US2212960A - Signal controlled switching circuits in telephone systems - Google Patents

Description

2,212,960 SIGNAIJ CONTROLLED swITcHING CIRCUITS IN TELEPHON SYSTEMS m w v N AQ L.. \\\Y, Aw 1 0 I p l Nmqlmw im I -I V.

Aug. 27, 1940. sci-[OTT Filed July 12, 1939 ril' VVE/V705 By' L. SCHOTT @6M-X Patented Aug. 27, 1940 UN'E'EB STATES PATENT ortica SIGNAL CONTROLLED SWITCHING CIR- CUITS IN TELEPHONE SYSTEMS Application July 12, 1939, Serial N0. 283,926

l0 Claims.

The invention relates to signal wave transmission systems and particularly to signal control switching circuits employed for controlling trans.- mission in telephone systems.

The invention will be specically described as applied to the voice-operated switching circuits, commonly called Vodas (voice-operated device anti-singing) circuits, employed for preventing singing and suppressing echoes in two-way telephone systems, but is adapted as well for use with other types of voice-operated switching circuits. As is well known, such systems are subject to variable interfering waves of different types as regards duration and frequency of occurrence, such as room or line noise and static, having a frequency spectrum similar to that of the voice signals but which are of a steady nature compared to such signals. This noise is often of such amplitude as to tend to produce false operation of the voice-operated switching circuits at the sensitivity required for their proper operation on voice signals.

The Vodas switching circuits at the terminals of a radio terminal system are intended to be operated properly only by the speech waves which pass through the terminal. To that end, the sensitivities of these circuits must be so adjusted as to make their proper actions possible. The receiving Vodas circuits are usually adjusted as sensitive as possible for the amount of noise or static present in the receiving signal path, whereas the transmitting Vodas circuits have their sensitivities adjusted in relation to the level of the speech current in the transmitting signal path. In the case of the former circuit, the sensitivity may, but not necessarily so, remain iixed for appreciable periods of time with predictable adjustments, whereas in the case of the latter circuit the sensitivity may vary widely for successive talkers over about a 40-decibel range.

Particularly, when an automatic device like the Vogad (volume-operated gain adjusting device) is used in the transmitting path to provide. in addition to its usual function of delivering substantially constant signal volume to the radio transmitter, that oi adjusting the sensitivity of the transmitting Vodas circuit to compensate for the varying signal input with strong and weak talkers, diiiiculties may be encountered from abnormal noise operations of the transmitting Vodas circuit, which may impose severe penalties on -circuit performance.

It is an object of this invention to eliminate to a large extent the noise operation difficulties of voice-operated switching circuits in such systems,

observed as detrimental under service conditions.

A more specific object is to protect the switching relays of a relay type, voice-operated switching circuit employed for preventing singing and suppressing echoes in a two-Way telephone system, against excessive false operation by noise of the diierent types encountered in such a system.

These objects are attained in accordance with the inventionA by a circuit arrangement comprisinga plurality of control circuits controlled fromv diiTerent parts of the voice-operated switching circuit, or from that circuit and the Vogad in combination, having such delays in operation as to be respectively responsive to abnormal false operation of the voice-operated switching circuit by noise of one of the different types, to adjust the sensitivity of the switching circuit sufficiently to cause its release.

The invention will be better understood from the following detailed description thereof when read in conjunction with. the accompanying drawing, the single iigure of which shows 'schematically, and in part diagrammatically, one terminal of a radio telephone system embodying one form of the invention.

First, the diierent types of noise difficulties whichl experience has indicated are most likely to be encountered under service conditions at the terminal of a radio telephone system will .be described briefly. These include:

(1) Roomnoise, the most troublesome cases of which are generally caused by the traiiic operator leavingher talking set connected to the terminal during idle circuit periods. This noise, generallyv below the normal talkingy level, may occasionally become high enough to operate the gain increaser ofthe Vogad and cause the gain in that device to rise, inV turn causing the vnoise'at the transmitting Vodas input to become high enough to operate the transmitting switching relays. In many cases, this noise has caused solid o-peration of the relays which thus blocks ofi completely any received transmission until the transmitting relays are released.

