US3890466A - Encoders for quadraphonic sound system - Google Patents

Abstract

Methods and apparatus for combining a plurality of channels of audio program information into two composite signals suitable for recording or transmission on a medium having only two independent tracks, and for presentation over the two loudspeakers of stereophonic playback apparatus which at the same time will admit of decoding into a plurality (e.g., four) separate signals corresponding to the originally encoded signals for presentation over corresponding loudspeakers. The two composite signals respectively contain input signals intended for presentation on loudspeakers positioned at the left front and right front corners of a listening area, and both composite signals contain, at reduced amplitude, signals intended for presentation on loudspeakers positioned at the left back and right back corners of the listening area. The encoder includes all-pass phaseshifting networks designed to transmit all frequencies in the frequency range of interest which are operative to cause the left back and right back components of one composite signal to be in quadrature relationship with corresponding components in the other composite signal.

Description

United States Patent 1 Bauer 1 June 17, 1975 1 ENCODERS FOR QUADRAPHONIC SOUND SYSTEM [75] Inventor: Benjamin B. Bauer, Stamford,

Related US. Application Data [63] Continuation-in-part of Ser. No. 328,814, Feb. 1, 1973, abandoned, which is a continuation-in-part of Ser. No. 251,544, April 21, 1972, abandoned, Continuation of Ser. Nos, 44,224, June 8, 1970, abandoned, Ser, No. 124,135, March 15, 1971, Pat. No. 3,821,471, and Ser. No. 288,829, Sept. 13, 1972,

OTHER PUBLICATIONS The Compatible Stereo-Quadraphonic SQ" Record, Bauer, Audio Magazine, Oct. 1971.

The Sansui 08 System, Audio Magazine, Oct. 1971. Discrete vs. SQ Matrix Quadraphonic Disc Bauer, Audio Magazine, July 1972.

The Joke System by Michael Gerzon, Hi Fi News, June 1970, pp. 843, 847.

Primary Examiner-William C. Cooper Assistant Examiner-Thomas D'Amico Attorney, Agent, or Firm-Spencer E. Olson [57] ABSTRACT Methods and apparatus for combining a plurality of channels of audio program information into two composite signals suitable for recording or transmission on a medium having only two independent tracks, and for presentation over the two loudspeakers of stereophonic playback apparatus which at the same time will admit of decoding into a plurality (e.g., four) separate signals corresponding to the originally encoded signals for presentation over corresponding loudspeakers. The two composite signals respectively contain input signals intended for presentation on loudspeakers positioned at the left front and right front corners of a listening area, and both composite signals contain, at reduced amplitude, signals intended for presentation on loudspeakers positioned at the left back and right back corners of the listening area. The encoder includes all-pass phase-shifting networks designed to transmit all frequencies in the frequency range of interest which are operative to cause the left back and right back components of one composite signal to be in quadrature relationship with corresponding components in the other composite signal.

33 Claims, 37 Drawing Figures SHEET TNP I PATENTEDJuu 17 ms 1 1 O ShEET 6 .707R (L L,

234 Mr) (H Jon (R 240 ENCODERS FOR QUADRAPHONIC SOUND SYSTEM This is a continuatiomin-part of now abandoned application Ser. No. 328,814 filed on Feb. 1, 1973 in the name of Benjamin B. Bauer (which, in turn, is a continuation-in-part of now abandoned application Ser. No. 251,544 filed on Apr. 21, 1972 as a continuation of now abandoned application Ser. No. 44,224 filed June 8, 1970, and of application Ser. No. 124,135 filed on Mar. 15, 1971, now U.S. Pat. No. 3,821,471) and of now abandoned application Ser. No. 288,829 filed on Sept. 13, 1972 in the name of Benjamin B. Bauer.

