US2761011A - Intercommunication system - Google Patents

Description

Aug. 28, 1956 HARDY INTERCOMMUNICATION SYSTEM 2 Sheets-Sheet 1 Filed Jan. 9, 1951 INVENTOR Rene Ila/1 By I g j ATTORNEYS United States Patent INTERCOMIVIUNICATION SYSTEM Ren Hardy, Paris, France, assignor to Sepco, Paris, France Application January 9, '1951,=Serial No.-205,052

4 Claims. (Cl. 179-1) As conducive to an understanding of the invention it is to be noted that where in order to afford communica-- tion between two positions, illustratively a local and remote station, a microphone and speaker are placed near each other at the local station and connected respectively through suitable circuits to a speaker and microphone at a remote station, which each of the respective circuits having amplifying means associated therewith, the amplitude of the sound emitted by each of the speakers may be raised to any suitable amount. However, as the sounds emitted by the speaker at one station will be picked up by the adjacent microphone and transmitted to' the speaker at the other station from which the sounds originated and as the sounds from such speaker will also be picked up by the microphone adjacent thereto, the original sounds will be built up as the cycle is repeated to the limit of the range of the amplifiers. The resultant oscillations cause a howl known as acoustic feed-back, which will drown out the conversation and render the conversation.

Where, in order to prevent such acoustic feed-back,

the microphone at each station is normally cut' OE and:

requires the pressing of a key for it to be used, the results are unsatisfactory in that it requires an aflirmative ac-,'

tion on the part of the user each time he speaks and if he should inadvertently fail to press the key when talking into the microphone, no sound would be transmitted.

Where systems are used which automatically out OK an amplifier associated with one of the microphonespeaker combinations when the microphone associated with the other amplifier is spoken into and such auto-' matic cut-cit is controlled by the voice of the person speaking, unless the normally de-energized amplifier can he energized substantially simultaneously with the spoken signal, a portion of such signal will be clipped rendering the speech unintelligible. In addition, where such last mentioned system is used, if upon energiza-' tion of one of said amplifiers and cut-off of the other amplifier, such condition should remain as long as the. first speaker is talking, there would be no possibility of the second speaker cutting in or interrupting the first speaker and consequently cross conversation, as is possible in conventional telephone communication, is prevented.

It is accordingly among the objects of the invention to provide an intercommunication system that will pick up sounds even at a considerable distance from the microphone at one station and amplify such sounds so that they may be heard even at a considerable distance from the speaker at the other station and whichwill function without acoustic feed-back that might render the sounds unintelligible, without the need for any manual. operation on the part of the listener or person speaking, which does not require the microphone and speaker at the respective stations to be spaced widely apart, which will permit normal conversation between the installation useless for normal ice parties so that either of. them may interrupt the other at any time. and will dependably reproduce the intelligence or speech without clipping or distortion thereof.

According to the invention these. objects are obtained by the use of a microphone at a local station connected to a speaker at a remote station and a speaker at a local station connected to a microphone at the remote station, with a speech amplifier associated with each of the microphone speaker combinations. Each of the speech. amplifiers is normally de-energized and has a control amplifier associated therewith, and the amplifiers are so connected that upon an audio signal input into one of the control amplifiers it will energize the associated speech amplifier and retain the other speech amplifier. de-energized, yet upon an audio signal input into the second control amplifier of greater amplitude than that intothe first control amplifier, the first speech amplifier will be cut off and the second speech amplifier energized.

