US2258662A - Sound transmission system - Google Patents

Description

Oct. 14, 1941. l Y w B -sNiw 2,258,662

SOUND TRANSMISSION SYSTEM A r-TDRNE y w. B. sNo w 2,258,662

SOUND TRANSMISSIQN SYSTEM Patented Qct. 14, 1941 SOUND TRANSMISSION SYSTEM William B. Snow, Summit, -N. J., assignor to Bell' Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Application April 5, 1940, Serial No. 328,078

8 Claims.

This invention relates to systems for the reproduction of sound and more particularly to systems winch reproduce the sound with the spatial distribution of the original sound. The invention applies to systems in which the sound is detected and transmitted for immediate reproduction and also to systems in which the sound is detected and recorded for subsequent reproductions.

Sound transmitting systems and sound recording and reproducing systems have been proposed heretofore in which the volume range capacity of the transmitting medium or the recording medium is effectively increased by compressing the volume range before transmission or re-V cording and expanding the volume range in a complementary manner before the reproduction of the sound. Typical systems of these types are disclosed respectively in Patents 1,565,548, December 15, 1925, A. B. Clark; and 1,623,756, April 5, 1927, C. F. Sacia. In systems of this character the effective amplification, that is, the gain or loss in the transmission channel, is varied 'in such a way that the louder sounds do not overload the system and the faintest sounds are amplified sufliciently to be well above the level of extraneous noise originating in the system.

It has also been proposed heretofore, to reproduce sounds in their original spatial relation by using a plurality of separate transmission channels with the reproducers disposed with respect to the desired virtual source of the reproduced sound in a manner corresponding to the relation of the microphones to the actual source of the sound. Systems of this character are shown, for example, in British Patent 23,620 of 1911, A. Rosenberg, and United States Patent 1,589,139, June 15, 1926, E. H. Foley. To produceV a satisfactory illusion of the spatial distribution of the sound, the transmission system, which may inf clude a sound recording and reproducing.` link, must be capable of reproducing a very wide range of both the volume and the frequencies of the sound.

The object of the present invention is a multichannel stereophonic sound transmission system capable of transmitting a very wide range of both volume and frequency of sound.

A feature ofthe invention is the use of volume compression and expansion individually `in each channel of the system to increase the effective volume range capacity of the system.

Another feature of the invention is the use of a constant frequency carrier current individual to each channel, varied lin accor degree of compression applied to sound.

A further feature of tnemventio is in use ofthe same constant frequencyivarying ampli--v tude carrier current to control both `tha-,com-

pressor in the transmitting channel and ,the ex,-

pandenin the-reproducing channel.

A further feature of the invention is the transe.

mission of all of the'control currents fromthe dance withthe side of the desired odd harmonic. 'Ihe upper various channels over a single transmission channel, or the recording of 'all of the control currents Fig. 1l alagrammauc'any discloses the sending end of a transmission system embodying the invention; and

Fig. 2 diagrammatically discloses the receiving' end of a transmission system embodying the in' vention. Y l

'I'he microphones l, 2 are appropriately disposed in the vicinity of the source of sound to form a stereophonic pick-up system. The sound waves are detected by the microphones l, 2 in the form of electrical waves which are amplied in the ampliers 3, l, compressed in the compressors 5, 6, further amplied if desired in the ampliers 1, 8 vand applied to separate transmission channels which may, if desired, be in the form of the recording devices '9 and l0. Carrier currents of constant frequency are generated by the oscillators Il, I2, modulated bythe control circuits I3, Il, and, after modulatiomrare transmitted over a common circuit which may, if desired, have the form of the common recording `device I5. The oscillator associated with any given channel generates a frequency which differs from the frequencies generated by the oscillators asso` ciated with the other channels. If desired, the v carrierf'frequenciesmay be harmonically related, in which case the individual oscillators may be replaced by a common oscillator and a known form of harmonic generator with suitable filters for each channel or by an oscillation generator capable of producing a plurality of harmonically related frequencies. Preferably the'carrier `frequencies form a series 4of odd harmonics, because, when thus related, the beat frequencies due to any intermodulation between the carrier frequencies will form a series of even harmonics which may easily be eliminated by the band-pass lters. :Ihe pass bands of the respective lters will pass the desired odd harmonic but are too narrow to pass the even harmonics lying on each thetransniitte y Other forms oi modulator, such as theone dis- 2 y 2,258,662 harmonics are also s'o phased with respect to the closed in United States Patent 1,936,176, Novemlower harmonics that they subtract from the ber 21, 1933, R. R. Scoville, may be used in place peak value of the lower harmonics thus reducing of the modulator shown.