CROSS-REFERENCE TO OTHER APPLICATIONS This invention is related to the subject matter of the following other co-pending applications, all of which are assigned to the assignee of the present invention and application: Ser. Nov 164,675 filed July 21, 1971 as a continuation-in-part of now abandoned application Ser. No. 40,510 filed May 26, 1970 now also aban' doned; Ser. No. 44,196 filed June 8, 1970, now U.S. Pat. No. 3,708,631; Ser. No. 271,470 filed July 13, 1972 as a division of application Ser. No. 44,196, filed June 8, 1970, now U.S. Pat. No. 3,794,780; Ser. No. 185,050, filed Sept. 30, 1971 as a division of now abandoned application Ser. No. 44,224 filed June 8, 1970, now U.S. Pat. No. 3,813,494; Ser. No. 251,636 filed May 8, 1972 as a continuation of now abandoned Ser. No. 81,858 filed Oct. 19, 1970, now U.S. Pat. No. 3,812,295; and Ser. No. 243,800 filed Apr. 13, 1972, now Pat. No. 3,761,628, Ser, No. 118,271 filed Feb. 24, 1971, now U.S. Pat. No. 3,784,744.

BACKGROUND OF THE INVENTION There is an increasing interest in multiple-channel recording and reproduction because of the variety of sounds and music forms that can be achieved thereby. 1n the early days of phonograph, only single channel or monophonic recording was used, and as early as 40 to 50 years ago, investigators realized the value of recording and transmitting two separate channels of information, which in modern parlance is known as binaural or stereophonic sound. However, even two channels of information are considered insufficient for full illusion of reality. For example, when a listener is placed in front of a symphony orchestra he hears sounds arriving from many different directions and from a variety of instruments, as well as reflections from the walls and ceiling, which gives him an accustomed illusion of space perspective. However, when reproduction is accomplished by utilizing only two channels it is difficult, if not impossible, to produce true reality with respect to spatial perspective. Early experiments have demonstrated that a minimum of three independent channels are needed to convey a satisfactory illusion of reality in the reproduction of orchestral music.

The modern stereophonic phonograph is capable of recording, or encoding, modulation along two separate channels, which geometrically are at 90 to each other and at 45 to the disc surface. It is usual practice to include a third, or center, channel by matrixing or combining it as an in phase phantom channel to the other two, which causes it to be recorded as lateral modulation parallel to the record surface. Upon reproduction, the third (or central) channel appears on the two loudspeakers of the stereophonic phonograph with equal loudness and in-phase relationship and an observer placed centrally between the loudspeakers perceives the illustion of the third channel being located between the other two. Although there have been attempts to reproduce the third or center channel on a separate loudspeaker, the results have not been entirely satisfactory, and most stereophonic systems, even though many stereo records carry a center channel, employ only two loudspeakers.

In the aforementioned co-pending application Ser. No. 164,675 of William S. Bachman there is described a system for providing third and fourth playback channels to otherwise two-channel systems by feeding third and fourth loudspeakers with signals respectively rep resenting the sum and difference between the left and right channel signals. The left and right loudspeakers may be located, for example, on opposite sides of a listening area, with the loudspeakers for the two virtual channels positioned at opposite ends of the listening area. Each loudspeaker displays the particular information fed to its channel accompanied by half-power sig nals from its adjacent channels. This system provides a pseudo-four-channel effect, but does not give a complete illustion of each channel appearing independently on its corresponding loudspeaker.

