exhibits losses such that the
amplitude of the signal B received at the first station is
significantly less than the stations own signal, A. The
improvement includes a differential amplifier which has
first and second inputs and an output. rrhe amplifier ampli~
fies the diference between signals appear.ing at its input
by a predetermined factor and produces this amplified dif-
ference signal at its output. First coupling circui.try couples
the transmission medium duplex signal, i.e. A~B, to the first
differential amplifier input. A second coupling circuit
couples the first station generated signal, A, to the second
differential amplifier input. Thus, the output for the di-f-
ferential amplifier is the amplified signal B. By proper
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choice of the gain of the diffential amplifier, the signal B
can be restored to its original level as transmitted from
the second station.
Unique automatic gain control clrcuitry processes the
output from the differential amplifier. The gain control
circuitry includes a controlled attenuator which has input,
output and control terminals. The attenuator receives
signals at its input terminal, attenuates these signals by a
predetermined factor dependent upon a control signal applied
at it~ control terminal and produces the attenuated slgnal
at its output tenminal. A control signal generator is
coupled to the output terminal of the control attenuator and
produces a predetermined control signal in response to the
signal levels at this output. A sample and hold circuit
couples between the control signal generator and the control
terminal of the control attenuator. The sample and hold
circuit is operable in a first mode to Gouple~the produced
control signal to the control terminal and is operable in a
~econd mode to store the instantaneous value of the control
signal and apply this stored signal to ~he control terminal.
Activity checker circuitry detects the presence of a received
~ignal at the attenuator input terminal and, in response
thereto, activates the sample and hold circuit to its first
mode, thereby allowing normal automatic gain control action.
If the activity checker indicates that a signal is not being
received, such a~ might o~ur during speech pauses, the
sample and hold circuit is activated to its second mode.
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More particularly, there i5 provided~
~ n automatic ~ain control circuit compri~ing con~
trolled attenu~tor means having an input, output and control
termi~als, said attenuator receivin~ ~ignals at itq input ~er-
minal, attenuating said ~ignals by a predetennined factor de-
pendent upon a control signal at said control terminal and pro-
ducing said attenuated ~ignal at ~aid output terminal; control
signal generating means coupled to ~aid output terminal for pro-
d~cing a prede~ermined control signal in response ~o the sign~l
lev~ls thereat; sample and hold means, coupled between said
con~rol ~ignal generating means and the control terminal o said
controlled attenuator, and being operable in a ~ir t mode to
couple said produced control ~i~nal to said control terminal
and ~eing operable in a second mode to store the in~tantaneous
value of said control signal and couple said stored signal to
said control termunal; and zero crossing dete~tor means for de-
tecting the number of zero crossings made by a received signal
at said attenuator input terminal and activating said sample and
hold means to said ~irst mode upon detecting anumber of zero
crossings corresponding to an information signal and activating
said sample and hold means to ~aid second mode upon detecting a
numb~r o~ zero crossings ~orxespo~ding to a nois~ signal.
Brl-~ _ o be Drawi~s
Fig. 1 illustxates the principle components of a two-
way dupl~xed communication system; and
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Fig. 2 illustrates the preferred embodiments of the
circuitry for controlling the signal levels of signals over
the duplexed system shown in Fig. 1.
De-tailed Description of the Preferred Embodiments
of the Invention
Fig. 1 illustrates a two-way communication system
comprised of a base station 10 and a remote station 12
connected by a transmission medium 14. In this, the
preferred embodiment of the invention, the transmiss:Lon
medium 14 is comprised of a balanced telephone line. Each
station 10, 12 has a corresponding headset 20, 30 containing
sending por~ions 20a, 30a, and listening portions 20b, 30b,
respectively, which an operator uses to both transmit and
receive audio signals. Thus r designating the signal
originating from the base station 10 as signal A, and the
signal from remote station 12 as signal B, the signal A+B
is duplexed on transmission line 14. Each signal is sent
on the line at a reference level o zero dBm. Due to line
14 losses, the signal received by each station is signifi-
cantly attenuated. Thus, ~or a 20 mile length of telephoneline, the signal level of B appearing at the base station
10 is likely to be a -20dBm. This results in two principal
problems. Firstly, the signal heard by the operator at the
base station in the headset 20 is very weak for the remote
; 25 station yet very strong for its own signal level, resulting
in signal masking. Secondly, as Fig. 1 illustrates, in
many applications the signal either from remote station 12
to base station 10 may be routed to a transmi-tter 26 for
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-transmission over an antenna system ~ to, possibly, a mobile
receiverO Thus, it is important that the signal levels A
and B be of approximately the same magnitude such that
transmission levels are optimum.
