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Publication numberUS3761628 A
Publication typeGrant
Publication dateSep 25, 1973
Filing dateApr 13, 1972
Priority dateApr 13, 1972
Publication numberUS 3761628 A, US 3761628A, US-A-3761628, US3761628 A, US3761628A
InventorsBauer B
Original AssigneeColumbia Broadcasting Syst Inc
Export CitationBiBTeX, EndNote, RefMan
External Links: USPTO, USPTO Assignment, Espacenet
Stereo-quadraphonic matrix system with matrix or discrete sound reproduction capability
US 3761628 A
Abstract
A sound system for producing at least four discrete sound outputs from respectively different directions from a listener wherein the audio and directional information representing the sound outputs are recorded on a stereophonic disc record, or similar record medium having a two-track capability. The system includes recording apparatus having two matrix encoders for separately and differently matrixing four input signals to produce two pairs of composite signals each containing three of the input signals with preselected amplitude and phase relationships, at least one of the signals having a different phase in each of the composite signals of a pair. The amplitude and phase relationships are such that four combinations of the two composite signals of each pair will yield four separate signals in which respective ones of the four input signals are predominant. The two composite signals produced by one of the encoders are recorded at baseband frequency on respective walls of a stereophonic disc record, and the two composite signals from the other encoder, which have different phase relationships than the corresponding composite signals of the other pair, are utilized to amplitude modulate respective carrier signals which are also recorded on the walls of the record groove.
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United States Patent [191 Bauer 51 Sept. 25, 1973 1 STEREO-QUADRAPHONIC MATRIX SYSTEM WITH MATRIX OR DISCRETE SOUND REPRODUCTION CAPABILITY [75] Inventor: Benjamin B. Bauer, Stamford, Conn.

[73] Assignee: Columbia Broadcasting System, Inc.,

New York, NY.

[22] Filed: Apr. 13, 1972 [21] Appl. No.: 243,800

[52] US. Cl 179/1 GQ, l79/100.4 ST

[51] Int. Cl H04r 5/00 [58] Field of Search 179/1 GO, 1 G, BT,

179/100.4 ST, 100.1 TD

[56] References Cited UNITED STATES PATENTS 3,686,471 8/1972 Takahashi 179/1 GQ 2,874,221 2/1959 Dauguet 179/15 BT 3,067,292 12/1962 Minter 179/100.4 ST

3,085,203 4/1963 Logan et 211.... 325/ 3,401,237 9/1968 Takayanagt... 179/100.4 ST

OTHER PUBLICATIONS Why the 4-Channel War Need Not Take Place by Feldman, Audio Magazine, July 1972.

Primary Examiner-Kathleen H. Claffy Assistant Examiner-Thomas DAmico Att0rneySpencer E. Olson [5 7] ABSTRACT A sound system for producing at least four discrete sound outputs from respectively different directions from a listener wherein the audio and directional information representing the sound outputs are recorded on a stereophonic disc record, or similar record medium having a two-track capability. The system includes recording apparatus having two matrix encoders for separately and differently matrixing four input signals to produce two pairs of composite signals each containing three of the input signals with preselected amplitude and phase relationships, at least one of the signals having a different phase in each of the composite signals of a pair. The amplitude and phase relationships are such that four combinations of the two composite signals of each pair will yield four separate signals in which respective ones of the four input signals are predominant. The two composite signals produced by one of the encoders are recorded at baseband frequency on respective walls of a stereophonic disc record, and the two composite signals from the other encoder, which have different phase relationships than the correspond ing composite signals of the other pair, are utilized to amplitude modulate respective carrier signals which are also recorded on the walls of the record groove.

Apparatus for reproducing the record includes filters for separating the baseband frequencies from the carrier frequencies, detectors for recovering the modulation from the carriers, and a pair of decoders, one for decoding the baseband frequency signals and the other for decoding the two composite signals recovered from the carriers. Each decoder delivers four output signals in which respective ones of the four input signals are predominant and each containing unwanted signal components. Corresponding signals from the two encoders are combined, and by virtue of differences in the initial encoding, the desired predominant signals are enhanced and the unwanted signal components are canceled, thereby to produce four discrete output signals, corresponding in all respects to the input signals, for application to respective loudspeakers suitably positioned in a listening area.

19 Claims, 8 Drawing Figures /0 C M T/ l 4- l DELAY Lb Ell/CODE? f/ R 2 HG r r/ /52 FILTERS /60 0} 05m)" 2 A94 //86 DELAY 0 LA) /70 /74 f E/VCOOER /78 M00 2 050.63% )F/LT'RS H62 358 /64 T1 2-- M00 -,l'f

/72 I 72 ms n92 STEREO-QUADRAPI'IONIC MATRIX SYSTEM WITH MATRIX OR DISCRETE SOUND REPRODUCTION CAPABILITY BACKGROUND OF THE INVENTION This invention relates to audio systems, and in particular to a sound system adapted to record four-or more individual channels of audio information containing directional information on a two-track record medium and to reproduce the recorded information as four discrete audio output signals having the directionality of the original input signals.

In commercial stereophonic systems, two independent signals respectively modulate the two channels (left and right walls) of a single groove record in two perpendicular directions. Thus, the groove variations in one channel represent one function of the two signals and those in the other channel represent another function of the two signals. Typically, the groove is cut with modulation in each wall of the groove representing each signal and with lateral modulation representing the sum of the signals and vertical modulation representing the difference between the signals.

Since the commercialization of stereophonic sound reproduction, efforts have been made to record three or more signals on a two-channel record for the obvious reason that the sound distribution of the reproduced information will improve as the number of signal sources increases. For example, three-channel disc recording systems have been proposed in which a selected fraction of a third signal corresponding to the sound detected by a centrally disposed microphone is added to the two signals corresponding to the sound detected by spaced left and right microphones to record a dependent sum, or center, channel on the stereophonic record. Alternatively, it has been proposed to subtract a selected fraction of the third signal from one, and add it to the other of said two signals and to thus record a dependent difference channel on the stereophonic record. However, such a difference channel is not compatible with monophonic players because the vertical groove modulation cancels for monophonic reproduction, nor with stereophonic players, which produce an unnatural sound during reproduction.

In co-pending application Ser. No. 169,219 filed Aug. 5, 1971 by the present applicant and others, and assigned to the assignee of the present application, a system is described for recording and reproducing four independent, discrete sound signals designated L,, R,, L and R, on a two-channel medium, such as a stereophonic disc phonograph record. In accordance with the therein described invention, modulated carrier signals are utilized to provide, in effect, two additional recording channels so as to permit recording of four signals independently on the otherwise two-channel medium. Briefly, two baseband channels in the frequency domain from about 20 to 15,000 Hz are respectively recorded on the left and right channels of the disc, and two single sideband amplitude modulated carrier signals spanning the frequency range 'between about 20,000 and 35,000 Hz are also recorded in the left and right channels, respectively. When the record is reproduced with a phongraph pickup sensitive to frequencies up to about 35,000 Hz, the baseband frequencies separated from the modulated carriers by means of filters, and the modulation on the carriers detected, four separate independent information-carrying channels are obtained, each having a capacity of approximately 15,000 Hz. These four channels are used to respectively record one of the aforementioned signals L,, R,, L and R in a completely discrete and independent manner.