A better illusion of each channel appearing independently on its corresponding loudspeaker is provided by the system described in the above-mentioned U.S. Pat. No. 3,708,631 which includes four gain control amplifiers through which the four separate channels of information are respectively applied to corresponding loudspeakers, and a logic control circuit which derives its signals from the left and right output terminals of the transducer for automatically controlling the gain control amplifiers to enhance the realism of four separate channels of information. While this system provides a significant improvement in the art of reproduction of recorded sound, it has a number of drawbacks as follows: (1) The system provides for both a single back channel and the information originating from this back channel, which is encoded as a difference signal or as a vertically oriented elliptical signal, has little or no component in the lateral or sum direction, and accordingly, as the record is played on a monophonic phonograph or transmitted over a monophonic radio station, the signal identified with the back direction is greatly attenuated or disappears altogether; (2) In the case of a stereophonic disc record, it is undesir able to apply information originating from the back in the vertical direction because it tends to make cutting and pressing of the record more difficult; (3) When a stereophonic disc record carrying back information as vertical modulation is played on a conventional stereophonic player, the signals corresponding to the back direction appear at the two loudspeakers out-of-phase, or significantly so, thereby causing a relatively unpleasant pressure in the ears" sensation; (4) 1n conventional stereophonic practice the two loudspeakers are normally placed in two adjacent corners of the listening room, and it is conventional in the production of fourchannel recordings to have the four sources originate from the four corners of the room or listening area. However, the systems described in the aforementioned co-pending applications are designed to preserve symmetry withh loudspeakers placed centrally of the four walls of the listening room. If the loudspeakers were placed in the corners, the aspect of the originally recorded sound would be shifted by 45, causing an inconsistency confusing to the listener. Also, since there are practical difficulties in finding suitable locations for loudspeakers centrally of the walls in most homes, it is preferable that the reproducing system permit the placement of the loudspeakers at the corners of the listening room.

SUMMARY OF THE INVENTION A principal object of the present invention is to provide methods and apparatus for combining four channels of program information, edited for presentation on four loudspeakers placed at the corners of a listening area, into two composite signals suitable for presentation over the two loudspeakers of stereophonic sound reproducing apparatus which at the same time will admit of decoding into four separate signals corresponding to the four signals originally encoded, for pre sentation over four corresponding sound-reproducing devices.

Briefly, the foregoing object is obtained by combining the four channels of information for convenience identified as L, for left front". R,fr right front", L,, for left back" and R for right back", to form two composite signals L (left total] and R (right total) for recording or transmission on a two-channel medium, such as a stereophonic disc record or a two-track magnetic tape. The encoding apparatus contains all-pass phaseshift networks designed to transmit all frequencies from about to about 20,000 Hz and summing networks interconnected in a manner such that upon linear matrixing of four equal-amplitude input signals designated L,, R,, L,, and R the L and R composite signals have the following characteristics:

1 The L, signal appears only in the L composite signal and the R; signal appears only in the R composite signal; thus the front signals are completely isolated from each other. When L, and R are applied to a ste reophonic cutter, L, produces a 45 modulation, while R; produces a 45 modulation, precisely as with conventional stereo.

2. The left back (L signal appears in both the L and R composite signals at reduced amplitude, 0.707 L in the preferred embodiment, and in quadrature with each other, with the L component in the L composite signal preferably leading the corresponding signal in the R signal. This causes a stereophonic cutter stylus to describe a circular motion in the clockwise direction, which when combined with the lengthwise motion of the groove becomes a clockwise helix.

3. The right back (R signal also appears in both the L and R composite signals at reduced amplitude, 0.707 R in the preferred embodiment, and in quadratuure with each other. The R component in the R sig nal leads the corresponding component in the L signal which causes the cutter stylus to describe a circular motion in the counterclockwise direction.

The front channel modulations are fully equivalent to the corresponding stereophonic modulations and provide the full channel separation on playback of which the pickup is capable, while the back channel modulations are of a character to cause the pickup stylus to describe a circular motion in one direction for a left back signal and in the opposite direction for a right back signal.

BRIEF DESCRIPTION OF THE DRAWING An understanding of the foregoing and additional aspects of this invention may be gained from consideration of the following detailed description, taken in conjunction with the accompanying drawing, in which:

FIG. I is a schematic diagram of a prior system for recording four channels of information on a stereophonic record;

FIG. 2 is a vector diagram useful in explaining the motion of the cutter stylus in response to application of left, right, center and difference signals;

FIG. 3 is a cross-sectional view of a fragmentary portion of a record showing four record grooves on a greatly enlarged scale, to illustrate the motion of the cutter in response to various signals;

FIG. 4 is a schematic diagram of a prior art stereo phonic playback system for providing the illusion of a third channel;

