US3679836A - Keyed stereophonic transmission system - Google Patents

Abstract

A stereophonic transmission system having two sources of audio signals in which the audio signal of an ultrasonic frequency is keyed in opposite phase for periods of substantially one-half cycle of said frequency to derive the respective trains of pulses amplitude-modulated by said audio signals and displaced in phase relative to each other by the width of the pulses. The trains of pulses are super-imposed and admitted simultaneously to the input of the transmission channel and subsequently separated into the pulse trains at the output of the transmission channel.

Description

United States Patent Svorec [45] July 25, 1972 s41 KEYED STEREOPHONIC 2,792,449 5/1957 30mm ..179/15 TRANSNIISSION SYSTEM 2,336,276 12/1943 Van Der Meulen ...1789/l5 BT 2,921,981 1/1960 Kidd ..l79/l5 B1 [721 Invent Rich 324 36th St, South 3,306,981 2/1967 l-lecht ..179/1s ST Bend, Ind. 46615 22 Filed; June 19 19 9 Primary Examiner-Kathleen H. Claffy Assistant Examiner-Tom D'Amico I 1 Appl- 834,748 Attorney-Marmaduke A. Hobbs and Kemon, Palmer,

Related U.S. Application Data Stewart Estabrook [63] Continuation of Ser. No. 388,979, Aug. 8, 1964, [57] ABSTRACT abandoned A stereophonic transmission system having two sources of 52] U S Cl 179/15 BT audio signals in which the audio signal of an ultrasonic [5]] Km Huh 5/00 frequency is keyed in opposite phase for periods of substam 58] Field Search 5 HT 1 325/36 15 BP tially one-half cycle of said frequency to derive the respective 325/15 1 5 trains of pulses amplitude-modulated by said audio signals and displaced in phase relative to each other by the width of the pulses. The trains of pulses are super-imposed and admitted [56] References Cited simultaneously to the input of the transmission channel and UNITED STATES PATENTS subsequently separated into the pulse trains at the output of the transmission channel. 3,018,335 1/1962 De Rosa ..l79/1G 2,007,809 7/1935 Nicolson 179/15 A 2 Claims, 10 Drawing Figures 7 jl/TFl/T KEYED STEREOPHONIC TRANSMISSION SYSTEM This application is a continuation of my copending application Ser. No. 388,979 filed August 8, 1964 now abandoned.

A main object of the invention is to provide a novel and improved apparatus for utilizing a single amplifier or transmission channel for handling two stereophonically related signals the apparatus involving simple components, being reliable in operation, and providing a substantial saving in cost as over previously used arrangements wherein separate amplifiers or transmission channels are employed for the respective stereophonically related signals.

A further object of the invention is to provide an improved means for transmitting a pair of stereophonically related signals through a single transmission channel, the means being inexpensive to fabricate, involving relatively simple and durable parts, and having very low power requirements.

A still further object of the invention is to provide an improved apparatus for utilizing a single amplifier or transmission channel for transmitting a pair of stereophonically related audio signals, the apparatus being adaptable for use in conjunction with a stereophonic phonograph pickup assembly, or similar stereophonic transducer, and being arranged so that the derived stereophonically related signals will be amplified without substantial distortion and then will be separately reproduced so as to reproduce the original stereophonic relationship of the input signals.

A still further object of the invention is to provide an improved apparatus for amplifying or transmitting a pair of stereophonically related signals, the apparatus requiring only a single amplifier, being arranged so that the amplifier will operate with the same fidelity to provide the required gain of both of the input signals, no balancing being necessary, and the apparatus being arranged so that the amplified signals are properly divided and reproduced in separate reproducing devices, such as loud speakers, or the like, in the same stereophonic relationship as the signals originally possessed when introduced into the system.

Further objects and advantages of the invention will become apparent from the following description and claims, and from the accompanying drawings, wherein:

FIG. 1 is a block diagram showing a typical apparatus according to the present invention.

FIG. 2 is a schematic wiring diagram showing the electrical components employed in the system illustrated in FIG. 1.

FIG. 3 is a block diagram showing a modified monaural to stereophonic amplifier conversion system according to the present invention, the elements shown being those required to convert a pair of stereophonically related input signals to a composite single-channel signal suitable for recording or transmission by any conventional means.