This result is achieved by matrixing (i.e., adding with linear coefficients) the four audio signals to form four matrixed signals, two of which when recorded at baseband will provide a reasonably pleasing sound display when reproduced on a conventional stereophonic phonograph, and the other two of which when employed to amplitude modulate carrier signals also recorded on the separate channels of the stereophonic record, will upon replay and detection provide signals which when combined with the signals derived from the baseband channels produce four output sound signals that are completely discrete and independent of each other.

More particularly, in this prior system the four signals are matrixed or combined as follows: the left baseband channel signal, for convenience designated L equals (L;+ L,,); the right baseband channel signal R equals (R, R,,); the left carrier channel, designated L equals (L, L,,); and the right carrier channel, designated R equals (R, R,,). The sum signals (i.e., L and R are recorded as the basebands of the two channels and the carrier signals amplitude modulated by the difference signals (i.e., L and R are recorded as the high-frequency signals on the corresponding channels of a two-channel stereophonic record. Upon reproduction of the disc with a pickup having a sensitivity up to the upper frequency of the modulated carrier, and demodulation of the carrier signals, the reproduced signals are matrixed or combined as indica'ted below to obtain the four original channels: L]: L11 Ln R]: R11+ Rn b 11 'rz b Tl T2 While the record prepared by the described technique provides discrete output signals, it is not compatible" in the sense that it can not be reproduced satisfactorily on a conventional sterophonic phonograph because its pickup usually is unable to reproduce frequencies above about 20,000 Hz; thus, only the baseband channels would be reproduced and those recorded in the carrier mode would be lost. Moreover, it has been determined by listening tests as well as by theoretical considerations that reproduction of the baseband channels is not entirely satisfactory because upon stereophonic replay the pair of signals L, and L,, appears to be localized in one loudspeaker and the other pair of signals R, and R, appears to be localized at the other, instead of filling the space between them; thus, space perspective is not satisfactorily portrayed.

SUMMARY OF THE INVENTION It is the principal object of the present invention to provide improved apparatus for recording four discrete audio information signals on a two-channel disc record and apparatus for reproducing the same to deliver four discrete output signals, which record is also compatible" with existing sound systems in the sense that it can be acceptably reproduced on a conventional stereophonic phonograph, as well as on certain existing quadraphonic reproducing apparatus of the matrixed type to deliver four output sound signals, albeit not discrete signals, for presentation on respective loud-speakers.

This and other objects of the invention are attained by utilizing a technique similar to that described in the aforementioned application in the respect that carrier signals are used to obtain two additional recording channels, but differing therefrom in the manner in which the four discrete input signals are 'matrixed prior to recording. Instead of the simple sum-and-difference matrixing used in the prior system, four discrete inputsignals, for convenience designated L,, R,, L,, and R,,, are matrixed in two different matrix encoders each adapted to produce two composite signals each containing at least three of the input signals with preselected amplitude and phase relationships, at least one of the signals having a different phase in each of the composite signals. The amplitude and phase relationships are such that four combinations of the two composite signals will yield four separate signals in which respective ones of the four input signals are predominant. The two composite signals produced by one of the encoders are recorded at baseband on respective walls of the record groove, and the two composite signals from the other encoder, which have the justdescribed properties but have phase-relationships between the signals contained therein differing from those in the first pair of composite signals, are utilized to amplitude modulate respective carrier signals which are also recorded on the two walls of the record groove. Designating the two composite signals from one encoder L and R respectively, and those from the other encoder L and R respectively, the L signal from one encoder is recorded at baseband and a carrier modulated with the L signal from the other encoder are recorded on the left groove wall, and the R signal from the said one encoder is recorded at baseband and a carrier modulated with the R signal from the other encoder are both recorded on the right groove wall.

The apparatus for replaying the record includes a stereophonic pickup capable of reproducing the highest of the modulated carrier frequencies, suitable filters for separating the baseband frequencies from the carrier frequencies, means for detecting the modulation on the carriers, and a pair of matrix decoders, one for dematrixing the two encoded baseband frequency signals and the other, having different connections, for decoding the two encoded signals recovered from the carriers. Each of the decoders is operative to deliver four output signals in which respective ones of each of the four input signals are predominant and accompanied by two others of the input signals but at lower amplitude. Corresponding output signals from the two decoders are then combined in respective summing networks, and by virtue of the differences in the manner in which the two encoders initially matrixed the four input signals, and the two decoders have decoded the matrixed signals, the desired predominant signals are enhanced and the lower amplitude signals are canceled, thereby to produce four discrete output signals, corresponding in every way to the input signals, for application to four loudspeakers positioned at the left front, right front, left back and right back corners of a listening area.

In addition to its discrete channel performance when replayed on the just-described reproducing apparatus, the record produced in accordance with the invention is compatible" in the sense that it may be replayed on a conventional stereophonic phonograph having a pickup sensitive over only the baseband frequency range with results superior than is obtained from a conventional stereophonic record because of the presence ofback signal information in addition to the left and right front signals in the two composite signals recorded at baseband, and in that its baseband channels can be reproduced on existing matrix-type quadraphonic playback systems to provide a high degree of four channel realism.

Thereproducing apparatus of the invention is also compatible with existing quadraphonic disc records having recorded thereon composite signals matrixed in a manner compatible with the baseband decoder; in this situation, the second decoder for the composite signals derived from the carriers would not be operative.

Furthermore, while the invention is summarized above and will be described herein in connection with a disc record, it is equally applicable to other recording media and/or broadcasting or transmission systems normally having two channels but can, by carrier techniques, be converted to have four channels. In a more general sense then, the invention is useful for recording or transmitting multi-channel information over a multichannel system, with good reproduction, using but two of the channels thereof.

BRIEF DESCRIPTION OF THE DRAWINGS Other objects, features and advantages of the invention will be evident, and a better understanding of its construction and operation will be had from the follow ing description and accompanying drawings, in which:

FIGS. 1 and 2 are block diagrams illustrating preferred embodiments of two encoding matrixes for separately matrixing four discrete channels of audio information;

FIGS. 3 and 4 are block diagrams illustrating preferred embodiments of matrix decoders for deriving individual signal components from composite signals matrixed by the encoders of FIGS. 1 and 2, respectively;

FIG. 5 is a block diagram illustrating a preferred embodiment of a recording system arranged according to the invention for recording four channels of audio in formation on a two-channel disc record;

FIG. SA is a block diagram illustrating in more detail a portion of the system of FIG. 5;

FIG. 6 is a diagram illustrating the frequency distribution of the baseband and single sideband carrier signals recorded on one channel of a two-channel stereophonic record; and

FIG. 7 is a block diagram illustrating a preferred embodiment of a reproducing system arranged according to the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT four channels, the input signals L;, R,, L and R, are

Upon replay and demodulation the signals L R L and R become available for recombination to recover the original signals. It is well known that in every case where equations of this form are linearly independent, they can be solved to retrieve the four original signals in the following form:

L? 11 41 iz ri l3 7'2 14 T2 It will be evident that in the matrixing technique utilized in the co-pending application, the a" coefficients in equations 1 through 4 are as follows: Eq. (1) 1, l, 0, Eq. (2) 1,l, O, 0; Eq. (3) 0, 0, I, 1; and Eq. (4) 0, 0, l,l. Solution ofequations l (2) and (4) reveals that the b" coefficients in Eq. are :z r0,0;in Eq. (6) k. %0, 0; Eq. (7)0, 0, A95; and Eq. (8) O, O, re-b. Substituting equations (1) to (4) into equations (5) to (8) using these coefficients produces the four original signals L,, R,, L, and R The principle represented by this analysis is extended and utilized in the present invention.