the maximum .load impressed on the recording A portion of the output of the modulator i3 device I5 and increasing the average load on the 5 is selected by the band-pass filter 42 and supplied recording device. to the ampliiler-detector 43 where it is converted 'I'he records produced by the recorders 3, I0 into aslowly varying unidirectional current. 'I'he and I5, may conveniently be placed side by side slowly varying unidirectional current in the outupon the same medium I5 which may have the put oi' they amplifier-detector 43 is supplied to `form oi' a photographic nlm. 'I'he records on l0 the compressor 5. The compressor 5 is a modithe medium I5 may be detected by the reproned form of the device disclosed in United States ducers I1, |3, ampliiled if desired in the ampli- Patent 2,035,263, March 24, 1936, R. W. Cushman ers Il, 20, expanded in the expanders 2|, 22, et al., and is essentially a balanced resistance further. ampliiled if desired in the amplifiers 23, pad formed by the series resistors 44. 45, 43, 41

24 and reproduced as sound waves by the repro- 15 and the shunt network formed by the resistors f ducing devices 25, 25. The reproducers 25, 25 43 and 45 in series with the non-linear resistance are related to the apparent source of sound in network 53. The slowly varying unidirectional the same way that the microphones I, 2 are recurrent in the output of the amplifier-detector lated to the real source of sound. All ot the 43 is supplied to the potentiometer 5| and ilows control currents are reproduced by the repro- 2o through the element of the network 55 back to ducer 21, amplified if desired by the amplifier 23 the amplifier-detector 43. These currents ilowand supplied to suitable band-pass iilters 29, ing in the network 50 change the resistanceV of which select the appropriate carrier frequency the network 50, thus varying the shunt resistance and supply the current to control the expanders of the resistance network and effectively varying 2|, 22. Ii desired the sound recorders and repro- 25 the loss introduced by the-compressor 5 in the ducers may be omitted, and the channels conchannel ofthe microphone I. 'I'he potentiometer nected directly from the input of the recording 5| may be adjusted to improve the balance of devices 9, and I5, respectively. to the output the network 50. The transformers and resistance of the reproducing devices I1, I 3 and 21, respecnetwork at the input and outputof the device 5 tisfely. 30 prevent the changes in the impedance of this A portion of the electrical waves in the output device from being reilected into the channel. of the ampliiler 3 are supplied to a conventional The compressor l, which is essentially a variatype of amplifier-detector 3| and are there conble loss device, may .be replaced by a variable verted into a unidirectional current varying as gain device such as disclosed in United States some function of the variations in amplitude of Patent 1,623,756, April 5, 1927, C. 1". Sacia. the electrical waves inthe output of the .ampli- The compressor I in the channel leading from iier 3. The varying unidirectional control curthe microphone 2 may be oi the same type as rents are supplied to the modulating device 3 the compressor 5 and is controlled by the output in the output of the oscillator I I. The moduof the modulator I4 which may be o! the same lating device I3 is a modified form oi' the device 40 character as the modulator I3. The varying disclosed in French patent ot addition 39,757 pubcarrier vcurrents from the modulators I3 and I4 lished March 12, 1932, and in United States Patare combined into one channel for transmission ent 2,034,703, March 24, 1936, F. W. Metzger. The and may. if desired. be recorded by a single modulator I3 comprises two series resistors 32, 33, recording device I5. respectively shunted by non-linear resistance ele- While a two-channel stereophonic system has ments 34, 35 having the property of changing for convenience been disclosed, it is, o! course, their resistance with the current ilowing through evident that three or more channels may be used, the element. The non-linear