FIG. 4 is a block diagram of a reconversion and reproducing system adapted to reconvert the composite signal derived in FIG. 3 and to reestablish the stereophonically related signals originally introduced into the system of FIG. 3.

FIG. 5 is a block diagram of a modified system, similar to FIG. 4, but being arranged so that the stereophonically related signals will be reproduced regardless of the inability of the transmission medium to transmit higher audio frequencies.

FIG. 6 is a wiring diagram showing the electrical components of the system illustrated in FIG. 3.

FIG. 7 is a schematic wiring diagram showing the electrical components of the system illustrated in FIG. 4.

FIG. 8 is a schematic wiring diagram showing the electrical components of the system illustrated in FIG. 5.

FIG. 9 is a diagram showing the respective keyed signals derived by the oppositely phased keying transistors employed at the input side of a conversion system according to the present invention.

FIG. 10 is a diagram illustrating a substantially equivalent electrical circuit for a transistor employed as a variable impedance device as in the system of the present invention.

A prime purpose of the present invention is to provide a system for utilizing a single transmission channel, such as an amplifier, or any other conventional transmission channel, to

convey a pair of audio signals, for example, a pair of stereophonically related audio signals, and to subsequently separate the respective signal components from the output of the transmission channel and to employ the separated signals to energize respective reproducing devices, such as loud speakers, or the like, to reproduce the original stereophonically related signals. The method of the present invention involves keying the respective input audio signals at an ultrasonic frequency but in opposite phase and for periods of substantially one-half cycle or less of said frequency, whereby to derive respective trains of pulses which are amplitudemodulated by the input audio signals and which are displaced in phase relative to each other by one-half cycle. The respective trains of pulses are then superimposed and simultaneously admitted into the input of the transmission channel. The output signal leaving the transmission channel is then keyed at the same frequency and in phase with the keying of the input audio signals, whereby to separate the output of the transmission channel into respective trains of pulses amplitude-modulated by the original input signals and keyed at an ultrasonic frequency, so that the separated signals may operate associated reproducing devices to reproduce the original pair of input audio signals.

Referring to the drawings, and more particularly to FIGS. 1 and 2, a typical apparatus for carrying out the method of the present invention may comprise a pair of sources of audio signals 11 and 12, a single-channel amplifier 13, and a pair of sound reproducing circuits 14 and 15. The amplifier 13 is provided with a signal input terminal 16 and a signal output terminal 17. The audio source 11 is connected to the amplifier input terminal 16 through a first keying device 18, and the audio source 12 is connected to the amplifier input terminal 16 through a second keying device 19. As will be presently described, the devices 18 and 19 are keyed at an ultrasonic frequency but in opposite phase, whereby to supply trains of pulses to the amplifier input terminal 16 which are amplitudemodulated by the respective sources 11 and 12 and which are superimposed on each other to define a composite signal suitable for amplification by the amplifier. Thus, as shown in FIG. 9, a first train of pulses 20 appears at the output of the keyer device 18 and a second train of pulses 21 appears at the output of the keyer 19. Since the pulses 20 and 21 are displaced in phase by degrees, the respective trains of pulses 20 and 21 may be combined to derive a composite single audio signal. Similarly, the signal leaving the amplifier 13 at the output terminal 17 thereof may be keyed in the same manner to separate the components of the composite signal and to derive respective trains of amplified pulses 20 and 21 which may be separately reproduced in the respective reproducing circuits 14 and 15. As shown in FIG. 1, the output terminal 17 of the amplifier 13 is connected to the respective reproducing circuits l4 and 15 through the respective keying devices 22 and 23 which are similar to the keying devices 18 and 19 and which are respectively operated in phase with said keying devices 18 and 19.

In the arrangement illustrated in FIGS. 1 and 2, a common ultrasonic oscillator 24 is employed to simultaneously operate the keying devices 18, 19 and 22, 23 at a common ultrasonic frequency and to maintain 180 degrees phase displacement between the keyers 18 and 19 and the keyers 22 and 23, and

to maintain the keyer 18 in phase with keyer 22 and keyer 19 in phase with keyer 23.