In a preferred embodiment of a four-channel recording system arranged according to the present invention, as shown in FIG. 5, a pair of matrix circuits l0 and 12 each receive four audio signals from a suitable fourchannel source, such as a master tape (not shown) or four individual microphones, via input terminals l4, l6, l8 and 20, respectively. The input signals are designated left front (L,), left back (L right front (R,) and right back (R,,) and indicate the location in a listening area in which they are to be reproduced.

The matrix circuits and 12, illustrated in FIGS. 1 and 2, respectively, are very similar in construction and operation and embody the invention described in appli cants co-pending application Ser. No. 124,135, filed Mar. 15, 1971 and assigned to the assignee of the present application. The encoder of FIG. 1 has four input terminals 14', 16, 18' and 20' (they correspond to the input terminals of the recording system of FIG. 5) to which the four signals L,, L R and R,, depicted as inphase signals (when of the same frequency) and of equal amplitude, are respectively applied. With the switches S and S in the positions indicated, the full L, signal is added in a summingjunction 22 to 0.707 of the R signal, the output of the summing junction being applied to a phase-shifting network 24 which introduces a reference phase-shift 111 which varies continuously with frequency. The full R, signal at terminal 20' is added in a second summing network 26 to 0.707 of the L, signal appearing at input terminal 16, and the output is passed through a second ill-network 28 which also provides the reference phase-shift 41. The L,, and R, signals are also applied to respective ill-networks 30 and 32, each of which provides a phase ahift of III-90 and wherein the all-functions are essentially the same. The

7 full signal appearing at the output of network 24 is output terminal 36 a composite signal designated L Similarly, the full signal from network 28 is added in summing junction 38 to O.707 of the signal from network 32 to produce at its output terminal 40 a composite signal designated R The composite signals L and R are conveniently characterized by the phasor groups 42 and 44, respectively, and may be recorded on a two-track tape recorder in the form of encoded signals for a subsequent'recording on a disc record, or they may be encoded directly on a disc record using a conventional stereophonic record cutter.

' It will be observed from examination of phasor groups 42 and 44 that encoder 10 positions the full L,

and R, signals in phase with each other, causes the R components in the two composite signals to be substantially in quadrature with each other, and the L,, components in the two composite signals to also be substantially in quadrature with each other. Additionally, the L, and R, signals at output terminal 36 lead and lag, respectively, the L and R, signals at output terminal 40, and the R signal component is aligned with and in phase with the L, signal at terminal 36 and the L, signal component is aligned with but is out-ofphase with the R, signal in phasor group 44.

By slight modification of the encoder of FIG. 1, illustrated in FIG. 2, the four input signals can be encoded in a different manner whereby the phasors representing its composite output signals are the complements of the corresponding phasors representing the composite signals delivered by the FIG. 1 encoder. More specifically, the full L, signal is added in a summing junction 46 to -0.707 of the R signal, the output of the summing junction being applied to a phase-shifting network 48 which introduces the reference phase-shift 41, which has the same function as in the encoder of FIG. 1. The full R, signal at terminal 20" is added in summing network 50 to 0.707 of the L,, signal at input terminal 16" and the output is passed through the ill-network 52, which also provides the reference phase-shift 111. The L, and R, signals are also applied to respective Ii-networks 54 and 56, each of which provides a phase-shift of ll-. The full signal appearing at the output of network 48 is added in a summing network 58 to 0.707 of the signal appearing at the output of network S4to produce at its output terminal 60 a composite signal designated L Similarly, the full signal from network 52 is added in summing junction 62 to 0.707 of the signal from network 56, to produce at its output terminal 64 a composite signal designated R As with the encoder of FIG. 1, in the phasor groups 66 and 68 representing the composite signals L and R respectively, the L, and R, signals are in phase and each have unity value, the L and R signal components in one of the composite signals are in substantial phase quadrature with the corresponding signal components in the other composite signal, the L and R, signals at output terminal 60 lag and lead, respectively, the L, and R,, signals at terminal 64, the R signal in phasor group 66 is aligned with but out-of-phase with the L, signal, and in phasor group 68 the L, signal is aligned with and in phase with the R, signal.

As will be seen from the description to follow, it is the differences in the relative positions of the phasors representing the composite left and right" signals delivered by the two different encoders, and the recognition that by decoding of the composite signals produced by each and appropriate combination of the decoded signals, it is possible to derive the signals L,, L,, R, and R, as discrete, completely independent signals. Although such decoding and combining occurs in the reproducing apparatus it will be ,helpful to an understanding of the invention to here describe the decoding technique, before continuing with the description of the recording system of FIG. 5.

Essential to achieving the objects of the invention was applicants recognition that by utilizing appropriate matrix decoders of the type described in his aforementioned co-pending application Ser. No. 124,135 to decode the signals produced by the encoders of FIGS. 1 and 2 it is possible to obtain two sets of four decoded signals wherein the primary signals L L R and R, are identical in both sets while their corresponding accompanying signals are equal and out-of-phase whereby when corresponding decoded signals of the two sets are added the desired signals are augmented and the accompanying signals are canceled to thereby obtain a set of four output signals corresponding in every way to the original signals supplied to the encoders. To this end, the decoder illustrated in FIG. 3 is used to decode the composite signals L and R produced by encoder l0, and the decoder illustrated in FIG. 4, which closely resembles the circuit of FIG. 3, is used to decode the composite signals L and R produced by encoder 12. The decoder of FIG. 3, which is also described in applicants co-pending application Ser. No. 118,272 filed Feb. 24, I971, includes a pair of input terminals 70 and 72 to which the composite signals L and R are respectively applied. The signal applied to terminal 70 is applied in parallel to and phase-shifted by a pair of IIJ-IIEIWOI'ICS 74 and 76, and thecomposite R signal applied to input terminal 72 is applied in parallel to iii-networks 78 and 80. These ill-networks are of the type previously described, the networks 74 and 80 introducing a reference phase-shift of ill and the networks 76 and 78 introducing a phase-shift of Ill-90 1 The output signals from networks 74 and 80 are respectively applied directly to the left front" output terminal 82 and to the right front output terminal 84. Equal portions of the outputs of networks 74.and 78 are summed in a summing junction 86, the output of which is applied to the right back output terminal 88, and equal portions of the outputs of networks 76 and 80 are inverted and added in a second summing network 90, the output of which is applied to the left back output terminal 92. As a result of the described phase-shifting and summing action four decoded signals designated L' L' R,, and R',, appear at output termainls 82, 92, 88 and 84, respectively, and have the composition depicted by phasor diagrams 94, 96, 98 and 100, respectively. It is seen that the predominant signals in the four decoded signals, namely, L,, L,,, R, and R,, have the same relative amplitude and phase as the corresponding signals applied to the encoder of FIG. 1, but that the predominant front signals are accompanied by reduced amplitude signals from the back pair of channels and the predominant back" signals are accompanied by reduced amplitude signals from the front pair of channels.

