US2416321A - Automatic volume control in voice frequency circuits - Google Patents

Description

Feb. 25,- 1947. E. T. JONES 2,416,321

AUTOMATIC VOLUME CONTROL IN VOICE FREQUENCY CIRCUIT Filed Deo. 8,A 1943 3l/VZlE/TOR Y M ATTOR EY Patented Feb. 25, 1947 AUTOMATC VOLUME CONTROL IN VOICE FREQUENCY CIRCUITS Elwyn Tudor J ones, Eltham, London, England, as-

signor, by mesne assignments, to International Standard Electric Corporation, New York, N. Y., a corporation of Delaware Application December 8, 1943, Serial No.' 513,369 I n Great Britain April 9, 1943 8 Claims. (Cl. 179-84) This invention relates to circuits for the oper-y ation of a signal receiving device by signal currents of a predetermined frequency lying within the voice frequency range whilst preventing its operation by unwanted currents, also lying within this same range of frequencies, both types of currents being supplied to the device from a single communication channel.

There are various limiter circuits known in the art which limit by chopping the peaks of the currents under control. The present invention makes use of automatic volume control instead of a limiter circuit, and this former device provides several advantages. Unlike limiter circuits the automatic volume control may be adjusted to cause the peaks in unwanted speech currents to depress the average speech currents which may reach the signal receiving device to a low value for a considerable length of time and in addition the same circuit provides a substantially constant level of the desired signalling currents without distorting the wave form thereof. This results in more reliable operation over a wide range of signal input levels because the low harmonic content of such non-distorted signal currents reduces filter llosses and makes possible the design of simpler and more effective Lguard circuits,

which prevent operation of the receiving device, which may be a relay when certain frequencie are present on the communication level. i

According to the present invention a circuit of the kind set forth above comprises a variable transducer, a filter passing signal frequencies and excluding frequencies outside the signal range and a signal receiving device connected together in series and a circuit for automatically varying the amplitude of the currents passed by said transducer in inverseproportion to the amplitude of the currents at the input of said filter, this inverse proportion being modified by means acting to neutralize the amplitude control when the energy of the input currents falls below a predetermined reference level, this reference level being either constant, or being made to vary when the signal current of predetermined modulation frequency is present upon the communication channel.

The nature of the invention will be better understood from the following description taken in conjunction with the accompanying drawings in which:

Fig. 1 shows a circuit according to the invention using a variable attenuator;

Fig. 1Al shows a modication Fig. 1; and 'f Fig. 2 shows the circuit of Fig.. 1 with an additional amplitude control.

Referring to the drawings, Fig. 1 shows a circuit for the operation of a signal device X by currents in the'voice frequency range connected of the circuit of at the terminals l, 2 to a circuit over which both speech currents and signalling currents pass'.v

The circuit comprises a variable attenuator VA, an amplifier A and a lter F in series. The attenuator and amplifier together constitute the transducer. Two diagonal points of a rectifier bridge R are connected to the secondary of a transformer L3 which forms'part of the filter F, and the two opposite diagonal points are connected to the signal device X. This deviceA may be of any suitable type, as used in the telecommu- The filter comprises inductance's LI, L2, L3 shunted by c-ondensers C I, C2, C3. The

nication art.

loss in the attenuator VA is controlled by acurrent derived from the amplified currents and passing through a rectifier Wl. The automatic volume control is prevented from acting below a certain minimum amplitude of received vcurrents by an opposing voltage V, acting through a reoti Iier W2, a condenser C4 being inserted to isolate this opposing voltage from the amplifier A and filter F. A condenser C5 is used to maintainthe automatic volume control in action over a predetermined period of time, as is usual. y The filter F is designed to pass frequencies intended to operate the signal device X and the tuned circuits LI, CI and L2, C2 are preferably tuned to frequencies outside that range. These circuits thus cause the filter F to present a high impedance to frequencies outside the signalling range. This provides an effective and economic guard circuit, since the high impedance of the filter to frequencies outside the signalling range increases the relative voltage magnication at such frequencies at the lter input and thus causes these frequencies to exercise an accentuated effect in increasing the loss through'the attenuator. In addition or as an alternative to this arrangement of the impedance of lter F, the effect of unwanted frequencies in the automatic volume control circuit may be accentuated by placingin front of the attenuator an equalising network, EN, which, paradoxical thoughr it may appear, is arranged to cause greater loss to signal frequencies than to unwanted frequencies and in particular to pass with small loss unwanted frequencies close to the signal frequency and harmonics of the signal frequency, rangement is shown in Fig. 1A. The operation of the equallzing network is similar to that of the filter in that the network tends to provide un` Wanted signals at the output of the amplifier which are greater in amplitude than the desired signal. Since the automatic volume control circuit operates on the difference in signal levels, it may be adjusted to provide attenuation to the unwanted signals so that they are reduced to the same level'resulting with the nlter alone. When the filter and the equalizing networks are used together their effects are additive,

This ,ar-

q in the same manner Aas in Fig. 1.