Referring now to FIG. 2, it will be seen that the respective sources 11 and 12 may comprise the respective transducing coils of a conventional stereophonic phonograph pickup of the magnetic type. The transducing devices 11 and 12 have a common ground connection 25 which may be connected to the cathode or the ground terminal of the amplifier 13, in a conventional manner. The ungrounded terminals of the transducer coils 11 and 12 are connected to the input terminal 16 of the amplifier 13 through respective transistors 26 and 27, which are employed as keying devices in a manner presently to be described. The output terminal 17 of the amplifier 13 is connected through respective additional transistors 28 and 29 to respective sound reproducing circuits including loud speakers 30 and 31. Thus, the amplifier 13 may include a conventional output transformer 32 having one terminal 33 of its secondary winding connected to one terminal of each of the loud speakers 30 and 31. The remaining terminal of the secondary of the transformer 32 constitutes the output terminal 17, which is connected to the remaining terminal of each of the loud speakers 30 and 31 through the transistors 28 and 29.

The ultrasonic oscillator 24 is of conventional construction and employs a transistor 34. The oscillator 24 comprises a transformer 35 having a first winding 36 and respective additional windings 37, 38 and 39 inductively coupled to the first winding 36. A battery 37 has its negative terminal connected to the collector 40 of transistor 34 through the transformer winding 36. The positive terminal of battery 37 is connected through a control switch 41 and the transformer winding 37 to the emitter 42 of transistor 34. The base 43 of the transistor is connected through a resistance 44 to the wire 45, which is connected to the positive terminal of battery 37 through switch 41, as shown. Resistance 44 may be of any suitable value, for example, a value of between 50,000 and 100,000 ohms. A condenser 46 is connected across the resistor 44, the capacity of the condenser being of the proper value, taken with the inductance of winding 36 and the mutual inductance thereof with respect to the additionAl windings of transformer 35, to provide the desired ultrasonic frequency for oscillator 24, namely, a frequency of the order of 20,000 cycles.

As will be readily apparent, feedback from the coil 37 to the inductance 36 will maintain oscillation, and the oscillator 24 will thereby develop a definite ultrasonic frequency.

The ungrounded terminal of the transducer coil 11 is connected to the collector 47 of transistor 26, and the emitter 48 of said transistor is connected to the amplifier terminal 16. The base 49 of transistor 26 is connected to one terminal of the transformer coil 38. The opposite terminal of the transformer coil 38 is connected to the base 50 of transistor 27.

The ungrounded terminal of the transducer coil 12 is connected to the collector 51 of transistor 27, whereas the emitter 52 of transistor 27 is connected to the amplifier input terminal 16. Coil 38 is center tapped at 53, and the center tap 53 is connected to the common emitter connection of the transistors 26 and 27, namely the amplifier input terminal 16. It will therefore be apparent that the emitter bases 49 and 50 will be constantly in opposite phase relative to each other and will be displaced 90 degrees in phase in opposite directions with respect to the common connections of their emitters 48 and 52.

As is well known to those skilled in the art, transistors are essentially impedance devices controlling current flow rather than voltage. I have discovered that when a voltage is impressed on a transistor across the emitter and base, the resistance of the transistor from the emitter to the collector varies in accordance with said impressed voltage across the emitter and base. With one polarity of said impressed voltage the resistance of the transistor from the emitter to the collector becomes very great. When the polarity is reversed, the resistance of the transistor from emitter to collector becomes very small. This enables the transistor to be used as a keying device, in accordance with the equivalent circuit thereof illustrated in FIG. 10. For example, considering the transistor 26, a variable resistance is defined between the emitter 48 and the collector 47, the magnitude of which is controlled by the magnitude and polarity of the voltage applied between the emitter 48 and the base 49. If the control voltage alternates, the transistor is alternately conducting and non-conducting in phase with the alternations of the control voltage applied across the emitter 48 and base 49.

Therefore, when transistor 26 conducts between its collector 47 and emitter 48, transistor 27 is non-conducting between its collector 51 and emitter 52, and vice versa. Therefore, the audio signals from the transducer coils 11 and 12 are keyed at the frequency of the oscillator 24 to provide the respective trains of pulses and 21 illustrated in FIG. 9 and described above, said pulses being admitted simultaneously to the amplifier at its input terminal 16 and being superimposed so that they define a composite audio signal which may be conveyed and amplified by amplifier 13 in its ordinary manner.