The decoder of FIG. 4 likewise has a pair of input terminals 102 and 104 to which the composite signals L and R initially encoded by endoder 12 are respectively applied. The L signal is applied in parallel to a pair of phase-shifting networks 106 and 108 which respectively introduce a reference phase shift 11; and a phase shift of ill-, and the R signal is applied in parallel to a pair of similar phase-shift networks and 112. As in the decoder of FIG. 3, the outputs of phaseshift networks 106 and 112 are respectively applied directly to the left front output terminal 114 and the right front output terminal 116. Equal portions of the outputs of networks 108 and 112 are summed in a summing junction 118, the output of which is applied to the left back output terminal 120, and equal portions of the outputs of networks 106 and 110 are inverted and summed in a second summing network 122, the output of which is aplied to the right back output terminal 124. Again, the four decoded signals appearing at the output terminals, represented by phasor groups 126, 128, and 132, respectively contain predominant signal components L';, L',, R, and R,, each accompanied by reduced amplitude signals from two other channels. Again, also, the predominant signals have the same relative amplitude and phase relationship that they exhibited at the input terminals of the encoder of FIG. 2. 7

Inspection of the two sets of phasor groups from the two encoders reveals that the predominant signals L,, L,, R and R, in both are identical in relative phase and amplitude to the corresponding signals at the input terminals of their respective encoders, but that their corresponding accompanying signals are equal and out-ofphase. For example, comparison of phasor groups 94 and 126 shows that the L; signal in both is equal and in phase, whereas the signals 0.70711, and 0.707L' in one are equal and out-of-phase with corresponding components in the other. Therefore, when the signal at terminal 82 of the FIG. 3 decoder is added to the signal at terminal 114 of the decoder of FIG. 4, the desired L, signal will be augmented (doubled) and the undesired accompanying signals 0.707L' and 0.707R', will be canceled. The same is true of the composite signals appearing at output terminals 92, 88 and 84 of the decoder of FIG. 3 and the corresponding signals at the output terminals 120, 124 and 116 of the decoder of FIG. 4. Thus, by matrixing the four orignal input signals with the differing encoders of FIGS. 1 and 2, and decoding the composite signals produced thereby with the differing decoders in FIGS. 3 and 4, respectively, and by combining the outputs of the two decoders in an additive mode, a set of four output signals is obtained which in every way correspond to the orignal signals supplied to the encoders; that is, signals which are completely independent of each other and contain the orignal directional information.

While two encoder circuits have been separately illustrated in FIGS. 1 and 2, and two decoder circuits have been shown in FIGS. 3 and 4, to graphically show how the signals are matrixed and dematrixed to attain the objects of the invention, inspection of these figures will show that the encoder of FIG. 2 is the morror image of the encoder of FIG. 1 and that the decoder of FIG. 4 is the mirror image of the decoder of FIG. 3. Stated another way, in the case of the encoders, if the four input signals are applied to the input terminals of the FIG. 1 encoder in the reverse order from that shown therein (i.e., with the L,, L,, R, and R signals applied to input terminals 20', 18', 16 and 14', respectively) the resultant composite signals L and R at output terminals 36 and 40 will have the characteristics depicted by phasor groups 66 and 68, respectively (FIG. 2). Conversely, if the four input signals are applied to the input terminals of the FIG. 2.in reverse order to that shown, the composite signals will be as depicted by phasor groups 42 and 44 (FIG. 1). Thus, the encoders l and 12 in the system of FIG. may be identical circuits with merely the order in which the input signals are applied and the output signals obtained differing. The encoders being readily implementable in integrated circuit form, this feature is of considerable importance in production in that one chip design is needed for the two different encoding functions.

Likewise, a single circuit, which may also be readily fabricated in integrated circuit chip form, can serve as either of the two decoders illustrated in FIGS. 3 and 4, simply by the choice of input terminals to which the two composite signals are applied. That is, if in the circuit of FIG. 3 the input signals L and R are reversed, the output signals at terminals 82, 92, 88 and 84 would be as depicted by phasor groups 126, 128, 130 and 132, respectively (FIG. 4). Similarly, reversal of the input signals to the FIG. 4 matrix would result in decoded signals corresponding to those illustrated in FIG. 3.

Continuing now the description of the recording system of FIG. 5, the composite signals L and R appearing at the output terminals of encoder are amplified by respective amplifiers 150 and 152 and filtered with respective filter networks 154 and 156, each of which has a high-frequency cutoff at a frequency f,, which is at or near the highest audio frequency of interest, typically 15,000 Hz. The output of filters 154 and 156 are respectively passed through suitable delay networks 158 and 160, the purpose of which will be described later, and applied to the left and right input terminals, respectively, ofa stereophonic cutter 162 in cooperative relationship with a disc 164. Thus, the two composite signals delivered by matrix 10 are recorded on the two walls of the disc groove in a manner entirely similar to that used in making a conventional matrixed stereophonic disc as more fully described in applicants aforementioned application and in his co-pending application Ser. No. 44,224 filed June 8, 1970, now abandoned. The signals L, and R, cause stylus motion at 90 relative to each other, these motions being identical to those found in a conventional stereophonic record. The quadrature and leading and lagging relationship of the L and R,, signals in the two composite signals results in the back" channels being recorded in the form of clockwise and counter-clockwise helixes. Thus, the right-front and left-front channels have a theoretically infinite separation as with a conventional stereophonic record, while the left-back channel and the right-back channel appear more or less centered and somewhat dispersed between the two loudspeakers when reproduced on a stereophonic player. And, of course, the described recording of the back channel information provides the capability of admitting to a decoding operation to convert the signals back into a discrete fourchannel program.