(2) Intermittent noises, whichmay or may not affect the Vogad but whose frequency of occurrence is such as to effectively. bridge the normal releases of the transmitting Vodas'relays, particularly the nal relay in the chain operating to disable the receiving voice transmission path, which is the slowest to release, causing excessive operation times for these relays.

(3) Steady noise of a type suchvas would hold 55 the transmitting master relay operated for an excessive period.

The Vodas and the Vogad operate `on these types of noise in a characteristic manner generally quite distinguishable from the manner in which they operate on pure speech current. The circuits of the invention to be described in connection with the drawing make use of the characteristic differences in operation to effect automatic release from the noise troubles when they occur.

The drawing shows schematically one terminal of a two-way radio telephone system comprising a four-wire circuit connecting a telephone line to a radio transmitter and a radio receiver which may be of any well-known type or construction. The four-wire circuit comprises a transmitting circuit TC leading to the radio transmitter i and a receiving circuit RC leading from the radio receiver 2. The input oi the transmitting circuit TC and the output of the receiving circuit RC are connected in wellknown manner by the hybrid coil H1 and associated balancing network N1 in substantially conjugate relation with each other and in energy transmitting relation with the two-wire line section 3 which may be connected with a two-way telephone line.

The transmitting circuit TC includes, in order, connected between the hybrid coil H1 and the radio transmitter l, a vario-repeater 5 which, with the associated control circuits to be described later, constitutes an automatic volume control device known as a Vogad, a delay circuit 6 and one or more ampliers. The receiving circuit RC includes, in order, connected between the radio receiver 2 and the hybrid coil H1 the variable loss pad l, a delay circuit` and a volume control device 9. The transmitting circuit TC is normally disabled at a point between the radio transmitter l and the delay circuit 5 by a short-circuiting connection Ii), whereas the receiving circuit RC is normally operative due to a normal low loss condition of the suppressor loss pad 7.

Connected across the circuit TC at a point between the vario-repeater 5 of the Vogad and the delay circuit 6, is the input of the transmitting Vodas circuit TV comprising an adjustable resistance loss pad li for obtaining desired initial adjustment of the sensitivity of the transmitting Vodas, a wave-operated switching device i2 which may be a voltage-operated amplier-detector circuit such as is well known in the art, and a plurality of mechanical switching relays including the master Vodas relay i3, connected to the output of the control device l2, so as to be simultaneously operatively energized in response to operation of the latter device by outgoing speech signals in the transmitting circuit TC, providing their energizing circuit has not been previously disabled by the receiving Vodas circuit RV in response to prior signals in the receiving circuit RC received from the distant radio terminal in the manner described below. The master Vodas relay I3, when operated, closes an energizing circuit for relay 3'! and the switching relays lli and I5 so that these relays are operated. Relay lil, when operated, opens its normally closed contacts to break the short-circuiting connection l() across the circuit TC so as to render that circuit operative to transmit the speech signals which meanwhile have been delayed in delay circuit 5. Relay l5 when operated opens its normally closed contacts to put the loss pad 7 in the receiving circuit RC in a high loss condition, so as eiectively to disable that circuit to prevent singing and suppress echoes.

Relays M, 3l and i5 have progressively lower bias winding effects for the purpose of their respective slow release actions which are otherwise controlled by the charging of condenser 53. After relay i5 releases, relay lll releases in about .l2 second, relay 3l in .15 second and relay i5 in .17 second. Relay 3l! releases and quickly stops the charging of condenser 53, thus insuring that the slower release of relay i5 is soon thereafter eiected.

The suppressor loss pad l may be of any type adapted to be changed from a low loss condition to a high loss condition by operation of a relay. The particular loss pad illustrated operating in this manner is similar to the one disclosed and claimed in the Silent Patent 1,749,851 issued March 11, 1930.