The winding 39 of transformer 35 is center tapped at 53, and the center tap 53 is connected to the emitters 54 and 55 of the respective output keying transistors 28 and 29. One terminal of the coil 39 is connected to the base 57 of transistor 28 and the opposite terminal thereof is connected to the base 58 of the transistor 29. Collector 60, of transistor 28 is connected to a terminal of loud speaker 30 and collector 61 of transistor 29 is connected to a like terminal of loud speaker 31. As above mentioned, the remaining terminals of the loud speakers 30 and 31 are connected to the common terminal 33 of the output transformer 32 associated with the amplifier 13.

It will be seen that the transistors 28 and 29 are operated in the same manner as the input keying transistors 26 and 27 and are operated in phase with said input keying transistors. The transistors 28 and 29 therefore operate as keying devices, similar to the transistors 26 and 27, with respect to the audio signal delivered by the secondary of output transformer 32. The respective loud speakers 30 and 31 thus receive the respective trains of pulses, similar to the pulses 20 and 21, which are keyed at an ultrasonic frequency, the same as that employed with the transistors 26 and 27, whereby the loud speakers 30 and 31 reproduce the original signals developed in the transducer coils 11 and 12. Since the frequency of the oscillator 24 is above the audible range, this frequency is not audible in the output of the loud speakers 30 and 31.

The input keying transistors 26 and 27 are preferably of the low noise type, for example, may be of a type similar to the 2Nl90 type. The output keying transistors 28 and 29 are preferably of the power output type, for example, may be of the 2N255 type.

FIGS. 3 to 8 illustrate examples of single-channel compatible stereo systems according to the present invention that may be used in broadcasting, recording, or similar activities. Thus, F IG. 3 diagrammatically illustrates a transmitting arrangement wherein the respective audio input signals are furnished through the keyers l8 and 19 through respective band pass filters 70 and 71 which are designed to pass frequencies in the audible range, for example, the frequencies between 20 cycles and 19,750 cycles. The keyers l8 and 19 are operated in opposite phase at an ultrasonic frequency which is furnished by a frequency multiplier 73. An oscillator 74 generates a subsonic frequency, for example, a frequency of l3.75 cycles per second which is delivered to the input of the multiplier 73. Multiplier 73 provides an output frequency which may be 1500 times that of the input frequency from the oscillator 74, namely, 20.625 kilocycles. The output signal from the multiplier 73 is applied in opposite phase to the respective keyers 18 and 19 in the same manner as previously described in connection with FIGS. 1 and 2 to key the input signals. The keyed signals are simultaneously delivered to the input of an amplifier 13'. As will be presently pointed out with reference to FIG. 6, the subsonic frequency of 13.75 cycles from the oscillator 74 and the ultrasonic frequency of 20.625 kilocycles from the multiplier 73 are also applied to the input of the amplifier 13. Thus, the output signal from the amplifier 13 contains the superposed keyed signals, amplified by the amplifier 13, and also contains the subsonic signal voltage of 13.75 derived from the oscillator 74 and the ultrasonic keying signal of 20.625 kilocycles furnished by the multiplier 73. Therefore, the output from the amplifier 13', shown at 75, contains the composite keyed audio signals, the subsonic signal of 13.75 cycles per second, and the ultrasonic signal of 20.625 kilocycles per second.

As will be presently apparent, the reasons for including the subsonic signal of 13.75 cycles in the output of the amplifier 13' is to provide a means for developing a properly synchronized keying signal in the reproducing portion of the system in the event that the transmission medium between the transmission portion of the system and the reproducing portion of the system has poor high frequency response. If this is the case, the ultrasonic keying signal of 20.625 kilocycles will be substantially attenuated and will not be furnished to the reproducing portion of the system. However, the subsonic frequency of 13.75 cycles will be present and may be multiplied in the reproducing portion of the system to develop the required ultrasonic keying signal. An arrangement for accomplishing this result is diagrammatically illustrated in FIG. 5 wherein the composite signal 75 is delivered to the input of an amplifier 76, the output of the amplifier being delivered through respective keyers 22 and 23 to loud speakers 30 and 31 similar to those illustrated in FIG. 2. The output from the amplifier 76 is also passed through a tuned filter 77 which is designed to pass only the subsonic frequency of 13.75 cycles per second. Said subsonic frequency is furnished to a conventional multiplier 73, similar to the conventional multiplier employed in FIG, 3, the multiplier providing an output ultrasonic signal voltage of 20.625 kilocycles which is applied in opposite phase to the respective keyers 22 and 23 to key the output signals delivered to the loud speakers 30 and 31 in synchronism with the keying of the input signals furnished to the amplifier 13.