' naldevice X.

3 Because the automatic volume control does not chopv the peaks of the signalling currents it is well suited for use with modulated (including interrupted) carrier signallingl currentsI since it provides volume control with-out loss of ercentage modulation` so that reliable functioning is possible over a wide lrange of input levels.

At the same time, the circuit may, as shown y age doubling network Ril and the resulting rectii'iedcurrent is applied to the control of the variable attenuator VA in opposition to the normal control circuit. ln this way all currents will be reduced to aflow value except a pro-perly modul lated signaler the right carrier frequency. Such asignal will produce a neutralising current and WilLrise to a high level and will operate the sig- When current derived from transformer T! is rectified, the resultant direct current voltage will vary asa function of the energy content of the modulation of the signal. Therefore the referrence level below which energy derived via 'C4 can function to effect control of the variable attenuator` will be raised during the persistence of such modulation currents, due to the increased value ofthe neutralizing current. This causes the attenuator to deliver maximum energy to the suc-` ceeding elements of the System, thereby operating signal device X so as to bring about the maximum effective sensitivity thereof. What is claimed is:

1. A circuit forthe operation of a Areceivingv device by signal currents within the voice frequency range whilst preventing its operation by unwanted currents comprisingv a variable trans` ducer, a filter passing signal frequencies and eX- cluding frequencies outside the signal range and a signal receiving device connected together in series and a circuit for automatically varying the amplitude of the currentsA passed by said transducer in inverse proportion to the amplitude of the currents at the input of said lter, above a predetermined minimum reference level.

2. A circuit as claimed in claim l comprising means for automatically maintaining the amplitude of currents passed by said transducer at a 'value determined by a peat: of received energy for a period oftime. Y

3. A'circuitas claimed in claim l in whichr said lter presents to said 'transducer such an impedance for frequencies outside the signal range as to accentuate the effect of the presence of said frequencies in reducing the amplitudeof currents passed by said transducer.

Ll. A circuit as claimed in claim l in whichsaid transducer is preceded by an equalising network causing greater loss at signal frequencies than l at unwanted frequencies.

5. A system for operating a receiving device by signal currents within a desiredY frequency range but'not by currents of frequencies outside i said range,said system comprising a transducer This frequency,

.Whose transmission efficiency may be varied, a

filter connected to receive signals from said trans` ducer and to pass only the desired frequency range, said receiving device being connected to receive signals from said filter, and an automatic `volume control circuit responsive to the amplitude of signals at the input of said filter for varying the transmission emciency of said transducer in inverse relation to the amplitude of said current, above a predetermined minimum reference level.

6. A system for operating `aireceiving device by signal currents within a desired frequency range but not by currents of frequencies outside said Y range, said system comprising a transducer having a variable transmission efficiency, a filter connected to receive energy from the output of said` transducer and to pass only the desired frequency range, said receiving devicebeing connectedA to receive signals from the output of said filter, and an automatic volume control circuit adapted to receive energy from a position between said transducer and said filter, said automatic volume control circuit including elements for developing a voltage responsive to the energy impressed on said volume control circuit and varying the transmission eiciency ofv said transducer in inverse relation thereto, above a predetermined minimum reference level.

"7. A system for operating a receiving device by signal currents within a desired frequency range but not by currents of frequencies outside said range, said system comprising a transducer having a variable transmission efficiency, a lter connected to receive energy from the output of said transducer and to pass only the desired frequency range, said receiving device being connected to receive signals dependent'on the output of said filter, -an automatic volume control circuit responsive to the amplitude of energy at a point between said transducerand Said lter, and a second volume control circuit responsive to energy at said receiving device, said second volume control circuit being' connected in opposition with the first mentioned volume contr-ol circuit, and a connection from said volume control circuits to the transducer for varying the transmission emciency of said transducer in inverse relation with the energy derived from said first volume control circuit, above a level determined bythe energy derived from said second volume control current. `8. A system according to claim 7 in which the second volume control circuit comprises circuit elements for separating from the signal at said receiving device, Currents having the frequency of signal modulation, rectifying said currents and ELVVYN TUDOR J REFERENCES CITED The following Yreferences are of record in the le of this patent:

UNITED STATES PATENTS Number Name vDate 2,334,468 Adams n v Nov. 16, 1943 2,084,135 u f Ford June 1 5, '1937-

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