The L and R signals delivered by encoder 12 are recorded in the left and right channels, respectively, as modulation on respective carrier signals. These two composite signals are first amplified by respective amplifiers 170 and 172 and then applied to respective modulators 174 and 176, each of which may have a self-contained source of carrier signal or, as illustated, they may be energized by an external carrier frequency generator 178. The function of the modulators 174 and 176, the operation of which will be described in more detail later, is to translate the frequency of the signals applied to the encoder 12 to a frequency domain above the baseband spectrum. Typically, the carrier frequency is of the order of 20 KHz and is amplitude modulated from about 5 to 35 KHz. The modulated carriers delivered by the two modulators are passed through respective filters and 182, each designed to reject the lower sideband (i.e., frequencies below a lower cutoff frequency f leaving essentially only the carrier and the upper sideband. The signals passed by the filters are delayed by respective delay networks 184 and 186 and then combined with the output signals from delay networks 158 and 160, respectively, for application to the cutter 162. The delay networks 158, 160, 184 and 186 are provided to equalize delays present in the four paths of the system due to the modulation process and filtering so that the signals recorded on the disc bear a relative time-domain relationship such that upon replay they can be decoded and recombined with a minimum of time-delay error.

Referring now to FIG. 5A, which illustrates in more detail a refinement of the modulatiing process shown in FIG. 5, and considering one of the modulators, say modulator 174, after amplification, the L signal is initially applied to an equalizer circuit 190 which attenuates the components of the composite signal ranging in frequency from about 20 Hz to about 1 KI-Iz to compensate both for the fact that some of the lower sideband signals is transmitted to the output of the processing circuitry because of the method employed for producing single sideband, and to improve the signal-tonoise ratio of the composite signal upon playback. The signal-to-noise ratio of the composite signal is improved by pre-emphasizing the high frequency components of the L signal (in which the spectral energy is less than for mid-range frequencies) and correspondingly de-emphasizing the signal during playback so that a flat frequency response is attained and the record noise subsequently attenuated. In a preferred embodiment, single sideband is generated by passing a double sideband signal with carrier through a high-pass filter having a cutoff frequency equal to the carrier frequency. Consequently, the audio level for low frequencies must be attenuated with respect to the high frequencies so that the modulation percentage in the double sideband region is the same as in the single sideband region for constant level input. These signals, ranging in frequency from about 17 KHz to 20 KHz will be transmitted, albeit with attenuation.

From the equalizer circuit 190 the signal L is applied to an envelope processor citcuit 192 which includes an amplitude modulator 194, a high pass filter 196, a full wave detector 198 and an inverter 200. As will have become evident, the present invention utilizes a single sideband modulator to record the signal L on the same channel (left) of the stereophonic record groove that the signal L is recorded. As is understood in the art, when a sideband is removed from an amplitude modulated carrier signal to develop a single sideband signal with carrier, the envelope of the single sideband signal is in the shape of a poid and thus represents a distorted replica of the modulating signal. In'

order to compensate for such distortion of the envelope of the single sideband signal, it is desirable to predistort the modulating signal in the processor 192.

Within the processor 192, therefore, the signal L is applied to one input terminal of the amplitude modulator 194 wherein the signal modulates a carrier signal having a frequency of the order of 20 KHZ. The developed modulated carrier signal is then applied to the high-pass filter 196 having a cutoff frequency of about 20 KHz which rejects at rapid rate frequencies below 20 KHz and passes without attenuation the upper sideband of the modulated carrier signal. The transmitted sideband signal is then applied to the full wave detector circuit 198 which comprises, for example, a full wave rectifier and low-pass filter and, accordingly, detects only the envelope of the transmitted sideband signal. Thus, the detected signal constitutes a distorted replica of the L signal.

The inverter 200 inverts the detected and distorted L signal and applies the inverted signal to one input terminal of an amplitude modulator 202, the first component in single sideband modulator 174. In the modulator 174, the inverted and distorted L signal modulates a 20 KHz carrier signal applied from the source 178, for example, to the other input terninal of the modulator. A high-pass filter 180 having a cutoff frequency f of about 20 KHz transmits only the upper sideband of the amplitide modulated carrier signal. By virtue of the inherent distortion in the modulation process, the envelope of the transmitted upper sideband signal is distorted with the result that the previous distortion is canceled and the envelope of the resultant signal is an exact replica of the signal L From the filter 180, the upper sideband signal with carrier may be amplified by a buffer amplifier 204, the output signal from which is applied to the input terminal of delay network 184. It will be understood that the other encoded signal, R is also processed by a separate like envelope processor and single sideband modulator prior to application to delay network 186.

Referring now to HO. 6, there is shown the frequency distribution of the L and L signals, the former being recorded as a regular audio signal and the latter being recorded as single sideband modulation with carrier C in the left channel of the record groove. The total bandwidth extends from about 30 Hz to 35 KHz. The distribution of' the frequency components of the right channel signal is identical to that shown for the left channel. Thus, the resulting disc has a pair of matrix-encoded composite signals recorded at baseband, and a pair of carriers single sideband modulated with differently encoded but related composite signals, the relationship between the coding of the two sets of composite signals being such that upon replay and decoding, the resulting two sets of four output signals each when combined produce four discrete signals corresponding to the input signals applied to the encoders.

In a preferred embodiment of a four-channel record reproducing system arranged according to the present invention, as shown in FIG. 7, a two-channel disc phonograph pickup transducer 210 includes a stylus 212 that responds to the modulations in the groove of a record disc recorded in the above-described manner to supply the L signal and the L single sideband signal with carrier, and the R signal together with the R single sideband signal with carrier, along a pair of conductors 214 and 216, respectively, to a pair of preamplifiers 218 and 220, respectively. The stereophonic pickup has a good frequency response up to the highest frequency of interest of the modulated carrier recorded on the record,namely, up to about 35 KHz. The output signal from preamplifier 218 is applied in parallel to a low-pass filter 222 designed to transmit audio frequencies up to a frequency f which is at or near the upper frequency of the baseband signals being reproduced, and to a high-pass filter 224 designed to transmit frequencies above the frequency f which, typically, includes a part of the carrier and all of the upper sideband of the modulated signal. The signal from preamplifier 220 is likewise applied in parallel to a low-pass filter 226 and a high-pass filter 228, each having characteristics corresponding to the characteristics of justdescribed filters 222 and 224, respectively.

The signals transmitted by low-pass filters 222 and 226, namely, L and R are applied to the input terminals and 72, respectively, of a decoder 230, having the configuration illustrated in FlG. 3, which decodes the composite signals in the manner described earlier to produce at the four output terminals thereof the signals L' L' R' and R',, containing the signal components depicted by phasor groups 94, 96, 98 and 100, respectively (FIG. 3).

The output signals from high-pass filters 224 and 228 are applied to respective detectors 232 and 234 which are operative to detect the modulation on the carriers and recover the original encoded composite signals L and R respectively. The output signals L and R from the detectors are applied to input terminals 102 and 104, respectively, of a second decoder 236, having the configuration illustrated in FIG. 4, which is operative to decode the composite signals in the manner earlier described to produce at its four output terminals the signals L' L' R and R',, containing the signal components depicted by phasor groups and 132, respectively (FIG. 4).