More specifically, according to one aspect ofthe invention, the speech amplifier associated with each of the microphone-speaker combinations is rendered inoperati've'by the lack of sufiicient potential on one of the elements of at least-one of its vacuum tubes. The control amplifier associated therewith is designed to provide, when actuated by the signal into the associated microphone, a sufiiciently high potential on such vacuum tube element to cause the stage in which it is included to function, thereby rendering the speech amplifier operative. The control amplifier is also designed to apply a disabling potential to the output stage of the control amplifier associatedwith the speech amplifier controlling the other microphone-speaker combination so that said control amplifier will not deliver an energizing potential to said speech amplifier and hence the latter will remain inoperative. The control amplifiers are also so designed, that eventhough the output stage of one of said control amplifiers may have such disabling potential applied thereto, as above described, if the input signal to the output stage of said disabled control amplifier should be sufliciently high, such as would occur if the person talking into the microphone associated with said disabled amplifier should speak in a loud voice, the 'cut-oif' condition of such output stage would be overcome and such output stage would function to provide a voltage which would actuate the speech amplifier associated therewith and disable the output stage associated with the other control amplifier. As a result, either of the parties" speaking may take, over the conversation by' merelyspeaking in a voice. louder than the other party.

In a specific embodiment herein shown, the speech amplifiers associated with each of the microphone-speaker combinations. is rendered inoperative by having the screengrid of the input stage thereof normally'either' at zero potential or at such a low potential that the tube will not function properly. The screen grid is connected to the 1 plate ofzthe' output stage of the control amplifier associated with said speech amplifier, said output stage being normally energized so that its plate voltage by reason of the voltage drop across the associated plate load resistor will be at a relatively low value, not sutficient' for normal operation of the input tube of the speech amplifier. The output 'stage' of the control amplifier is controlled by suitable circuits which include an input or amplifier stage fed by the microphone associated with the speech amplifier. The output of such input stage is desirably rectified to produce a negative signal which, when impressed on the grid of the output tube of the control amplifier will cut the latter ofi; As a result of the sudden decrease inplate current, the plate potential will rise rapidly to a point suflicient to energize the input stage of the speech amplifier so t-h-atthe latter'will function properly. In order'simultaneously to retain the other speech amplifier inoperative,

the plate of the output tube of the first named control amplifier is connected to the control grid of the output tube of the control amplifier associated with said last named speech amplifier. Consequently, when the output tube of the first named control amplifier is cut off, a relatively high positive potential will be applied to the control grid of the output tube of the last named control amplifier so that the latter will continue to conduct thereby preventing any likelihood of sufficient plate potential being developed by such tube which would render operative the speech amplifier controlled thereby.

In the accompanying drawings in which are shown one or more of various possible embodiments of the several features of the invention,

Fig. 1 is a detailed circuit diagram of the communication system, and

Fig. 2 is a circuit diagram of a switching arrangement which may be utilized with the system shown in Fig. 1.

Referring now to Fig. l of the drawings for a detailed analysis of the circuit by means of which the principles of operation of the invention herein are performed, the system desirably includes a local station, preferably a master station having its own amplifiers. The local station has a microphone connected through a conven tional microphone transformer 11 and lead 12 to the control grid 13 of vacuum tube 14 in the input stage of the associated speech amplifier 15. Vacuum tube 14 which illustratively is a conventional pentode, has its plate 16 connected through coupling capacitor 17 to potentiometer 18 and thence through the movable arm 19 of such potentiometer to the control grid 22 of a vacuum tube 23 which also desirably is a pentode, connected in conventional manner. The plate 24 of tube 23 is connected by lead 25 to a speaker transformer 26, the output of which is connected by leads 27 to a speaker 28 at a remote station which illustratively is a secondary station having no amplifier units associated therewith.

Desirably, one of the elements of the input vacuum tube 14, illustratively the screen grid 32. thereof, is not supplied with sufficient potential and hence the input stage of such amplifier in which tube 14 is incorporated, will not function or will function with very low gain so that the speech amplifier will be inoperative.