FIG. 4 diagrammatically illustrates a reproducing system wherein the transmission medium has good high frequency response so that the ultrasonic keying frequency contained in the transmitted signal at 75 is not substantially attenuated. In the arrangement of FIG. 4, the transmitted signal 75 is applied to the input of an amplifier 76, and the output of the amplifier is applied through respective keyers 22 and 23 to the respective loud speakers 30 and 31. The output from the amplifier 76 passes through a tuned filter 77' which is tuned to pass the ultrasonic keying frequency of 20.625 kilocycles. The keying frequency is then applied in opposite phase to the keyers 22 and 23 in the same manner as shown in FIG. 2, whereby the signals applied to the loud speakers 30 and 31 are keyed in synchronism with the respective input signals applied to the input of the amplifier 13'. The loud speakers 30 and 31 will therefore substantially reproduce the audio signals originally furnished to the system with the same stereophonic relationship possessed by said audio signals.

FIG. 6 is a wiring diagram of the arrangement illustrated in FIG. 3, wherein the input keyers comprise respective transistors 26 and 27 similar to those employed in the circuit of FIG. 2. The subsonic oscillator 74 is conventional in design and employs a transistor 34'. The oscillator output is available at a first output winding 80 of the transformer 81, and is also available at a second output winding 82 of said transformer. The signal available from the output winding 80 is applied to one grid 83 of a pentagrid converter input tube 84 of the amplifier 13'. The signal available from the output coil 82 is applied to the input of the conventional multiplier 73, the multiplier being provided with the output transformer 85 having the respective output coils 86 and 87. The output signal available from the coil 86 is applied to another grid 88 of the amplifier input tube 84. The output coil 87 is center tapped at 89, said center tap being connected to one terminal 90 of the primary of a transformer 91 employed to couple the audio input signal to the amplifier tube 84. One terminal of coil 87 is connected to the base 49 of the keying transistor 26 and the opposite terminal of said coil is connected to the base 50 of the keying transistor 27. The emitters 48 and 52 of said transistors are connected to the terminal 90, whereby the control voltages applied between the emitters and the bases of the respective transistors are opposite in phase, providing the alternate keying of the signals in the manner above described. Thus, the audio signals from the respective band pass filters 70 and 71 are keyed in opposite phase by the transistors 26 and 27, in the manner above described, and are applied as a composite signal to the terminal 90 of transformer 91.

As above mentioned, the output from the multiplier 73 is an ultrasonic frequency, which in the specific case above described is 20.625 kilocycles. The composite signal containing the respective keyed audio input signals is coupled to the input of the amplifier 13' by a connection from the secondary terminal 92 of transformer 91 to another grid 93 of the amplifier tube 84. Therefore, the output from tube 84, shown at 75, contains the superimposed keyed audio input signals, the subsonic frequency of 13.75 cycles per second, and the ultrasonic keying frequency of 20.625 kilocycles.

FIG. 7 is a wiring diagram of the arrangement illustrated schematically in FIG. 4. Thus, the output of the amplifier 76 is coupled to the reproducing circuits by an output transformer 94 having the secondary windings 95 and 96. The secondary winding 95 is center tapped at 97 and the center tap is connected to to respective emitters 54 and 55 of the output keying transistors 28 and 29, a suitable biasing battery 56 being provided in circuit between the emitters and the center tap 97 to render the transistors 28 and 29 normally conducting. One terminal of winding 95 is connected to the base 57 of the transistor 28 and the opposite terminal of said winding is connected to the base 58 of the transistor 29. The collectors 60 and 61 of the respective transistors 28 and 29 are each connected to one terminal of a respective loud speaker 30 and 31, as in FIG. 2. The remaining terminals of the loud speakers 30 and 31 are connected to one terminal of the output winding 96. The remaining terminal of output winding 96 is connected to the emitters 54 and 55, as by a wire 98. Thus, the respective loud speakers 30 and 31 are connected to the output winding 96 of output transformer 94 through the respective collectors and emitters of the transistors 28 and 29. The transistors are keyed in opposite phase, as in FIG. 2, and are synchronized with the input keying transistors 26 and 27 of FIG. 6. The filter 77' has respective tuned circuits 99 and 100 which are connected between the emitters 54, 55 and the respective bases 57 and 58 and which are designed to pass only the ultrasonic keying frequency of 20.625 kilocycles.