The signals at corresponding output terminals of decoders 230 and 236 are then summed at respective summing junctions 238, 240, 242 and 244, in the manner described earlier in connection with FIGS. 3 and 4, to yield at the output conductors 246, 248, 250 and 252 four discrete signals L,, L, R and R,, respectively. These four signals may then be applied, with or without further amplification, to four separate loudspeakers (not shown) positioned at the left front, left back, right back and right front corners, respectively, of a listening area to reproduce sounds corresponding in every way to the original signals applied to the encoders to provide completely independent transmission of the four channels.

It will have become evident that a disc record recorded in accordance with the invention, and the reproducing apparatus illustrated in FIG. 7, offer the important advantage that the record can be satisfactorily reproduced with other existing types of playback equipment, and that the reproducing apparatus is capable of reproducing existing records recorded in other modes. Considering the record first, as has been mentioned earlier, it can be replayed on a conventional stereophonic phonograph because of the presence in the encoded baseband signals of all left and right front signals so encoded as to provide a satisfactory stereophonic presentation on two loud-speakers. The record can also be replayed on existing stereo-quadraphonic playback apparatus of the kind illustrated in aforementioned application Ser. No. 124,135 which includes a single matrix decoder having the construction illustrated in FIG. 3 and capable of decoding the composite baseband signals to derive four output signals in which the L,, L,,, R, and R, signals are respectively predominant. It follows that the record can also be reproduced on a stereo-quadraphonic logic decoder of the kind also illustrated in application Ser. No. 124,135 which includes in addition to a matrix decoder, separate gain control amplifiers for the four output signals from the decoder and a logic and control circuit operative in response to the signals instantaneously present to adjust the gains of the amplifiers to enhance channel separation and thus the realism of four channel reproduction.

Conversely, a conventional stereophonic record will be satisfactorily reproduced on the playback apparatus of FIG. 7; in this case the left and right signals are transduced and simply passed by low-pass filters 222 and 226 to the input terminals of decoder 230 which, because of the direct connections (except for the reference phase shift) of the input terminals to the left-front and right-front output terminals, will deliver them to the appropriate loudspeakers. Likewise, existing stereo-quadraphonic records recorded in accordance with the teaching of application Ser. No. 124,135 (i.e., using an encoder of the kind shown in FIG. 1), and being distributed by Columbia Records, when replayed on the apparatus of FIG. 7 will produce at the output terminals of decoder 230 four output signals in which Ly, L R, and R signals are respectively predominant.

Although not required for playback of records recorded in accordance with the present invention, the capability of the playback apparatus of FIG. 7 to reproduce existing stero-quadraphonic records may be extended by further including gain control amplifiers 260, 262, 264, and 266 in the output lines 246, 248, 250 and 252, respectively, and logic and control circuitry 268 of the kind shown in application Ser. No. 124,135 for example, for adjusting the gains of the amplifiers to enhance channel separation. The logic and control circuit is operative in response to signals derived from decoder 230 to deliver appropriate control signals to the control electrodes of the gain control amplifiers.

Inasmuch as the just-described logic control is desired only when replaying existing stereo-quadraphonic records on which the encoded signals are recorded at baseband, the system of FIG. 7 further includes means fpr automatically disabling the logic circuit 268 when a record according to the present invention is being played and for activating the logic when the existing type of stereo-quadraphonic record is being played. To this end, the output terminals of high pass filters 224 and 228 are connected to the input terminals of respective isolating amplifiers 270 and 272, the output signals from which are rectified in respective rectifiers 274 and 276. The signals delivered by the rectifiers are summed in a summing or combining junction 278, the output of which is connected through the actuating coil 280 of a relay 282. The contact 284 of the relay is connected in series with suitable circuitry within logic and control circuit 268 which is operative when the contact is closed to actuate the logic. Thus, if an existing type of stereo-quadraphonic record is being played, the absence of any significant carrier signal output from filters 224 and 228 will cause the relay contact to remain closed and the logic to function in the manner described earlier.

However, when a record according to the present invention is played, the amplified and rectified carrier signals passed by filters 224 and 228 cause sufficient current to flow in relay coil 280 to open the contact 284 and disable the logic to prevent application of variable control signals to the gain control amplifiers 260 and 266 and instead establish a constant bias on the amplifiers so that they function as fixed gain amplifiers. In this case, then, the two decoders operate in the manner described earlier to produce discrete or independent output channels L,, L,,, R and R, at the output terminals of the respective amplifiers.

It will be evident to ones skilled in the art that the described carrier-actuated relay may be refined by the use of tuned circuits to avoid operation of the relay merely by surface noise or extraneous signals that may be present in the record. Also, it is advisable to design the rectifiers 274 and 276 to have attack and release time constants to preclude actuation of the relay by spurious pulses that may be found on the record. It will be evident, too, that the described carrier-actuated switch may also be used to adjust the gain of amplifiers 260, 262, 264 and 266 in a predetermined manner so as to accomodate the level of response of the system to the needs of the various types of records that may be played.

Although the invention has been described as utilizing combinations of encoders and decoders of specific design, it is possible to modify the encoders, for example, without changing the decoders, and vice versa, and still realize the benefits of the invention. For example, by switching the switches S and S in the encoders of FIGS. 1 and 2 to the other contact from that shown, the full left-front and right-front signals are caused to bypass their respective dJ-networks and are applied directly to an input terminal of summing junction 34 and 38 (FIG. 1) or 58 and 62 (FIG. 2), respectively. The L,, and R, signals are applied (in the case of the FIG. 1 encoder) to summing networks 22 and 26, ill-networks 30 and 32 and summing junctions 34 and 38, as before.

The effect of this modification is illustrated in the phasor diagrams 42 and 44 associated with FIG. 1 and in phasor diagrams 64 and 66 associated with FIG. 2. Considering the phasor diagrams 42 and 44, it is seen that when the L, and R, signals by-pass their respective ill-networks 24 and 28, these phasors L, and R,in effect are moved in a leading direction by the phase angle #:(f), which for a given frequency may introduce the illustrated phase shift ill". Since the ill-functions in all of the ip-networks are substantially the same, the resulting phasors L, and R, in phasor groups 66 and 68, therefore remain in the same relative phase relationship with respect to each other, but are shifted relative to the phasors 0.707R and 0.707L,, in said phasor groups, the latter two sets of phasors, however, being unaffected.