resistance elements the three or more control currents being combined 34,35 may conveniently comprise a plurality ofthe and transmitted in a single channel, or recorded known copper, copper-oxide couples. The slowly by a single recording device I5; varying unidirectional current from the ampli- In Fig. 2 the proper control current in the outfier-detector 3| flows through the resistor 35 to put of the ampliiler 23 is selected by the bandthe junction of the resistors 31 and 38 and there .pass iilter 25 and supplied to an amplier-detecdivides equally. One half of the current flows tor 52 where the control current is converted into through the resistor 31 through the resistors 33 .55 a slowly varying unidirectional current, which and 35 in parallel and through the resistor 39. ilows from the amplifier-detector 52 through the The other half of the current ows through the network formed by the expander 2| back through resistor 35, the resistors 32 and 34 in parallel the ampliiler-detector 52. The expander 2| is oi' and the resistor 40, the combined current ilowing essentially the same construction as the modulathrough the resistor 4I back to the amplier- 6'0 tor device I3 and operates in a similar manner: detector 3|. 'Ihe unidirectional current owing thus a detailed description oi' the operation of in the resistors 34 and 35 changes the resistance this network iS believed t0 be unnecessary. The of these resistors and thus changes the loss sufpotentiometer and variable resistance shown in iered by the constant frequency current from the circuit of the device 2| assists in balancing the oscillator in passing through the device the Circuit The expander 2| 'ots the current I3. The current at the output of the device I3 .transmitted through the deviceinamanner which will then have the form of a constant frequency is the inverse oi' the eilects of the compressor l carrier current varying in amplitude in accordon the current transmitted through that netance with the variations in the unidirectional work. Inother words, the compressorlinsertsa current from the amplifier-detector 3|. The 70 loss in the channel which increaseswith an intransformer and resistance pads at the input crease in theam'plitude of the current transand output of the device I3 prevent the variamitted through the channel. while the expander tions in the impedance of the device I3V from 2| introduces a loss into the reproducing channel being reilected into the other part of the circuit. which decreaseswith an increase in the amplitudeotthecurrentiromthemicrophone w\ 3. In a transmission system, arsource of sound,

As the compressor Bris controlled by a slowly varying unidirectional current which is a function of the electrical wave from the amplifier 3, it is evident that this compressor could be controlled directly by the output of the amplifierdetector 3|. In the present system, however, the output of the amplier-detector 3| is rst converted into the form of a slowly varying carrier which is selected by the band-pass filter 42 and then converted back into a unidirectional current by the amplifier-detector 43 and it is the output of the amplifier-detector 43 which controls the compressor 5. A very real advantage is gained by this method of control. If the compressor 5 were to be directly controlled by the output of the amplifier-detector 3|, and the expander 2| is controlled by the carrier currentfrom the modulator I3, this would require that the modulator I3 and oscillator Il operate as strictly linear devices so that there would be exact proportionality between the output of the amplifier-detector 3| and the output of the ampliiier-detector 52. However, in the present system, both the compressor 5 and the expander 2| are controlled by unidirectional current derived from the same carrier output of the modulator device I3, thus it is not necessary that the oscillator vIl and modulator device |3 be linear in their operation and one diflicult circuit requirement has been eliminated. In either case, it is, of course, essential that the expander 2| should be a fairly exact inverse of the compressor 5.