Associated with the speech amplifier 15 is the control amplifier 33, which as shown in Fig. 1 has an input tube 34, illustrating a pentode connected in conventional manner, the control grid 35 of which is connected by lead 36 to the output of the microphone transformer 11. The tube 34, which functions as a conventional pentode amplifier tube, has its plate 37 connected by lead 38 through coupling capacitor 39 to the potentiometer 42, the movable arm 43 of which is connected by lead 44 to the control grid 45 of an amplifier tube 46, illustratively a triode connected in conventional manner. The plate 47 of tube 46 is connected through coupling capacitor 48 and lead 49 to the plate 52 of a diode detector tube 53, the cathode 54 of which is connected to the movable arm 55 of a potentiometer 56, the latter being connected between a source of potential 57 and ground. The plate 52 of diode 53 is also connected by lead 58 to resistors 59 and 62,. which are connected respectively by leads 63, 64 and lead 65 to opposite sides of a capacitor 66 and by lead 67 to ground. Lead 65 is also connected by lead 68 to the control grid 69 of the output tube 72 of the control amplifier 33, said tube 72 illustratively being a triode connected in conventional manner as to be normally conducting. The cathode 73 of tube 72 is connected to the movable arm 74 of a potentiometer 75 which is positioned between a source of potential 76 and ground. The plate '77 of tube 72 is connected through load resistor 78 to a source of potential 79 and also by lead 82 through resistor 83 and lead 84 to the screen grid 32 of tube 14 in speech amplifier 15, a capacitor 86 being associated with the resistor 83 in conventional manner to prevent the varying audio signals from affecting the positive screen grid potential,

As the speech amplifier 87 at the local station is identical to the speech amplifier 15, it is not shown in detail being diagrammatically represented by the block outline and has a microphone 99 at the secondary station asso ciated with the speaker 91 at the local station. Speech amplifier 87, in manner similar to speech amplifier 15, feeds the input stage of a control amplifier similar to control amplifier 33 and diagrammatically represented by the block 92 and, through circuits identical to those previously described in control amplifier 33 (and bearing the same reference numerals primed), feeds the control grid 93 of the output tube 94 of said control amplifier 92.

As is clearly shown in Fig. l, the plate 77 of 'tube 72 is connected by lead 95 through resistor 96 to the control grid 93 of tube 94 and in like manner the plate 97 of tube 94 is connected by lead 98 through resistor 99 to the control grid 69 of tube 72. The plate 97 of tube 94 is also connected through a plate load resistor 101 to a source of potential 102 and by lead 103, resistor 104 and lead 165 to the screen grid (not shown) of the vacuum tube in the input stage of speech amplifier 87.

In the operation of the circuit shown in Fig. 1, when the party at the local station talks into microphone 10 a signal will be impressed substantially simultaneously on the control grids 13 and 35 of vacuum tubes 14 and 34. As insufficient potential is normally applied to the screen grid 32 of vacuum tube 14, the speech amplifier 15 will remain substantially inoperative. However, as tube 34 is functioning in normal manner, the signal impressed upon control grid 35 will be amplified and such amplifier signal will be applied to the control grid 45 of tube 46, the amount of signal fed to grid 45 being determined by the adjustment of potentiometer 42.

Tube 46 will also amplify the signal and such signal will be detected and rectified by the diode 53, which will permit the positive portions of the signal to pass to ground and the negative portions of the signal, which will be filtered by the network 59, 62 and 66, will be impressed upon the control grid 69 of the normally conducting output tube 72. Tube 72 is normally biased at such a point that it will not be cut off by relatively weak signals, such as from outside noises which may be picked up by the microphone 16, and impressed on the grid 69 of tube 72 through the circuits of the control amplifier above described. The negative input to the grid 69 of tube 72, caused by the detection and amplification above described of the voice input, will cause the tube 72 to cut ofi. As a result, little or no plate current will fiow through the plate load resistor 78 and the voltage on the plate of tube 72 will be substantially at the applied potential 79. Such high potential will be impressed through the lead 82, resistor 33 and lead 84 to the screen grid 32 of tube 14 in speech amplifier 15. As a result tube 14 will function normally to amplify the signal originally impressed on its grid 13 and such amplified signal will be heard through the speaker 28 at the remote station.