This seemingly minor modification provides significant improvement in certain recording environments. For example, a center total signal applied equally to all of the input terminals l4, l6, l8 and 20 of the encoder of FIG. 1 with the switches in the positions illustrated would if applied directly to a four-channel reproduction system, appear as an overhead signal; thus, a corresponding signal recorded for two-channel stereophonic reproduction should appear as a generally center signal in the stereophonic field. However, with the unmodified FIG. 1 encoder it can be shown that a center total" signal at input terminal 14 is characterized by the addition of the three fundamental phasors L O.707R,,, and 0.707L which results in a relatively large center total signal in the left" channel. Similarly, at the output terminal 40 the center total signal is made up of the phasor sum of the three basic signals R 0.707R and 0.707L,,, which is smaller than that appearing at output terminal 36. Thus under the circumstances mentioned above, instead of obtaining two signals of generally equal magnitudes, the encoder of FIG. 1 (unmodified) produces two composite signals of generally unequal magnitudes. By-passing the ill-networks 24 and 28 (by switching switches S and S to the other position) in essence introduces a differential reference function tb" between the front signals L, and R; and the rear signals R and L Therefore, the phasors corresponding to these sets of signals are positioned differently with respect to each other at different frequencies, resulting in an action similar to that of a comb filter in equalizing the magnitudes of the phasors resulting from application of a center total signal equally to all four input terminals of the encoder. 7

Referring now to the phasor diagrams in FIGS. 3 and 4, it is seen that if all of the phasors L,, R,, 0.707L, and 0.707R, therein are shifted in phase by the same angle 11:", as shown in FIGS. 1 and 2, their relative position in the phasor groups will be changed with respect to the phasors L,,, R,,, 0.707L and 0.707R by the same phase angle ill", but since all of the phasors will have been shifted simultaneously (as shown by the dashed line phasors) by the same angle ill" (omitted for clarity in FIGS. 3 and 4), the positions of the phasors L,, R,, 0.707L, and 0.707R, are relatively unchanged. Because of this relatively unchanged position of these phasors, when the two sets of four output signals appearing at output terminals of the FIG. 3 and FIG. 4 decoders are summed together in the manner shown in FIG. 7, the transferred signal components 0.707 L, and 0.707L will still cancel each other. Therefore, the illustrated modification of the encoder will not interfere with the operation of the overall system. The essential criteria is that the coding sequences of the two pairs of encoded signals be complementary so as to permit positioning for cancellation of the unwanted signals when the two sets of decoded signals are added together or otherwise respond to the theory described in connection with equations (1 )-(8).

Although the recording and playback apparatus described above utilize the single sideband modulation principle, other types of modulation can be used to modulate the carrier. For example, it is possible to use conventional (double sideband) amplitude modulation, in which the carrier would be placed at about 35 KHz with the sidebands extending from to 50 KHz. Also, frequency or phase methods of modulation, or other forms of modulation, could be used without departing from the spirit of the invention.

it should also be noted that the switchable encoder arrangement shown in FIG. l,for example, may be used to record stereo-quadraphonic records of the existing type (non-carrier) in the manner described in aforementioned application Ser. No. 124,135.

Furthermore, the method and apparatus disclosed herein is equally applicable to the transmission of discrete independent channels over suitable broadcasting facilities, while retaining all the benefits of the compatible quadraphonic matrix system herein described. The implementation of this method is carried out, for example, by encoding the four channels with the encoder of FIG. 1 for transmission over the broadcasting channels which are received by conventional stereophonic reception systems, e.g., an F.M.-multiplex receiver, and by encoding again the four channels using the encoder of FIG. 2 for transmission on two additional channels or sub-carriers which are received by an auxiliary receiver circuit. Upon detection and rematrixing in a manner similar to that shown in FIG. 7 four discrete output signals are produced. Other modifications of this alternative application of the invention will be evi dent to those skilled in the art.

I claim: 1. A sound system for transmitting four input audio information signals designated L;, L, R,, and R, on a transmission medium having first and second channels each of which has a baseband frequency channel and a modulated carrier channel and for reproducing corresponding substantially identical audio information output signals, comprising, incombination, an encoder and a decoder, said encoder comprising means for coupling to said first channel of said medium a first combined signal containing a first composite signal at baseband frequency containing said L L and R,, signals to the extent they are present and a carrier signal modulated by a second composite signal containing said L,, L and R signals, the L and R, signals in said first composite signal being in phase opposition with corresponding sig nals in said second composite signal, and means for coupling to said second channel of said medium a second combined signal containing a third composite signal at said baseband frequency containing said R L, and R signals to the extent they are present and a carrier signal modulated by a fourth composite signal containing said R L and R signals, the L,, and R signals in said third composite signal being in phase opposition with the corresponding signals in said fourth composite signal, and said decoder comprising filter means for separating said first and said third composite signals at baseband frequency from the modulated carrier signals in each of said first and second combined signals,

first and second detector means for respectively detecting the modulation from the modulated carrier signals from said first and second combined signals and operative to derive said second and fourth composite signals, respectively,

first decoding means to which said separated first and third composite signals are applied,

second decoding means to which said derived second and fourth composite signals are applied,

said first and second decoding means each being operative in response to the signals applied thereto to derive four output signals respectively containing dominant signal components L R L,, and R each accompanied by sub-dominant signals, and

means for combining the four output signals from said first decoding means with the corresponding four output signals from said second decoding means and operative to cancel said sub-dominant signals and to produce four discrete output signals L], L R and R 2. A sound system according to claim 1 wherein said L, and R signals in said first and second composite signals are substantially in quadrature with the L and R signals in said third and fourth composite signals, respectively.

3. A sound system according to claim 2 wherein the amplitudes of said L, and R signals in said composite signals are reduced by a factor of approximately 0.707 relative to the amplitudes of said L, and R, signals.

4. A sound system according to claim 3 wherein said L, and R, signals in said composite signals are substantially in phase with each other.

5. A sound system according to claim 3 wherein at least one of said L and R signals is either in phase or in phase opposition with the dominant signal in each of said first, second, third and fourth composite signals.

6. For use with a sound system wherein four discrete audio information signals designated L,, L,,, R, and R, to the extent they are present are recorded or transmitted on first and second channels of a two-channel medium each channel of which has a baseband frequency channel and a modulated carrier channel as first and second combined signals, respectively, the first combined signal containing a first composite signal at baseband frequency containing said L L and R signals and a carrier signal modulated by a second composite signal containing said L,, L, and R, signals, the L and R signals in said first composite signal being in phase opposition with corresponding signals in said second composite signal, and the second combined signal containing a third composite signal at baseband frequency containing said R,, L, and R,, signals and a carrier signal modulated by a fourth composite signal containing said R,, L,, and R signals, the L, and R signals in said third composite signal being in phase opposition with the corresponding signals in said fourth composite signals, a decoder for recovering said four audio information signals comprising, in combination,

filter means for separating said first and third composite signals at baseband frequency from the modulated carrier signals in each of said first and second combined signals,

first and second detector means for respectively detecting the modulation from the modulated carrier signals from said first and second combined signals to derive said second and fourth composite signals, respectively,

first decoding means to which said separated first and third composite signals are applied,

second decoding means to which said derived second and fourth composite signals are applied, said first and second decoding means each being operative in response to the signals applied thereto to derive four output signals respectively containing dominant signal componentsl R;, L, and R each accompanied by sub-dominant signals, and

combining means for combining the four output signals from said first decoding means with the corresponding four output signals from said second decoding means and operative to cancel said subdominant signals and to produce four discrete output signals L,, L,,, R and R 7. Apparatus in accordance with claim 6 wherein said filter means comprises first and second low-pass filters each having a cutoff frequency at approximately the highest baseband frequency for respectively transmitting said first and third composite signals at baseband frequency, and

for respectively transmitting the modulated carrier signals in said first and second combined signals.