FIG. 5 is a schematic diagram ofthe system described in the aforementioned US. Pat. No. 3,708,631 for recording four channels on a two-track stereophonic record;

FIG. 6 is a greatly enlarged illustration of a record groove illustrating the effect of applying the difference" signal to the left and right channels through a phase-shift network;

FIG. 7 is a schematic diagram of one embodiment of an encoder according to the invention for combining four independent signals intended for ultimate presentation on four separate loudspeakers;

FIG. 8 is a vector diagram useful in explaining the operation of the encoder of FIG. 7;

FIG. 9 is a plan view ofa listening area illustrating the location of four loudspeakers therein and the phasor diagrams of the signals appearing on the four loudspeakers;

FIG. 10 is a pair of phasor diagrams useful in explaining the principles of the invention;

FIG. 11 is a schematic diagram of another encoder according to the invention;

FIG. 12 is a schematic diagram of decoder apparatus described in US. Pat. No. 3,784,744, useful in explaining the efficacy of the encoder of FIG. 11;

FIG. 13 is a schematic diagram of an alternative form of encoding apparatus embodying the invention;

FIG. I4 is a schematic diagram of still another alternative form of encoding apparatus embodying the invention;

FIG. 15 is a diagram similar to FIG. 8 illustrating the motion of the cutting or playback stylus of stereophonic recording or reproducing apparatus in response to signals encoded in accordance with the invention;

FIGS. 16 and 17 are phasor diagrams useful in explaining the operation and advantages of the encoder of FIG. 14;

FIG. 18 is a schematic diagram of a modified form of the encoder of FIG. 14, and FIGS. 18A, [88, and 18C are diagrams illustrating the operation thereof;

FIG. 19 is a schematic diagram of an encoder described in applicants co-pending US. Pat. No. 3,761,628, and FIGS. 19A, 19B and 19C are diagrams illustrating the operation thereof;

FIG. 20 is a schematic diagram of still another alternative form of encoding apparatus, and FIGS. 20A, 20B, and 20C and 20D are diagrams useful in explaining its operation;

FIG. 21 is a schematic diagram of still another encoder, and FIGS. 21A, 21B, and ZIC are diagrams useful in explaining its operation; and

FIG. 22 is a schematic diagram of encoding apparatus embodying the features of the three encoders illustrated in FIGS. l8, l9 and 20, and FIGS. 22A and 22B are phasor diagrams useful in explaining its operation.

DISCUSSION OF THE PRIOR ART By way of background for better understanding the present invention, the current method of recording stereophonic signals including a third or center channel, and a method of reproducing the signals over a stereophonic two-loudspeaker system will be described with reference to FIGS. 1-4. The currently provided left (L), right (R) and center (C) signals are applied to the two terminals of a stereophonic cutter 10 having a cutting stylus ]2 which is adapted to cut a groove in the lacquer of a master disc 14, revolving on a recording turntable (not shown). The C signal is supplied through a signal splitter I6 of known configuration resulting in application of equal portions thereof, equivalent to 0707C, to each of the L and R terminals in-phase. AS is well known in the groove cutting art, the tip of the cutter is capable of motions contained within a surface generally perpendicular to the disc in the manner portrayed by the vector diagram of FIG. 2. When a left signal L is applied, the stylus executes motions along the arrow L, which is at an angle of 45 to the horizontal, and when an R signal is applied, the stylus motion is along the arrow R, at an angle of 45 to the horizontal. Application of 0707C to each of the L and R terminals in-phase causes motion of the stylus along the arrow C, equal in magnitude to 0.707 (L i-R), which is of the same magnitude as either L or R, but directed horizontally. It will be appreciated that instead of applying the L, R and C signals directly to the cutter, as shown in FIG. 1, they may, in keeping with common practice, first be recorded on a two-track master tape recorder and the output of the tape reproducer used to drive the record cutter. Discussion of the difference signal D illustrated in FIGS. 1 and 2 will be deferred until later.