As the voltage is applied to the screen grid 32 of tube 14 from the control amplifier 33 almost simultaneously with its input into the microphone 119, there will be no clipping of the signal and hence no distortion thereof, for the speech amplifier will be energized by the action of the associated control amplifier substantially simultaneously with the signal input thereto.

1 plate potential to be relatively low. As a result, little or no potential will be applied to the screen grid of the input amplifier tube in speech amplifier 87 and conse quently the? latter willremain inoperative. Consequently there is little or nolikelihood of acoustic feed-back" being developed by'the' sounds that would, in the absence of: the present. invention", be picked up by the remote microphone 90.

If the party at the remote station should desire to break into the conversation, he need merely talk into-his microphone 90 in a voice louder than the amplitude at which the first party is speaking. As a result, the input stage of the control amplifier'92 associated with the" secand speech. amplifier 87 will amplify such loud voice in. the manner previously described with respect-to control amplifier 33 and the signal: will be detected and rectified by the diode 53 so that a negative signal of relatively great amplitude will be: impressed on the control grid 93 of vacutun tube 94 of the control amplifier 921 This negative: signal will be: sufficient to overcome the positive bias on such tube. afiected both through potentiometer 75 and the connection from the platev 77 of vacuum tube 72 to the control grid 93-. As a= result, vacuum tube 94 will be cut off causing. the positive voltage on the plate 97 thereof to rise rapidly. and? as such positive voltage will be impressed" on the' control grid 69 of vacuum tube 72throughlead 98" and resistor 99, such positive voltage will tend to make vacuum tube 72 conduct heavily as it will overcome the negative rectified signal from diode 53; As aresult of the" voltage drop across: plate load resistor 78 of vacuum tube 72, the voltage on plate 77 of tube 72 will: drop.v Hence, the positive bias through lead-.95 and resistor 96 to control grid 93 of tube-94will be reduced, thereby rendering such tube less. conductive until it finally'cuts. off. At this time the voltage on: the plate of tube 94 will be very high and consequently the positive voltage on: the grid 69 of tube 72 will be. sufiicient so: that said tube. will conduct strongly.

It is apparent, therefore, that as the voltage. on the plate of tube 72 drops and that on the plate of tube. 94 rises the screen grid voltage of speech amplifier will be reduced and such speech amplifier will be rendered inoperative. Simultaneously the screen grid voltage of speech amplifier 87 will be increased and such speech amplifier will be rendered operative.

Once the second party has takenover and cut oti the first party, the second party may then speak in a normal voice for continued conversation. As the operations above: described occur very rapidly, it is. apparent that as far as the parties are concerned they will. be: maintaining a conversation that approximates almost: a face to face meeting and either may interrupt the; other. as desired by merely speaking in a louder voice.

Itis to be understood that the circuitsherein described could bev incorporated in a systemin which the local station is a master station as shown having amplifiers associated therewith and having a number of remote stations. connected to thelocal station, which remote stationsv may also be master stations havingtheir own amplifiers, or secondary stations without amplifiers.- In such a system it is desirable that the party at the local: station be able to call one or more of the remote stations with but a few simple manual operations before starting the conversation and thereafter hold a.conversation with the remote station or stations called. It is also desirable that either the secondary or master remote stations be able to call the local master station and that suchi'conversation also may be limited by the local master station to only itself and the station calling.