8. Apparatus according to claim 7 wherein said carrier signals are amplitude modulated and said first and second detector means are amplitude modulation detectors respectively connected to receive the signals transmitted by said first and second high-pass filters.

9. Apparatus in accordance with claim 8 further including first, second, third and fourth gain control means to which the four output signals from said combining means are respectively applied,

control circuit means operative in response to said separated first and third composite signals to selectively control the gains of said gain control means, and

means operative in response to the presence of carrier signals at the outputs of said first and second high-pass filters to disable said control circuit, 10. A method for recording four audio information input signals on a two-channel stereophonic disc record, and for reproducing said four audio information signals as discrete signals, comprising,

encoding said four input signals by combining them in preselected amplitude and phase relationships to form first and second composite signals at baseband frequency each including at least three of said input signals, encoding said four input signals by combining them in preselected amplitude and phase relationships to form third and fourth composite signals said third and fourth composite signals respectively including the same at least three of said input signals as are included in said first and second composite signal,

modulating first and second carrier frequency signals with said third and fourth composite signals, respectively,

recording said first composite signal at baseband frequency and said first modulated carrier signal on one channel of a stereophonic disc record and recording said second composite signal at baseband frequency and said second modulated carrier signal on the other channel of said stereophonic disc record;

transducing from the stereophonic disc record first and second combined signals recorded on the respective channels thereof,

separating said first and third composite signals at baseband frequency from the modulated carrier signals in each of said first and second combined signals,

detecting the modulation from the separated modulated carrier signals to derive said second and fourth composite signals, respectively,

decoding said separated first and third composite signals to produce a first set of four composite output signals containing dominant signal components corresponding to respective ones of said four input signals each accompanied by sub-dominant signals, decoding said derived second and fourth output signals to produce a second set of four composite output signals containing dominant signal components corresponding to respective ones of said four input signals each accompanied by the same subdominant signals as accompany corresponding ones of the four output signals resulting from decoding said first and third composite signals but in phase relationships such that the sub-dominant signal components contained in the four output signals of the first set are in phase opposition with corresponding sub-dominant signal components contained in corresponding ones of the four output signals of the second set, and combining the four output signals of said first set with the corresponding four output signals of said second set for canceling said sub-dominant signals and producing four discrete output signals corresponding to respective ones of said four input signals.

1 1. In a sound system for transmitting a multichannel program including first, second, third and fourth program signals to the extent they are present to respective separate loudspeakers on a transmission medium having first and second channels each of which has a baseband frequency channel and a modulated carrier channel, the combination comprising:

first and second matrix encoders each having first,

second, third and fourth input terminals to which said first, second, third and fourth signals are respectively applied and first and second output terminals, each said encoder including means connected between said first and fourth input terminals and said first and second output terminals, respectively, operative to transfer substantially equal amplitude proportions of said first and fourth signals to said first and second output terminals, respectively, and

means connected between both said second and third input terminals and both said first and second output terminals operative to transfer substantially equal reduced amplitude proportions of said second and third signals to both of said first and second output terminals and including phase-shifting networks operative to provide a substantially constant differential phase-shift angle between said second and third signals at said output terminals over the frequency range of interest;

first means for coupling to the first channel of said transmission medium a first combined signal containing a first composite signal from the first output terminal of said first matrix encoder at baseband frequency and a carrier signal modulated by a second composite signal from the first output terminal of said second matrix encoder; and

second means for coupling to the second channel of said transmission medium a second combined signal containing a third composite signal from the second output terminal of said first matrix encoder at baseband frequency and a carrier signal modulated by a fourth composite signal from the second output terminal of said second matrix encoder.

12. Apparatus according to claim 11 wherein said differential phase-shift angle has a value of substantially 90 and said second signal at one of the output terminals of each of said matrix encoders leads said second signal at the other output terminal of the respective matrix encoder and said third signal at said one output terminal of each encoder lags said. third signal at the other output terminal of the respective matrix encoder.

13. Apparatus in accordance with claim 12, wherein said second and third signals in said first and third composite signals are substantially in phase opposition with the second and third signals in said second and fourth composite signals, respectively.

14. Apparatus in accordance with claim 13, wherein said first and second signals in said first, second, third and fourth composite signals are substantially in phase with each other.

15. Apparatus in accordance with claim 14,,wherein the amplitudes of said second and third signals in said composite signals are reduced by a factor of substantially 0.707 relative to the amplitudes of said first and second signals.

16. in a sound system for transmitting or recording four discrete audio information signals designated L,, L,,, R, and R, on a transmission or recording medium having first and second channels each of which has a baseband frequency channel and a modulated carrier channel wherein first and second combined signals containing predetermined matrixed combinations of said four audio information signals are applied to the firstand second channels of said medium, respectively, and including means for deriving from said first and second combined signals first and second sets of four output signals, corresponding ones in each set containing dominant signal components L,, R,, L, and R each accompanied by sub-dominant signals, and means for combining the four output signals of the first set with the corresponding four output signals of the second set to produce four discrete output signals L,, L, R,, and R transmitting or recording apparatus, comprising:

means including matrix encoding means for coupling to said first channel of said transmission or recording medium a first combined signal containing a first composite signal at said baseband frequency containing said L;, L, and R signals to the extent they are present and a carrier signal modulated by a second composite signal containing said L L and R, signals, the L, and R, signals in said first composite signal being in phase opposition with the L and R signals in said second composite signal, and

means including said matrix encoding means for coupling to said second channel of said transmission or recording medium a second combined signal con taining a third composite signal at said baseband frequency containing said R,, L, and 'R signals and a carrier signal modulated by a fourth composite signal containing said R,, L, and R signals, the L, and R signals in said third composite signal being in phase opposition with the L, and R,, signals in said fourth composite signal.

17. Apparatus according to claim 16, wherein said L, and R signals in said first and second composite signals are substantially in quadrature with the L and R signals in said third and fourth composite signals, respectively, and the L and R signals in said first and third composite signals are substantially in phase opposition with the L, and R, signals in said second and fourth composite signals, respectively.

' 18. Apparatus according to claim 17, wherein said L, and R, signals in said first, second, third and fourth composite signals are substantially in phase with each other.

19. Apparatus in accordance with claim 18, wherein the amplitudes of said L, and R signals in said first, second, third and fourth composite signals are reduced by a factor of substantially 0.707 relative to the amplitudes of said L, and R, signals.

l i i i i

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Classifications
U.S. Classification369/90, 381/23
International ClassificationH04S3/00
Cooperative ClassificationH04S3/006
European ClassificationH04S3/00B
Legal Events
DateCodeEventDescription
Jan 4, 1988ASAssignment
Owner name: CBS RECORDS, INC., 51 WEST 52ND STREET, NEW YORK,
Free format text: ASSIGNMENT OF ASSIGNORS INTEREST.;ASSIGNOR:CBS INC.;REEL/FRAME:004809/0935
Effective date: 19871130