The receiving Vodas circuit RV, the input of which is connected across the receiving circuit RC at a point between the loss pad 'l and the delay circuit 8, comprises the adjustable resistance loss pad it for obtaining a desired initial sensitivity adjustment of the receiving Vodascircuit, the wave-operated control device il' which may be a voltage-operated amplier-detector circuit like the transmitting device i2, and a chain of switching relays including a master relay i8 which is operated in response to operation of the control device Il by applied voice waves, and a relay i5 operatively energized by operation of the master relay i3 and operating to break the energizing circuit for the relays including the master Vodas relay i3, controlled from the output of the transmitting control device i2 in the transmitting Vodas circuit TV.

The Vogad in the transmitting circuit TC may be of any oi the well-known types. The particular Vogad illustrated is essentially the same as disclosed in Mitchell et al. Patent 2,019,577 issued November 5, 1935. It includes the vario-repeater 5 comprising two three-electrode amplifying vacuum tubes i connected in push-pull in the transmitting circuit TC, having a gain regulating condenser Zii common to the control grid-cathode circuits of the two amplifying tubes, and asssociated control circuits comprising the gain decreaser circuit 2i, the gain increaser circuit 22 and the gain increase disabler circuit 23.

The gain decreaser circuit 2i includes the three-electrode vacuum tube detector 2li having its input connected across the transmitting cir-' cuit TC beyond the output of vario-repeater 5, y

and its output connected across the gain-regulating condenser 2u. This circuit is arranged to operate in response to an increase in the level of the waves in the output of vario-repeater 5 above a certain desired value to reduce the charge on the condenser 25, so as to cause the gain of the vario-repeater 5 to be decreased in proportion to the increase in the output level thereof over the desired value.

The gain increaser circuit 22, the input of which is coupled to the transmitting circuit TC at a point in front of vario-repeater 5 by the hybrid coil H2 and associated balancing network N2, includes the noise discriminating lter 25 in its input, the three-electrode vacuum tube detector s 26 having its input connected to the output of filter 25, relay 27 connected to the output of detector 25, so as to be operatively energized by operation of the latter, and the gain increase relay 28 adapted to be operatively energized by operation of relay 2l, relay 28 operating to connect the positive pole oi the grounded battery 29 to gain-regulating condenser 20 so as to increase its positive charge and thus raise the gain of vario-repeater 5. The gain increaser circuit 22 is arranged to operate in response to an increase in the level of the voice waves in the transmitting circuit TC in front of vario-repeater 5, above a certain minimum value to increase the gain of vario-repeater proportionately.

The gain increase disabler circuit 23, the input of which is connected across the transmitting circuit TC at the same point as the input of the gain decreaser circuit 2l, includes a three-electrode vacuum tube detector 3Q and the gain increase disabler relay 3l operatively responsive to operation oi detector 33 to disable the gain increaser circuit 22 by opening the energizing circuit for the gain increase relay 28. The gain increase disabler circuit 23 is arranged to operate in response to the waves in the ,transmitting circuit TC beyond the output of vario-repeater 5 when their level reaches the desired value so as to prevent further increase of the gain of vario-repeater 5 under control of the gain increaser circuit 22.

In addition, the Vogad includes circuits for preventing change in the gain of the vario-repeater 5 in the transmitting circuit TC by the associated gain control circuit when the receiving Vodas circuit RV has been operated in response to received signals or noise in the receiving circuit RC. These circuits include a relay 54 in circuit with the relay i@ operating simul. taneously therewith in response to operation of the receiving Vodas master relay i8, and the relays dfi and it arranged to be supplied with' energizing current from batteries d6 and 41, respectively, when relay 5 is operated. Relay 44 is designed to be quick-operating and slow-releasing so that it will be operated quickly in response to operation of the receiving Vodas master relay ldeither by speech signals or noise Waves, and will remain operated during the usual pauses in speech transmission. hand, is designed to be slow-operating but quickreleasing, so that it will not respond to operation of the receiving Vodas master relay I8 by momentary speech or noise impulses but will respond to operation of relay i8 in response to continuously received speech waves.