The type of groove modulation resulting from the just-described procedure is shown in FIG. 3. When only the left signal L is applied, the groove is modulated in accordance with the arrow L, which is essentially confined to one wall of the groove. Similarly, when the R signal is applied, the modulation is in the opposite wall of the groove in the direction of the arrow R, which, it will be noted, is perpendicular to the arrow L. Application of equal amounts of the center signal C to the L and R lines causes both walls of the groove to be simultaneously and equally modulated in the directions indicated by the arrows L=0.707C and R=0.707C, resulting in horizontal or side-to-side translation indicated by arrow C.

Apparatus for reproducing a stereophonic record carrying L, R and C signals recorded in this manner, schematically illustrated in FIG. 4, includes a stereophonic pickup having a cartridge 18 and a stylus 20 which enters the groove in the record and is actuated by the groove modulations to deliver output voltages on the L and R terminals. If only L signal modulation is present in the groove. an output signal appears only at the L terminal and is amplified by a suitable power amplifier 22 and reproduced by a loudspeaker 24. Similarly. when only R signal modulation is present in the groove, an output voltage appears at only the R terminal of the pickup, which is amplified by power amplifier 26 and applied to its respective loudspeaker 28. When the groove has lateral modulation consisting of the presence of equal amounts of left and right signal, then equal signals, namely, 0.707C, appear at both the left and right loudspeakers, resulting in the appearance of a phantom source C (shown surrounded by a dashed line circle) midway between loudspeakers 24 and 28. However, this illusion is preceptible only to the centrally located observer 30; when he moves to either side, the C signal is heard over the nearest loudspeaker unless special precautions are made to adjust the directional characteristics of the loudspeakers with respect to the position of the observer.

It will be noted that the described three-channel record is compatible because the L, R and C signals all have a horizontal component and thus will be heard when played on a monophonic player, which is sensitive only to lateral modulation, albeit their relative intensities will not be in the exact balance initially intended by the recording director since the horizontal components of L and R are 0.707 of C. In reality, in spite of the introduction of a third channel, the abovedescribed system reproduces only two independent channels of information. The third channel, C, is contained in both the left and right channels and the listener will, therefore, usually hear it reproduced from the loudspeaker nearest to him. This center channel may be presented on a separate loudspeaker system, as shown in dotted lines in FIG. 4, and amplifiers are commercially available for this purpose. This permits the observer to percieve the center information without having to locate himself equidistant from the left and right speakers, although such a center channel loudspeaker tends to cause the sounds of the other two loudspeakers to appear to be pulled in toward the center.

Reverting to FIGS. 1-3, a fourth channel, D, may be introduced to the two-channel stereophonic system by dividing it into equal parts by a signal splitter 32 and applying them in phase-opposition to the left and right channels. As shown in FIG. 2, application of the D signal in this manner causes motion of the stylus in the vertical direction, along the arrow D, to an extent specified as 0.707 times the amount of D contained in the left and right channels subtracted from each other; i.e., 0.707 (L-R). As seen in FIG. 3, this causes the left and right motions of the stylus to be out-of-phase relative to each other, resulting in up and down motion. When vertical modulation is reproduced by the system of FIG. 4, the loudspeaker cones are driven in opposite directions, resulting in out-of-phase sound pressures applied to the ears of the listener, and since this condition of pressure on the ears does not correspond to any known normal listening experience, the observer is unable to localize the sound. The difference signal D appears at some indefinite point in space, shown as D in a dashed circle, and the listener is unable to locate its whereabouts. Furthermore, some listeners of such outof-phase sound have complained of a peculiar pressure in the ears sensation. This is in part overcome, however, by the system described in the aforementioned Bachman application Ser. No. l64,675 wherein the difference signal, as well as the center signal, are reproduced on separate loudspeakers.