What is claimed is:

1. In a transmission system, a source of sound, a plurality of detectors for converting sound waves from said source into electrical waves, a plurality of transmission channels respectively associated with said detectors, means individual to each of said channels for controlling the transmissionlof electrical waves through said channel, a plurality of sources of electrical currents of different constant frequencies, one source being associated with ach transmission channel, means individual tc each of said sources of constant frequency current for controlling the output of said source in accordance with a function of the electric wave transmitted through the channel, means for combining the output of all of said control means in a single transmission channel separate from the channels transmitting said electrical waves.

2. In a transmission system, a source of sound,

ulated output, and means for deriving a currentV varying with the envelope of the amplitude variawaves from said source into electrical waves, a

plurality of transmission channels respectively associated with said detectors, means individual to each channel for compressing the range of amplitude of the electrical waves transmitted through the channel, a. plurality of sources of electrical current of constant frequency respectively associatedwith said channels, means for deriving Ifromy the electrical Waves in each channel, a current varying with a function of the amplitude variation of the Waves in the channel, means individual to each source of current of constant frequency for controlling the output of said source in accordance with the current derived from the Waves in the associated channel,

vmeans for controlling the compressor in the a microphone forv converting sound waves from said source into electrical waves, a-reproducer for convertingsaid electrical waves into sound, and a transmission channel connecting said microphone and said reproducer and having a compressor associated with said microphone and an expander associated with said reproducer, a carrier current generator associated with said chan..` a

nel, means associated with the output of said microphone for deriving a current varying with a function of the amplitude variations of the electric waves from said microphone, means operated by said derived current for controlling the output of said carrier current generator and means for controlling said compressor and said expander by the controlled output of 'said carrier current generator. 1

4. In a sound recording system, a plurality of spaced microphones, a recorder for each microphone, a separate circuit including control means connecting each microphone with the corresponding recorder, a plurality of sources of current of different constant frequencies, means for deriving currents varying with the envelope of the currents in the separate circuits `and respectively modulating the outputs of said sources, and means for. recording all of said modulated outputs in a single record track and respectively controlling the control means in one of said circuits by the modulated output of the source corresponding to said circuit.

5. In a sound recording system, a microphone, a recorder, a circuit including amplitude control means connecting said microphone and said recorder, means for deriving a current varying with the envelope of the amplitude variations of the current from said microphone, a source of current ofconstant frequency, means for modulating the output of said source with said envelope current, means for making a record of said modtions of said modulated currentand controlling said amplitude control means with said current.-

6. In a sound recording system, a plurality of microphones, a recorder for each microphone, a separate circuit including control means connecting each microphone with the corresponding recorder, V.a plurality of sources of currents of different constant frequencies, means for deriving currents varying with a function of the currents in the separate circuits and respectively modulating the outputs of said sources, and means for recording all of said modulated outputs in a single record track.

'7. In a transmission system, a plurality of sources of electrical waves, a plurality of transmission channels respectively associated with saidsources, means individual to each of said channels for controlling the transmission of electrical waves through said channel, a plurality of sources of electrical currents of dierent constant frequencies, one source being associated with each transmission channel, the frequencies of said sources of constant frequency currents forming a series of odd harmonics, the upper harmonics being so phased with respect to the lower harmonics that the peak value of the lower harmonics is reduced, means individual to each of said sources of constant frequency current for controlling the output of said source in accordance with a function of the electric waves transmitted through the channel, and means for combining the output of all of said control means in a single transmission channel separate' from the channels transmitting said electrical waves.

8. In a sound reproducing system, a recordingr medium comprising a plurality of records ot sound and a. separate record of a plurality oi carrier current control records, a pick-up device individual to each of said sound records, a sound reproducer individual to each of said sound records, a transmission channel containing control means respectively connecting each pick-up to the corresponding reproducer, a common pick-up for all said control records, and means for selecting from the output of said common pick-up the carrier current corresponding to a given channel and for controlling the control means in said channel with said current.

WILLIAM B. SNCW.

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