Relay lill, when operated, causes relay 48 to bel 2S of gain increaser 22 preventing its operation to increase the gain of vario-repeater 5 while the receiving Vodas circuit RV is locked up regardless of whether this is caused by received speech signals or noise.

Relay 45, when operated, causes a relay 50 to be operatively energized by current from battery 5l to short-circuit the input of the ygain decreaser circuit 25, disabling that circuit and removing battery El from the grid of the gain increase disabler tube 3l? thereby holding the gain increase disabler circuit 23 operated and effectively preventing any increase in the Vogad gain during the period in which the receiving Vodas circuit RV is operated by received speech Waves. y

The volume control device 9 in the receiving circuit RC may be a manually adjusted arrangement, or is preferably an automatic adjusting arrangement such as disclosed in British Patent Relay 45, on the other' No. 381,831, Which functions at all times byinserting suicient loss to maintain a safe margin wherein received energy in circuit RC which is so weak as to fail.y to operate control RV will also, at least, just fail to cause false operation of control TV when parts of this energy reach circuit TC' as echo energy. Y

The transmission devices and circuits in the radio telephone control terminal as shown in the drawing, as described above, are such as are commonly used in radio control terminals. The remaining portions of the circuit illustrated in the drawing constitute one embodiment of the noise protection circuit of the invention. As illustrated, this noise protection circuit includes three delayed operating relay control circuits A, B and C, shown within the dot-dash box labeled Noise protection control, controlled respectively from the transmitting Vodas master relay circuit including relay i3, the transmitting Vodas relay 3l which operates in a manner closely related to the operations of the switching relay circuit including relays i4 and l5, and in combination from relay 3l and the gain increaser circuit 22 of the Vogad in the transmitting circuit TC, each of which delayed operating relay circuits controls the operation of the quickoperate quick-release action relay 32, which, in turn, controls the transmitting Vogad gain.

The relay circuit A includes a master relay 33 in circuit with the master relay i3 of the transmitting Vodas circuit TV and operating simultaneously therewith, and a relay 34 adapted to be operatively energized from battery 35 When relay 33 is operated. The operation of relay 3d causes operating current to be supplied to the winding of the action relay 32 from battery 36. By suitable design of relay Sli to make it slow to operate in the required' degree, the relay control circuit A is made to cause operation of the action relay 32 in response to continuous operation of master relay 33, and therefore of the Vodas master relay i3, by the output current of the transmitting amplifier-detector I2 for a time interval of about 2 seconds after the transmitting Vodas master relay i3 operates.

The relay control circuit B includes relay 3l, which is in circuit with and a part of the transmitting Vodas switching system with relays lll and l5 and operates simultaneously with the latter relays when operating current is supplied from battery 38 in response tooperation of the master Vodas switching relay I3, and the relay 39 arranged to be supplied with operating current from the battery ill when relay 3l operates. Operation oi the relay 39 also causes energizing current to be supplied to the Winding of the action relay 32 from battery 36. By proper design oi the relay 39 to make it slow-operating to the required degree, the control circuit B is made to cause operation of the action relay 32 in response to continuous operation of relay 3l for a time interval of about l0 seconds.

The relay control circuit C includes relay 3l' and relay 4l which is supplied with energizing current from battery 'l2 through the back contacts of the gain increase relay El (when that relay is unoperated) and the front contacts of relay 3l when the latter relay operates under control of the transmitting Vodas master relay E3. The operation oi relay 4l also causes energizing current to be supplied to the winding oi action relay 32 from battery 3G. By proper design of the relay di to make it slow-operating to the required degree, the control circuit C is made to cause operation of the action relay 32 in response to continuous operation of relay 3l for a time interval of about 2 seconds provided the Vogad gain increaser relay 2 has not operated.