To afford better compatibility with monophonic and conventional stereophonic players, while at the same time improving the illusion of four separate channels during playback, the difference signal D may be applied in the manner suggested in applicants article en titled Some Techniques Toward Better Stereophonic Perspective", IEEE 'IRANSZ-K'TION'S OF AUDIO. Vol. ALl-l I, No, 3, lvlaydune, I963. In keeping therewith, and as is illustrated in FIG, 5, instead of applying the difference signal equally and oppositely to the left and right channels as in the circuit of FIG. I, the D signal is applied through an acoustical phase shift network 32 which splits the incoming signal into two equal amplitude signals D, and D each containing all ofthe frequencies of the D signal, but displaced in phase with re spect to each other. Relative phase displacements in the range of l l() to l70 have been successfully used, with an angle of l35 being particularly suitable. It can be readily demonstrated that when the two signals are thus displaced relative to each other. the tip of the stylus instead of undergoing a purely up and down motion as shown in FIG. 3, executes the elliptical motion illustrated in FIG. 6. The limits of stylus motion are shown by the dashed lines and the direction of motion of the ellipse depends on whether D leads D or vice versa. The important consideration is that the groove has a horizontal component defined by the horizontal width of the ellipse, whereby both monophonic and stereophonic phonographs will reproduce all four signals; that is, the record with four separate channels will be fully compatible with the older playback systems, albeit with monophonic systems the signal D is attenuated by about 8 db.

The realism of reproduction of four separate Chtlt'l nels of information recorded as described above is enhanced by the control system described in the aforementioned US. Pat, No. 3.708,63l which derives signals from the left and right terminals ofthe transducer, separates the composite signals into their respective components, compares the magnitudes of these components, and actuates gain control amplifiers in the respective loudspeaker circuits in concert with the loud ness of the respective components in a manner to give a realistic illustion of four separate independent sources of sound.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to FIG. 7, there is shown in schematic form a first embodiment of an encoding or matrixing system for combining four independent signals, intended for ultimate display on four separate loudspeakers, into two composite signals for recording or trans mission on a two-track medium, such a stereophonic disc record or a two track tape. The encoder includes four input terminals 40, 42, 44 and 46 for receiving four separate input signals which, for convenience, will be designated left front (L left back (L right back (R and right front (R respectively. These designations signify the locations in a listening area of the four loudspeakers on which the signals are intended for ultimate presentation. These signals are identified by vertical arrows of equal length which signify, for purposes of the analysis to follow, that the incoming signals are assumed to be of equal magnitude and referred to the same phase reference. The encoding matrix includes six alLp- ass phaseshift networks 48, 50, 52, S4, 56 and 58 designed to introduce a substantially constant phase shift to the applied signal over the frequency range of interest without altering their magnitudes. Each of the networks has a reference phase shift d1, which is a function of frequency, the two phase shifters and 56 introducing only the reference phase shift, Phase shifters 48 and 58 provide a phase shift equal to (il:+45), and networks 52 and 54 provide a phase shift equal to lil1+9tll It is to he noted that according to the convention used herein the phasc shift angles produced by phase-shifters 48-58 are lagging angles. In other words, a network with phase-shift of (ill-+90") produces an out put lagging 90 behind that produced by a network with phase shift (with).

The signals L, and R,, respectively identified with the left front and right front loudspeakers are applied via their respective terminals 40 and 46 through their asso ciated all-pass networks 48 and S8 to respective summing circuits and 62. The left back signal L,, is ap plied to both of phase shift networks 50 and 52, the output signal from the former being applied to summing circuit 60 with attenuation corresponding to the multiplicand 0.707, and the output signal from network 52 is applied to summing circuit 62 with the same attenuation. The right back signal R,-, is similarly applied to both of phase- shift networks 54 and 56, the output signals from which are respectively applied with 0,707 attenuation to summing circuits 60 and 62. The suinming circuits 60 and 62, which are of conventional design and well known to ones skilled in the art, are operative to produce respective composite signals L and R at their corresponding output terminals 64 and 66. These signals may be applied to the left and right termi nals of a stcreophonic disc record cutter, for example, or to the two recording heads of a two track tape recording apparatus, or to any other known two'track medium, in a manner which will be apparent to ones skilled in the art.