Although any suitable switching arrangement may be used for this purpose, it is preferred to use the arrange ment shown in Fig. 2. which desirably incorporates the circuits shown in Fig. 1. Thus the master local station has the speech amplifiers 15 and 87 andthecontrolamplifiers 33 and 92 associated therewith, the control amplifiers being diagrammatically represented. by the single block outline. Speech amplifier. 15 which has the microphone 10' associated-therewith is connected by leads 111 and 1 12 to contact 113 of a double pole double throw switch 114 of the push button type, the contact 115- associated with contact 113 being connected by lead 116 to-terminal117. The speech amplifier87 at the local master station which has the speaker 91 associated therewith, has its input connected. by lead to the contact 126 of a single'pole double throw switch 127, the associated contact 128 being connected by lead 129 to a source of power 131 and thence to ground; The movable swit'ch-arm13-3of switch 127, which. is normally retained against contact 126; is

connected by leads 134-and 135 to contact 136 of switch 114 at .the master station. The contact 137 associated with contact 1361 is connected by lead' 138 to a terminal 139; Contact137 of'switch 114 is also connected by lead 141 to contact 142 of switch 114, said contact 142 having anassociated contact 143 connected through resistor 144-, lamp 145 and lead 146'to buzzer 147 and then to ground.

Withthe' switching arrangement thus described'a remote master station may readily be connected to thelocal master stationiby means of transmission lines 151' and: 152 connecting the terminals 117, 117 and 139, 139'. As the remote master station shown on the right hand side of Fig. 2 is'identical to the'remote-master station shown on theleft hand side ofFig. 1' it Will not be described in detail, parts corresponding to the already described local master station having the same reference numerals primed.

In order that the local master station may be'connected toa remote'secondary'station and that such remote secondary station may callthe localmaster station, thecircuit shown in the lower'part of Fig. 2. is desirably provided.

Thus, a second double pole double throw switch 153 also desirably'of the pushbntton type is'provided at the local station; the contact 154 of said switch beingcom nected. by lead 155 through resistor 156, lamp 157 and lead 158 to the junction between lamp 145 and buzzer 147. The contact 159 associated with contact 154 is connected. by lead: 161 to contact 162 of switch 153 and also by lead 163 to terminal 164, the contact 165 associated with contact 162 being connected by leads 166- and 167 to the junction 168 between leads 134-and 135. The contact 169 of. switch 153-is connected by leads 1571 and 17.2 to the junction 173 between leads 111 and 112, and the contact 174 associated with contact'1'69 is connected by lead 175 to terminal 176.

The remote-secondary station shown in Fig. 2 desirably comprises the microphone 90showu in Fig. 1',.connected inconventional manner and having a lead 177 connected to contact178 of a single pole double throw switch 179; The associated contactt181 of said switch is connected by lead.182 to a buzzer 183 and thence to ground. The movable. arm 184 of. switch 179-which illustratively engages contact 181, is connected by lead 185 to contact 186 of asingle pole double throw switch 187. The movable arm 188 of switch. 187 which normally engages contact 186 is connected by lead 189 to terminal 164' of said secondary station. The other contact 192 of switch 187 is connected by lead 193 to terminal 194, and the speaker 28 of said secondary station, which is connected in conventional manner, is connected by lead 195' to terminal 176'.

In the installation of the equipment the remote sec ondary station is connected to the local master station by means of transmission lines 196 and 197 and 198' which connect the terminals 164, 164'; 176, 176 and1'94, 194 respectively, the terminal: 194 at the local station being connected by lead 199 to lead 129 as at 201.

Although-the local master station has been shown with provision for connection thereto of. but two remote stations, Le, a master station and a secondary station, it is of course to be understood that any suitable number of remote stations, either master or secondary could be con;

nected to the local master station by merely duplicating the switching ararngement shown.