The relays 34, 39 and di are designed to be quick-releasing so that the quick-releasing action relay S2 will return quickly to the unoperated condition when the transmitting Vodas switching relays release.

The operation of the control circuit C will be prevented if the gain increaser circuit 22 has been previously operated by applied voice waves from the transmitting circuit TC, to open the back contacts of the gain increase relay 2l', thereby breaking the connection of battery i2 to the Winding of relay lll.

Operation of the action relay 32 in response to operation of any one of the control circuits A, B or C will cause the relay contacts to be closed to connect the discharge resistance i3 in shunt with the gain-regulating condenser 2li of the Vogad. Then, the condenser will be discharged through that resistance to reduce the gain of the vario-repeater 5 in an amount dependent on the time during which the action relay 32 remains operated, that is, until the amplitude level oi the waves in the output of the variorepeater 5 impressed on the transmitting Vodas circuit TV is suflciently reduced to cause the release of the master relays i3 .and 33. Preferably, the time constant of the circuit (discharge resistance' 43) controlled by the action relay $2 should be made such that the gain ci the Vogad is reduced several decibels more than is sumcient to release the Vodas relays in order to avoid any appreciable eiiects like hunting.

The manner in which the circuits of the invention as described above operate to provide protecticn against the various types of noise diiculties occurring in the operation of a radio terminal may be explained as follows:

Considering the room noise problem, it will be assumed, as is generally the case, that the transmitting Vogad gain is at some moderately low value as adjusted properly by the last speech currents transmitted. With the transmitter of the talking party left open during a lull in conversation, it will be assumed that a burst of room noise then occurs in suiiicient magnitude to operate the Vogad gain increaser meinentarily, whereupon the Vogad gain may increase some amount. From this, or from successive but generally infrequent bursts of room noise, the gain of the Vogad may become high enough to canse virtual lock-up of the transmitting Vodas relays E3, 3i, it and it on the lower background of room noise. When that occurs and persists for as long as 2 seconds, the slow-operate (2 seconds) relay control circuit C will operate to cause operation of the action relay 32, provided the gain increase relay 2l has not in the meantime been operated to disable that circuit.

The gain increaser in any Vogad should have a sensitivity so that it will operate at least on the strong peaks of any speech for which the Vogad is intended to adjust, and these peaks should ocur at least once every second during continuous now of speech current. It is prac tically unavoidable that the gain increaser will operate occasionally, but certainly not so regularly on random bursts of noise. On speech then, the control relay circuit C is effectively disabled by the gain increase relay il', whereas on the usual types or room noise this relay circuit is seldom operated again within 2 seconds so 'Vodas circuit as soon as possible.

that the control C is permitted to operate the action relay 32 and reduce the Vogad gain as it should. Thus, the undesirable room noise operation can be eectively and automatically detected and compensated for, in most cases Within 2 seconds of its occurrence.

Certain other types of noise, however, may not be eective in operating control circuit C. It has been observed that in ordinary conversation,

'the speech ows are so broken or grouped that the Vodas relays 3l, ld and i5 are practically never operated continuously for as long as 10 seconds. Thus, any type of noise, intermittent or steady, Which, in so far as the Vodas relays 3l, id and E5 are concerned, tends to bridge the gaps between the speech flows in an outgoing continuous train of speech, will operate control circuit B and thus operate the action relay 32 within atime interval of l seconds after the Vodas switching relays 3l, M and i5 become operated. In the absence of speech currents in the transmitting circuit TC, the relay control circuit B would be eiiective, of course, on any intermittent noise whose gaps between the successive operating peaks did not exceed .15 second in a continuous lll-second period, the time of .i5 second being the slow-release time of the transmitting Vodas relay 37, required in connection with the particular transatlantic radio telephone terminal to which the circuits of the invention were applied.

t has been further observed that in normal operation, the transmitted speech currents will not hold the transmitting Vodas master relay l3- operated solidly for a continuous period as long as 2 seconds. Thus, any steady noise tending to hold the master relay i3 operated continuously for 2 seconds would operate relay control circuit A and thus the action relay 32. This type of noise would also, of course, operate relay control circuit B if it persisted for seconds, but when it is so denitely and quickly detected it is desirable to make any necessary correction in the sensitivity oi the transmitting This type of noise would also be detected in 2 seconds by control circuit C if the noise at the input of the Vogad was low enough in magnitude so as to fail to operate the gain increaser 22.