Although the encoding apparatus has been thus far described in terms of four input signals, if it is desired to have a signal appear centrally in the reproducing systern, a center signal designated by the arrow labeled C, may he applied equally and in phase to terminals 40 and 46, or to the terminals 42 and 44, or to all four terminals simultaneously, as indicated by the curved ar rows. It will be evident that the C signal will be subjected to the phase shift of those of networks to which it is applied, in the example of FIG. 7 to networks 4858, and will become part of the composite signals LT and R1.

The nature of the composite signals appearing at terminals 64 and 66 will be seen from the phasor diagrams adjacent the terminals. It is seen that each of these signals contains a predominant front loudspeaker signal, L and R;', respectively, both of which are shifted in phase relative to input signals L, and R, by NIH-45). The L -signal further includes signals L and R,,' at to each other, with the signal leading. and in a 45 relationship with L,'. The C signal appears C in both composite signals in the same relative phase position as the signals I and Rf,

It is significant to note that the R signal contains in addition to the signal R the two signals L,, and R,,' at 90 to each other. It will be seen, however, that they are reversed in phase relative to the L signal, with R,,' leading and signal I.,,' lagging relative to the corresponding signals on terminal 64. As noted earlier, however, the signal C is again in the same relative position with respect to the corresponding signal on terminal 64.

Another significant feature of the composite signal is that the L,, and R,, components in one composite signal are in quadratuure with the corresponding components in the other composite signal, and that the L, component in the L composite signal leads the corresponding component in the R signal and that the R component in the R signal leads the R component in the L signal.

Since the signals L, and R, usually will be incoherent signals, if recorded on a stereophonic disc record they will appear independently as separate modulations of the left and right channels. The signals C being in phase at both terminals 64 and 66 will cause lateral modulation of the disc record. The fact that signal L,,' at terminal 64 leads the L,,' signal at terminal 66 by 90 will cause modulation of the record groove in a clockwise advancing spiral, in the manner of a right-hand screw thread. Similarly, because signal R, at terminal 64 lags behind signal R, at terminal 66 by 90 will result in a counter-clockwise helix, in the manner ofa left hand thread. Thus, it is seen that the five signals applied to the matrix system of FIG. 7 may be applied to a stereophonic disc record as five distinct types of modulations, namely, modulation of the left and right walls of the groove, lateral modulation, and clockwise and counter-clockwise helical modulation.

The form of modulation on the disc record, as viewed from the point of view of the cutter tip, looking in the direction of motion of the groove, is illustrated in FIG. 8. The L, signal causes motion at 45 to the horizontal. the R, signal causes motion at 45 to the horizontal, and the C signal causes lateral or horizontal modulation. These three modulations, it will be recognized, are identical with those which obtain in the cutting of a conventional stereophonic record. As a significant departure from conventional practice, there is, additionally, clockwise circular modulation L corresponding to the left back loudspeaker signal, and counterclockwise circular modulation R corresponding to the right back loudspeaker signal. Since the L, and R modulations have a significant horizontal component (as projected on the line C) it is evident that they will produce equivalent signal components in the horizontal mode, therefore assuring full compatability with a monophonic phonograph player. An important advantage of this method of combining the input signals is that the stereophonic record or tape can be replayed over any stereophonie or monophonic player with full and complete reproduction of all of the sounds recorded on the record.

Upon playback, the composite signals L and R-,- depicted by the phasor diagrams of FIG. 7 may be decoded by the decoder described in above-mentioned US. Pat. No. 3,813,494 to produce four output signals for display on four loudspeakers 70, 72, 74 and 76 positioned, as illustrated in FIG. 9, at the left front, right front, left back and right back corners, respectively, of a listening area. Phasor diagrams of the signals appearing on each of these loudspeakers produced by the referenced decoder are presented adjacent their respective loudspeaker. It will be observed that the signals L,", R,", L," and R predominate in loudspeakers 70, 72, 74 and 76, respectively. The signals from other channels appearing in each of the main channels are about 3 dB lower in level than the principal signals and, accordingly, tend not to be prominent in the mind of the listener; rather, he will hear primarily the four independent channels being presented on the four loudspeakers.

