CA1110704A - Level control circuitry for two way communication system - Google Patents

Abstract

LEVEL CONTROL CIRCUITRY FOR
TWO WAY COMMUNICATION SYSTEM

ABSTRACT OF THE DISCLOSURE

Signals carried over a multiplexed transmission medium between first and second stations suffer transmission losses such that the signal one station generates has a substantially greater magnitude than the signal it receives from the other station. A differential amplifier is utilized to extract and amplify the weaker signal thereby establishing signal balance. Following the differential amplifier is an automatic gain control stage which includes novel sample and hold and activity checker circuitry. The activity checker responds to the frequency of input signals to either allow variable gain control action or, when the frequency is representative of noise, to hold the gain control at its instantaneous value prior to the onset of noise.

Description

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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~2--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 _ It is an object of this invention, therefore, to provide in two way communication systems, improved level controlling `~

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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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-4a-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 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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-l:L-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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Claims (3)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE PROPERTY
OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. An automatic gain control circuit comprising con-trolled attenuator means having an input, output and control terminals, said attenuator receiving signals at its input ter-minal, attenuating said signals by a predetermined factor de-pendent upon a control signal at said control terminal and pro-ducing said attenuated signal at said output terminal; control signal generating means coupled to said output terminal for pro-ducing a predetermined control signal in response to the signal levels thereat; sample and hold means, coupled between said control signal generating means and the control terminal of said controlled attenuator, and being operable in a first mode to couple said produced control signal to said control terminal and being operable in a second mode to store the instantaneous value of said control signal and couple said stored signal to said control terminal; and zero crossing detector 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 first mode upon detecting a number of zero crossings corresponding to an information signal and activating said sample and hold means to said second mode upon detecting a number of zero crossings corresponding to a noise signal.

2. The automatic gain control circuit of claim 1 wherein the zero crossing detector means comprises: zero transi-tion detector means for determining the number of zero transi-tions of the signal at the attenuator input terminal, and com-parator means for producing an output signal to activate said sample and hold means to its first mode if and only if the number of zero transitions is less than a predetermined minimum.

3. The automatic gain control circuit of claim 2, wherein: said zero transition detector means includes means for producing a pulse of predetermined time duration in response to each zero crossing of the signal at the attenuator input terminal and integrator means for integrating said pulses to produce a DC signal having magnitude representative of the average number of said pulses occurring over a time interval, and wherein said comparator means comprises a voltage comparator having the input from said integrator means at one input and a predetermined level DC signal at its other input such that for integrator produced DC signals representative of the zero transi-tions at the attenuator input being below said predetermined level said comparator activates said sample and hold means to its first mode, said integrator produced signal otherwise acti-vating said sample and hold means to its second mode.