When any oi these three noise protector circuits A, B or C causes operation of the action relay 32, a connection through its contacts connects the discharge resistance Il?. across the gain-regulating condenser C, which operates to reduce the charge on the gain-controlling condenser 2@ in the Vogad so as to make the necessary reduction in the Vogad gain. When the gain is reduced sufficiently, the Vodas relays I3, 3l, Hi and l5 will release, in turn causing release of the action relay 32 and stopping further unnecessary reduction of the Vogad gain. The time action of this gain decreasing arrangement can be adjusted by the choice of the value of the resistance l2 in the leakage path of the gain control condenser 29, so that in relation to the slowuelease time ci relay 3l (and the action relay 32), the gain reduction will be desirably carried several decibels beyond the just release point and thus provide a necessary margin of safety. This time action can be arranged, of course, so that following an action of the noise protection circuits, the Vogad is immediately available to make any desired gain change.

To obtain optimum protection against noise difculties, it has been found experimentally that important only because noises of the types neces-` sary to cause their operation occur rather infrequently.

It is to be understood that the values for the delayed operating times of the control circuits A, B and C speciiied above are given by way of example only, and that these operating times may diler radically from the values given, depending cn the constants of the particular Vodas circuits with which these circuits are employed.

The action relay 32, of course, may be ar,- ranged to operate on a variable gain amplier or Vogad located in the transmitting Vodas circuit TV to produce the required change in sensitivity to cause its releas-e from abnormal operation on the diiT-erent types oi noise, instead of on the Vogad used in the transmitting circuit TC, in a manner similar to that described for the latter device. Also, similar results can be obtained by causing the operation of the action relay to switch Xed losses of the proper values into the transmitting circuit TC or the transmitting Vogad circuit TV. This may be accomplished, for example, by providing resistance pads of the proper values in these circuits, which are normally short-circuited, and having the action relay 32,`when operated, remove the short circuits.

Other mcdications of the circuits illustrated and described which are within the spirit and scope cf theinvention will occur to persons skilled in the art. v

What is claimed is:

l. In combination with a circuit transmitting signal waves and subject to interfering noise waves of different types, and an associated waveoperated switching device operating differently on said signal waves and sai-d noise waves, means for minimizing false operation of said svi/'itching device by the applied noise waves, comprising a plurality of control circuits unresponsive to operation oi said device by the signal waves but respectively responsive to characteristic abnormal operation of said device by applied noise waves of a diierent one of said types, to adjust the sensitivity oi' said device to prevent further false operation oy the applied noise waves.

2. In combination with a wave-operated switching device supplied from a source of signal waves intermittent in character and subject to comparatively steady interfering noise waves of different types, means for minimizing false operation of said device by said noise waves comprising a plurality of control circuits respectively'responsive to continuous operation of said device for diierent abnormally long time intervals, each characteristic of operation by one of said different types of noise, and eachlonger lthan would occur in response to applied signal waves alone, and means responsive to operation of any one of said control circuits to adjust the sensitivity of said device to prevent its further false operation by the applied noise waves.

3. In combination, a line transmitting voice signal waves and subject to comparatively steady interfering noise waves of diierent typesas regards duration and frequency of occurrence, a voice-operated switching device connected to said line, a plurality of control circuits respectively operatively responsive to continuous operation of said device for an abnormally long time interval longer than would occur in response to voice signal waves alone, caused by `a different one of saiddifferent types of noise, and means vresponsive to operation of any one of said control circuits to reduce the sensitivity of said device sufficiently to prevent further false operation'by the applied noise waves.