FIG. 8 is a wiring diagram showing the arrangement schematically illustrated in FIG. 5. The output from the amplifier 76 is applied to the primary winding of the output transformer 94'. Transformer 94' has the respective output coils 95' and 96. The output coil 96 is connected to the respective loud speakers 30 and 31 through the emitters of the respective keying transistors 28 and 29, The connections of the loud speakers 30 and 31 are the same as in FIG. 7. A suitable conventional filter 77 is connected across the output winding 95 of transformer 94' and is arranged to pass only the subsonic frequency of 13.75 cycles. The winding 95' is connected to the input of the multiplier 73, with the filter 77 arranged to pass only said subsonic frequency. The subsonic frequency of 13.75 cycles is multiplied in the multiplier 73 1,500 times and is applied to the primary winding of a coupling transformer 101. The coupling transformer has a secondary winding 102 and is connected through a suitable biasing battery 56 to the emitters 54 and 55 of the transistors 28 and 29. One terminal of the output winding 102 is connected to the base 57 of transistor 28 and the remaining terminal of said winding is connected to the base 58 of the transistor 29. The collectors 60 and 61 of the transistors 28 and 29 are each connected to a terminal of a respective loud speaker 30 and 31, the remaining terminals of the loud speakers being connected to one terminal of the output winding 96, as by a wire 104. The remaining terminal of the winding 96 is connected to the emitters 54 and 55, as 8 mentioned. The battery 56 is provided in order to render the transistors 28 and 29 conducting when a normal monaural signal is received so that this signal will be simultaneously reproduced by both speakers 30 and 31. However, the magnitude of the keying control voltages available from the output winding 102 is sufficient to cancel the voltage of the biasing battery 56 and to render the transistors 28 and 29 alternately nonconducting. Thus, the output reproducing circuits containing the loud speakers 30 and 31 are keyed in opposite phase in synchronism with the keying of the input signals by the keying transistors 26 and 27 in FIG. 6. Since the subsonic frequency of 13.75 cycles is available, the reproducing circuit of FIG. 8 is independent of the high frequency response of the transmission medium.

it is possible to make a conventional stereophonic disc record which is compatible with the system of the present invention and also with a conventional stereo system. Thus, the pulses of FIG. 9 may be recorded on one channel track of the stereo disc and the pulses 21 may be recorded on the other channel track. Then if the disc is played back in the conventional stereo fashion, the respective stereophonically related audio signals will be transmitted through the respective amplifiers to the associated speakers of the system in the conventional manner. However, when the disc is played back into the system of the present invention, as in FIG. 2, for example, the two channels provide the outputs consisting of the pulses 20 and the pulses 21, which are superimposed and which provide the composite signal at the wire 75 in FIGS. 3, 4 and 5. Of course, the composite signal should include the sub-multiple and ultra-sonic keying frequencies of FR}. 3.

The various methods of stereo transmission by radio may be similarly made compatible with the system of the present present invention, by transmitting superimposed pulse chains comprising the pulses 20 and the pulses 21 of FIG. 9. A conventional receiver which cannot separate the pulses will simultaneously reproduce the pulses from both channels, so that applicants system is compatible with the procedures of conventional radio broadcasting and reception.

While certain specific embodiments of an improved method and means of utilizing a single transmission channel to convey a pair of audio signals have been disclosed in the foregoing description, it will be understood that various modifications within the spirit of the invention may occur to those skilled in the art. Therefore, it is intended that no limitations be placed on the invention except as defined by the scope of the appended claims.