Interfacing signal circuitry 22 couples the transmission
line 14 to the headset 20 and, through the automatic gain
control circuit 24, to the transmitter 26. The gain control
circuitry 24, the preferred embodiment o~ which is described
more fully hereinbelow with respect to FigO 2, operates to
maintain that two signals, A and B, at a relatively constant
level, outputting these signals to -the headset 20 and the
transmitter 26. Transmitter 26 may be keyed by a push-to-
talk ~witch 29 -~hich, pre~erably, is mounted to the heaclset 20.
Coupled to the remote station 12 is a page command
circuit 32. In many applications the remote station 12 may
wish to address the transmitter 26 and send a message over
antenna 28 to a selected one or ones o~ mobile stations.
This selection includes sending a paging tone or signal which
indicates to those specified mobiles that a transmission is
in progress. The conventional means or carrying the page
command over telephone lines, such as line ].4, is by
application o~ the tones with a DC current to the lines
~hich, via the interfacing switching circuitry 22 in the
~ase station 10, maintains the proper paging tone levels.
Thus, the system o~ Fig. 1 illustrates a two-way
duplexed communication system allowing intercom operation
between a .remote station 12 and base station 10 and RF
transmission capabilities both by base station 10 and by
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remote control rom remote station 12.
Fig. 2 illustrates, in block diagram form, the preferred
embodiment of the level controlling circuitry according to
the invention used in the base station ].0 of Fig. 1. ~ere,
the balanced, d.uplexed line 14 is coupled ~o a winding of
an interstage transformer 50. Applied to another winding of
interstage transformer 50 is the output of line driver
amplifier 52 which amplifies the signals A produced by the
headset 20 of the base station. Thus, the signal A
originating from headset 20 is amplified in line amplifier
52 and inductively coupled through interstage coupliny
transormer 50 to the balanced line 14. A tertiary winding
54 electro-magnetically couples a portion of the signal
A~B to the noninverting input 60a of a differential amplifier
60. Coupled to the inverting input 60b of the differential
amplifier 60 is the signal A as supplied by the headset 20
and routed through a switch 62. Switch 62 is activated hy a
push-to-talk switch 29 on headset 20 to couple the signal A
to the inverting input 60b of the differential amplifier 60
only.when khe system is in the .intercom mode, the switch
being open during radio transmission.
Differential amplifier 60 is of conventional design and
amplifies the difference between signals appearing at its
input 60a, 60b, by a predetermined factor, producing these
amplified signals at its output. Here, with switch 62
closed, the output from the differential amplif.ier 60 is
equal to [(A-~B)~A~ x the gain factor of the amplifier. Thus,
the output from the differential amplifier 60 is signal B
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at an amplified level. For the above example wherein the loss
in signal B due to the 20 mile telephone line i5 20db, the
differential gain of amplifier 60 can be designed to be 20db
or greater whereby the output from the d.i~ferential amplifier
60 contains a level of signal B at e~ual to or greater than
the level of signal A. Thus, the use of the tertiary winding
54 and the differen-tial amplifier 60 substantially restores
the signals A and B to their desired levels.
The output from the differential amplifier 60 feeds
to unique automatic gain control circuitry, indicated generally
at 70. Gain control circuitry 70 includes an input attenuator
80 which has an input terminal 80a, an output terminal 80b
and a control terminal 80c. Acting in the conventioIIal
manner, the attenua~or 80 responds to control signals received
at its control terminal 80c to vary the attenuation of signals
received at input terminal 8Oa, producing these attenuat.ed
signals at its output terminal 80b. Following the attenuator
80 is a fixed g.ain amplifier 84, the output from which feeds
to an integrator circuit 86. Integrator 86 generates a DC
voltage which is representative of the average level of AC
levels appearing at the output of amplifier 8~. These DC
control signals are ~ed to the input 90a of sample and hold
circuit 90. Sample and hold circuit 90 has been provided
with an output 90b which couples to the control input 80c of
the cont~ol attenuator 80 and a mode control input 90c.
When the input signal appearing at mode control 90c activates
the sample and hold signal 90 to its first mode, the DC
control signals generated by the integrator 86 are continu-
ously fed to the sample and hold output 90b whereby they are
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used to control the attenuator 80 in the conventional manner
of prior art at~enuators. Thus~ for increasing or decreasing
signal levels appearing at the output o~ amplifier 84, a
corresponding change occurs in the DC control signal 86 there-
by altering the attenuation of attenuator 80 to maintain thesignals at the output of amplifier 84 at a desired output
level. However, when the sample and hold circuit 90 is
activated to its second ~ode the overall feedback loop from
integrator 86 to the control input of 80c of attenuator ~0
is open. Now that instantaneous value o~ control signal
from integrator 8~ which occurred prior to the transition of
the sample ancl hold circui.t 90 from its first to its second
mode is stored and is cont:inuousl.y applied to the control
input 80c of the attenuator 80. As is described more fully
hereinbelow, this operation of sample and hold circuit 90
preven~s noise bursts or dropouts which would otherwise
exist in the gain control system.