4. In combination, a circuit transmitting voice signal waves and subject to comparatively steady noise waves of diierent types as regards duration and frequency of occurrence, a Wave-operated switching device connected to said circuit, and means to minimize false operation of said switching device by said noise waves comprising a plurality of control circuits having diiierent delays in operation, each operatively'responsive to continuous operation of said switching device for a dii-ferent interval of time longer than would occur in response to applied voice signal waves alone, respectively characteristic of operation by noise waves of a diiierent one of said types, and

means responsive to operation of any one of said control circuits to reducev the sensitivity of said switching 4device sufficiently to cause its release. Y

5. In a two-way telephone system comprising oppositely directed one-way transmission paths for the telephone signals transmitted in opposite directions, which are subject to comparatively steady .interfering noise waves of dierent types as regards duration and frequency of occurrence, and a voiceoperated switching circuit connected to one path, operating to control transmission in said paths so as to prevent singing and suppress echoes, means to minimize-false operation vof said device by said noise waves comprising a plurality of control circuits respectively operatively responsive to continuous operation of portions of said switching circuit for diierent intervals of time longer than would occur in response to appliedtelephonic signals alone, e4 spectively characteristic to operation by a different one of said types of noise waves, and means responsive to operatic-n of any one of said control circuits to reduce the sensitivity of said device suiciently to cause its release.

6. The system of claim 5 in which said voiceoperated switching device comprises an ampliiier-detector connected to one oi said one-way paths, a master relay circuit controlled by operation of said ampliiier-detector and a switching relay circuit responsive to operation'oi said master relay circuit to properly control transmission in said paths, .and said control circuits comprise relay chains having different delays in operation respectively determined by said different types of noise waves, the operations of which are respectively controlled from said master relay circuit and said switching relay circuit. Y

7. In a two-way telephone system comprising oppositely directed one-way vpaths for the telephonie signals transmitted in opposite directions, which are subject to comparatively steady interiering noise waves of different types as regards duration and frequency of occurrence, a voiceoperated switching circuit consisting of an ampliiier-detector connected to one of said paths so as to be supplied with the telephonic signals and noise waves therefrom, a master' relay circuit responsive to operation of said amplifier-detector, and a switching relay circuit responsive to operation of said master relay circuit to disable the cluding a gain increaser portion, in said one path for automatically regulating the sensitivity of said switching circuit in accordance with the level of the telephonie signals` in said one path, and means for minimizing false operation of said switching circuit by the applied noise waves comprising a plurality of control circuits having respective delays in operation determined by the characteristics of the diiilerent types of noise waves, one of these control circuits being con trolled from said master relay circuit and two others from said switching relay circuit, and means controlled by operation of said gain increaser of said Vogad for preventing operation of one of said two other control circuits.

8. The system of claim 7, in which the delay time of said control circuit controlled from said master relay circuit is of the order of 2 seconds, the delay time of said one of said two other control circuits controlled from said switching relay circuit is of the order of 2 seconds, and the delay time of the second of said two other control circuits is of the order of 10 seconds.

9. The system of claim '1, in which the delay in operation of said one control circuit controlled from said master relay circuit is determined by the duration of comparatively steady noise such as would hold said master relay operated for an excessive period, the delay in operation of one of said two other control circuits is determined by the duration ci' the usual room noise, and the -delay in operation of the other of said'two other control circuits controlled from said switching relay circuit is determined by the duration of intermittent noise whose frequency of occurrence is such as to eiectively bridge the normal releases of the relays in said switching relay circuit.

l0. In combination with a voice-operating system comprising a plurality of wave-responsive devices which separately or in combination respond differently as regards duration of continuous operating intervals to a given train of voice waves, means to indicate continuous operation of the respective devices for predetermined time intervals longer than predetermined longest sustained operating intervals in response to normal voice waves, respectively characteristic of operation by different types of noise waves, and means responsive to said indicating means to adjust said system to make it unobjectionably responsive to noise waves.

LIONEL SCHOTT.

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