- In the operation of the switching arrangement above described, if the party at the local master station desires to call the remote master station, he need merely press down on the push button of switch 114 so the movable contact arms 202 and 203 thereof will engage contacts 136,137 and 113, 115 respectively. The party at the master station may then press key 127 so that the switch arm 133 thereof engages contact 128. As a result a circuit will be completed from power supply 131 through lead 129, contact 128 and switch arm 133, leads 134 and 135, through bridged contacts 156 and 137, lead 138, terminal 139, transmission line 152, terminal 139, leads 138' and 141' through the normally bridged contacts 142 and 143' of switch 114 at the remote master station, through resistor 144 and lamp 145, lead 146', through buzzer 147' to ground. As a result, the lamp 145 at the remote station will be illuminated and the buzzer 147' will vibrate to notify the party at the remote station that he is being called. Such party may then press down on the push button of switch 114 which will cause the contact arm 202 thereof to break the circuit between contacts 142 and 143' and close the circuit across contacts 136 and 137 and also move the contact arm 203 of switch 114 across the contacts 113 and 115' to bridge the latter. As a result, the lamp 145 and buzzer 145 will be cut out of circuit and a circuit will be completed from the terminals 117 and 139' of the remote master station to the speaker and microphone 91' and 10' thereof respectively. The party at the remote master station may then speak into the microphone 10' and answer the call. The circuit for microphone 10 will be through lead 125', contact 126 of switch 127, contact arm 133' normally engaging said contact 126, leads 134' and 135', n

through the bridged contacts 136' and 137' of switch 114', lead 138, terminal 139', transmission line 152, terminal 139, lead 138, through the bridged contacts 136 and 137 of switch 114 at the local master station, leads 135 and 134, through movable contact arm 133 and contact 126 normally engaged thereby, lead 125, through speech amplifier 87 to speaker 91 at the local station. The party at the local master station may then reply by speaking into the microphone 10 which is connected to the speaker 91 at the remote master station through the speech amplifier 15, leads 111 and 112, bridged contacts 113 and 115, leads 116, terminal 117, transmission line 151, terminal 117, lead 116', bridged contacts 115 and 113 and leads 112 and 111.

As a result of such switching arrangement a conversation may be held between the parties at the local and remote station, and the amplifiers and 87 with the associated control amplifiers 33 and 92 will function in the manner previously described to permit the equivalent of face to face conversation. It is to be noted that with this switching arrangement as the microphone and speaker of the remote station are normally disconnected through open switch 114 the party at the master station would not be able to hear any sounds from the remote station unless so desired by the party at the remote station.

If the party at the remote master station desires to call the local master station he need merely press the push button associated with switch 114' and then press the key 127'. As a result, a circuit would be completed from the source of power 131' at the remote master station through lead 129', contact 128', contact arm 133, leads 134' and 135', bridged contacts 136' and 137 of switch 114, lead 138, terminal 139', transmission line 152, terminal 1'39, leads 138 and 141, normally bridged contacts 142 and 143, resistor 144, lamp 145 and lead 146, through buzzer 147 to ground. The party at the local master station hearing the buzzer, need merely press the push button associated with the illuminated lamp 145 to complete the circuit from his microphone 10 and speaker 91 to terminals 117 and 139. A conversation may then be held in the manner previously described.

If the party at the local master station desires to call the remote secondary station he need merely first press the push button associated with switch 153 to move the movable contact arms 211 and 212 thereof to bridge contacts 162, 165 and 169, 174 respectively. He then would press the key 127 to move the movable contact arm 133 thereof into engagement with contact 128. This would complete a circuit from power supply 131 through lead 129, contact 128, movable contact arm 133, leads 134, 167 and 166, bridged contacts 165 and 162, lead 163, terminal 164, transmission line 196, terminal 164, lead 189, movable contact arm 188 in the secondary station, contact 186 which it normally engages, lead 185, contact arm 184 illustratively engaging contact 181, through buzzer 183 to ground. The party at the remote secondary station then need merely move contact arm 184 to engage contact 178 and talk into microphone 91] which is connected to speaker 91 at the master station through speech amplifier 87.

The key 127 at the local master station ordinarily need not be used to call the secondary station, for upon pressing of the push button of switch 114, for example, at the local master station, a circuit will be completed to the speaker at the secondary station. However, in the event the party at the master station does not wish to call out into his microphone he can give warning that he is desiring to call by the use of such key.