Also coupled to the di~ferential amplifier 60 is an
activity checker 100. The first stage of activity checker
100 is a zero crossing detector, or limiter 10~. AC signals
applied to the input of the zero crossing detector 102
result in sharp transition s~uare waves produced at the zero
detector output. The negative transitions o~ the output
from the zero crossing detector trigger a ~ollowing mono-
stable multivibrator stage 104. This stage produces a 100
microsecond output pulse corresponding to each input negative
zero crossing transition.
The pulse output ~rom the monostable multivibrator 104
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is integrated in an integrator 106. Thus, the integrator
106 produces at its output a DC level representative of the
frequenc~ pulses generated by the monostable multivîbrator
104.
The DC output from integrator 106 is applied to the
first input lQ8a of a voltage comparator 108. Applied to
the second 108b of voltage comparator 108 is a reference DC
voltage Vref. If the DC output from integrator 106 is below
the reference level, Vref, the output from the comparator,
which couples to the mode input control 90c of sample and
hold circuit 90, activates sample and hold circuit 90 to its
fixst mode. If, however, the DC voltage ~rom integrator 106
is greater than Vref, sample and hold c.ircuit 90 is activated
to its second modeO
Operation of the activity detector is based on the fact
that the primary signals from a received transmission such
as voice, have fundamental frequencies o approximately 500Hz.
Signals ~rom extraneous sources, such as noise, have consid-
erably higher fundamental frequencies. Thus, since the
monostable 104 produces a pulse for every zero crossing, the
number of pulses produced by monostable 104 and, thus, the
DC output of integra-tor 106, will be high for these extraneous
signals~ Therefore, the value of Vre~ is selected such that
the sample and hold circuit 90 operates in its firs~, con-
tinuous gain control mode, only for signals detected as beingdesired information signals. Otherwise, the sample and hold
circuit 90 is activated to its second mode, indicative of the
absence of a received signal.
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By detecting activity on an input l:ine, and discriminating
activity from noise or extraneous signals, the instant auto-
matic gain control circuit provides a significant advantage
over gain control circuits known in the prior ar~. As
mentioned hereinabove, in prior art circuits the yain control
might reduce a-ttenuation, and thus increase overall gain,
during pauses between spoken words. This would result in a
noise burst once the next word were received and increase
background noise duriny pauses. The instant system eliminates
such noise bursts, due to the fact that the activity checker
100 operates the sample and hold circuit to its second mode
during such pauses, thereb~ maintaining gain at khe desired
level.
~lso coupled to the balanced telephone line 14 is a
page detector llOo As mentioned with respe¢t to Fig. 1/ if
a remote station desires to send a page signal it generates
a DC signal on the telephone lines 14. Page detector 110
senses the presence of this DC signal and upon its reception
activates a monostable multivibrator 112. Monostable multi
20 vibrator 112 activates a switching attenuator 11~, which
couples to the output of switch gain 84, to de-emphasize
resulting signals to the system. In such systems, transmission
of voice information is pre-emphasized to improve signal to
noise per~ormance, whereas it is desirable to send the
paging tones at a constant amplitude. Thus, for the duration
of broadcast page tones, the swi~ching attenuator provides
de-emphasis to the control tone signals. ~hen the switching
attenuator 114 is not activated by monostable 112, corres-
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ponding to the absence of paging tones, the attenuator 114
passes the audio signals without any frequency shaping.
Thus, the switching attenuator 114 comprises the output
of the AGC, as shown in block 24 in Fig. 1, which is there-
after fed to subsequent sta~es including the transmitter,and the fedback audio signal to the headset~
Referring again to switch 62, when the push-to-talk
switch 29 is activated by the operator, indicating a desire
for radio ~requency transmission, switch 62 open circui~s,
whereb~ the onl~ input to the differential amplifier 60 is at
its first input 60a. The reason for including switch 62 is
that, due to phase shits ln the coupling transformer 50~
the signal A appearing at the first input 60a of the differ-
ential ampliier 60 is slightly phase shifted from that
appearing at the second input 60h. Thus, the output from
the differential amplifier may contain suppressed harmonics
of the signal A. Therefore, switch 62 open circuits the
input line to differential ampliier input 60b thereby
maintaining a high ~uality level of signal A.
In summar~, improved level controlling circuitry has
been described for use in a two-way, duplex communication
system. While preferred embodiments of the invention have
been described in detail~ it should be apparent that many
modifications and variations thereto are possible, all of
which fall within the true spixit and scope of the invention.
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For example, whereas the preferred embodiment of the
invention illustrates a system employinq a base station
coupled with a single remote, any number of remotes could
be coupled in the system.
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