I claim:

l. in a stereophonic transmission system having only two sources of audio signals: A first pair of transistors, each having an emitter, a collector and a base, a pair of audio signal sources, a signal transmission channel, means connecting each source to the input of said channel through the collector and emitter of a respective transistor, an ultrasonic sine wave alternating voltage generator, means connecting the output of said generator in opposite phase across the emitter and base of the respective transistors, whereby to key the audio signals from said sources in opposite phase at the frequency of said generator, deriving a composite signal comprising keyed components from said audio signal sources at the input of the signal channel, a second pair of transistors, each having an emitter, a collector and a base, a pair of sound reproducing channels, means connecting the output of the transmission channel to said reproducing channels through the collectors and emitters of the respective second pair of transistors, means injecting a subsonic sub-multiple of the ultrasonic alternating voltage of said generator into said signal transmission channel, and means for deriving from said subsonic sub-multiple sine, continuous wave alternating voltages at the same frequency as said ultrasonic alternating voltage generator, and means for applying said sine wave alternating voltages in opposite phase across the emitters and bases of the second pair of transistors, whereby to separate the keyed audio signal components.

2. in a stereophonic transmission system having only two sources of audio signals: a first pair of transistors, each having an emitter, a collector and a base, a pair of audio signal sources, a signal transmission channel, means connecting each source to the input of said channel through the collector and emitter of a respective transistor, an ultrasonic sine wave alternating voltage generator, means connecting the output of said generator in opposite phase across the emitter and base of the respective transistors, whereby to key the audio signals from said sources in opposite phase ultrasonic the frequency of said generator, deriving a composite signal comprising keyed components from said audio signal sources at the input of the signal channel, a second pair of transistors, each having an emitter, a collector and a base, a pair of sound reproducin channels, means in ecting a subsonic sub-multiple of the u trasonic alternating voltage of said generator into said signal transmission channel, means connecting the output of said signal transmission channel to said reproducing channels through the collectors and the emitters of the respective second pair of transistors, means to multiply said subsonic submultiple and to derive oppositely phased ultrasonic continuous sine wave alternating voltages of the same frequency as the output of said generator, and means applying said lastnamed alternating voltages across the emitters and bases of the second pair of transistors, whereby to key said reproducing channels in opposite phase at said frequency and to separate the keyed audio signal components.

Claims (2)

1. In a stereophonic transmission system having only two sources of audio signals: A first pair of transistors, each having an emitter, a collector and a base, a pair of audio signal sources, a signal transmission channel, means connecting each source to the input of said channel through the collector and emitter of a respective transistor, an ultrasonic sine wave alternating voltage generator, means connecting the output of said generator in opposite phase across the emitter and base of the respective transistors, whereby to key the audio signals from said sources in opposite phase at the frequency of said generator, deriving a composite signal comprising keyed components from said audio signal sources at the input of the signal channel, a second pair of transistors, each having an emitter, a collector and a base, a pair of sound reproducing channels, means connecting the output of the transmission channel to said reproducing channels through the collectors and emitters of the respective second pair of transistors, means injecting a subsonic sub-multiple of the ultrasonic alternating voltage of said generator into said signal transmission channel, and means for deriving from said subsonic sub-multiple sine, continuous wave alternating voltages at the same frequency as said ultrasonic alternating voltage generator, and means for applying said sine wave alternating voltages in opposite phase across the emitters and bases of the second pair of transistors, whereby to separate the keyed audio signal components.

2. In a stereophonic transmission system having only two sources of audio signals: a first pair of transistors, each having an emitter, a collector and a base, a pair of audio signal sources, a signal transmission channel, means connecting each source to the input of said channel through the collector and emitter of a respective transistor, an ultrasonic sine wave alternating voltage generator, means connecting the output of said generator in opposite phase across the emitter and base of the respective transistors, whereby to key the audio signals from said sources in opposite phase ultrasonic the frequency of said generator, deriving a composite signal comprising keyed components from said audio signal sources at the input of the signal channel, a second pair of transistors, each having an emitter, a collector and a base, a pair of sound reproducing channels, means injecting a subsonic sub-multiple of the ultrasonic alternating voltage of said generator into said signal transmission channel, means connecting the output of said signal transmission channel to saId reproducing channels through the collectors and the emitters of the respective second pair of transistors, means to multiply said subsonic submultiple and to derive oppositely phased ultrasonic continuous sine wave alternating voltages of the same frequency as the output of said generator, and means applying said last-named alternating voltages across the emitters and bases of the second pair of transistors, whereby to key said reproducing channels in opposite phase at said frequency and to separate the keyed audio signal components.

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