It is to be noted that with the circuits above described the party at the local station cannot eavesdrop on the remote secondary stat-ion for the latter need merely have the movable contact 184 of switch 179 disconnected from contact 178 and engaging contact 181. Thus the microphone of the secondary station will be out of circuit and hence no sounds can be transmitted from the secondary station to the master station until contact arm 184 is moved.

If the party at the secondary station should desire to call the party at the local master station he need merely press the key 187 so that movable contact arm 188 engages contact 192. As a result a circuit will be completed from the power supply 131 at the local master station through lead 199, terminal 194, transmission line 198, terminal 194, lead 193, contact 192, contact arm 188, lead 189 to terminal 164', transmission line 196, terminal 164, leads 163 and 161, normally bridge contacts 159 and 154 of switch 153, resistor 156, lamp 157, lead 158 through buzzer 147 at the master station to ground.

The party at the master station on hearing said buzzer need merely press the push button associated with the illuminated lamp 157 to complete the circuits heretofore described from microphone 10 and speaker 91 at the master station to speaker 28 and microphone at the secondary station.

As many changes could be made in the above construction, and many apparently widely difierent embodiments of this invention could be made without departing from the scope of the claims, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

Having thus described my invention, what I claim as new and desire to secure by Letters Patent of the United States is:

l. A communication system comprising two speech amplifiers, each having an amplifier tube having a screen grid, two normally conducting control amplifiers connected respectively to said speech amplifiers, a microphone designed to receive an audio signal and connected to the input of each of said speech amplifiers and its associated control amplifier, a speaker connected to the output of said speech amplifiers, each of said control amplifiers having a normally conducting stage having a tube with a plate load resistor, means connecting the plates of said tubes to the screen grids of the associated speech amplifier respectively, means in each of said control amplifiers, actuated by the signal into the associated microphone to cut off the normally conducting tube, whereby upon drop of the potential across the associated load resistor, the voltage on the plate will rise and such potential will be supplied to the screen grid of the amplifier tube of the associated speech amplifier to render said tube and said speech amplifier operative, and means controlled by said rising plate potential of one of said control amplifiers to retain the speech amplifier associated with the other control amplifier inoperative, whereby upon a signal into one of said microphones of greater intensity than the signal into the other of said microphones, the speech amplifier associated with said first named microphone will be rendered operative and the speech amplifier associated with the second named microphone will be rendered inoperative.

2. The combination set forth in claim 1 in which each of said control amplifiers comprises a normally conducting input stage, a diode detector associated with the output of said input stage and connected to rectify the signal therefrom and pass the negative portion thereof, the output of said diode being connected to the control grid of the normally conducting stage of said control amplifier whereby said negative signal will cutoff said tube to cause the plate potential thereof to rise.

3. The combination set forth in claim 1 in which the means controlled by said rising plate potential to retain said other speech amplifier inoperative and the means to render the speech amplifier associated with said first named microphone operative comprises a connection from the plate of the normally conducting stage of one of said control amplifiers to the control grid of the normally conducting stage of the other of said control amplifiers and a connector from the plate of the normally conducting stage of said last named control amplifier to the control grid of the normally conducting stage of said first named control amplifier.

4. The combination set forth in claim 3 in which each of the connections between the plate of the normally conducting stage of said control amplifier and the screen grid of the associated speech amplifier includes a voltage dropping resistor.

References Cited in the file of this patent UNITED STATES PATENTS 2,205,142 Hoard June 18, 1940 2,319,680 Hasenberg May 18, 1943 2,323,307 Campbell July 6, 1943 2,347,080 Campbell Apr. 18, 1944 2,515,726 Montani July 18, 1950 2,536,820 Prahm Jan. 2, 1951 2,545,466 Jeanlin Mar. 20, 1951 2,545,476 Levy Mar. 20, 1951 2,623,949 Jeanlin Dec. 30, 1952

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