While it is important in the encoding process to main tain a 90 relationship between the phasors 0707 L,, and 0.707 R,,, it will be seen from FIG. 10 that the positions of the phasors L, and R, relative to the L, and R components may be arbitrarily chosen insofar as decoding is concerned. That is, any decoder designed to decode the composite signals L and R shown in FIG. 7 will also satisfactorily decode the signals L and R shown in FIG. I0, regardless of the size of the angles a, and 01 between phasors L, and 0.707 L and between phasors R, and 0.707 R,, respectively. Since the as (45 in FIG. 7) are established by llJ- l'IfiIWOI'kS 48 and 58, by suitable design of these networks it is possible to place phasors L; and R, at any desired position with respect to the other two phasors in the group. An especially beneficial relationship is established by making both the angles or=90 so that in the phasor group portraying L the phasor L; coincides with phasor 0.707 R and in the other phasor group, the phasor R; coincides with phasor 0.707 L This relationship is readily obtained by modifying llI- I'IGIWOI'kS 48 and 58 so that instead of providing a phase-shift (tl1+45) they provide a phase-shift (tl1+90). An important benefit of this modification is that the encoding function can be performed with only four llJ-IIEIIWOI'kS, instead of the six required in the encoder of FIG. 7.

Referring now to FIG. I], an encoder embodying this improvement has four input terminals 80, 81, 82 and 83 to which input signals L], L,,, R, and R, represented by phasors corresponding to the same signals depicted in FIG. 7, are respectively applied. Rather than being applied directly to a Ill-network as in the system of FIG. 7, input terminals and 82 are connected to a summing junction 84 which is operative to add a unity measure of signal L, to 0.707 of signal component R Similarly, terminals 81 and 83 are connected to a second summing junction 85 which is operative to add a unity measure of signal R to 0.707 of signal L,,. Terminal 8] is also connected to the input of a ill-network 86 which introduces a relative phase-shift of ll! to the L, signal, and terminal 82 is connected to the input of a second (IIH'OQ) network 85. The output signals from summing junctions 84 and 85 are respectively applied to the input terminals of ill- networks 87 and 88, both of which introduce a relative phase-shift of (1l1+90).

The full output of -network 87 is added in summing junction 90 to 0.707 of the output of network 86, and similarly, the full output of ill-network 88 is added at summing junction 91 to 0.707 of the output of ill-network 89. As a consequence of this phase-shifting and combining of signals, there appears at output terminal 92 a composite signal L depicted by phasor group 94, and at output terminal 93 a composite signal R depicted by phasor group 95. It will be observed that there is a one-to-one correspondence between phasor groups 94 and 95 and the corresponding phasor groups in FIG. 10 if the angle a in the latter group is set at 90.

That the encoder of FIG. 11 is compatible with decoders intended for use with the signals encoded in accordance with the system of FIG. 7 is demonstrated by the comparative analysis presented in FIG. I2, as applied to the decoder described in US Pat. No. 3,784,744. This decoder includes a pair of input termi- 1 l nals 100 and 102 to which the composite signals L-, and R are respectively applied. The signal applied to terminal 100 is applied to both and is phase-shifted by a pair of II - HCIWOTkS 104 and 106, and the composite R,- signal applied to input terminal 102 is applied to both of lIJ- IICIWUI'kS 108 and 110. These ill-networks are of the type previously described, the networks 104 and 110 introducing a phase-shift of(tlr'+0) and networks 106 and 108 introducing a phase-shift of(tl|+90). It will be noted that the reference angle is designated ill instead of d1, as used in the encoder: this is to call attention to the fact that the reference phase-shift in the decoder need not be the same as in the encoder, provided the same reference phase-shift is used in all four of lIJ-HEI- works 104, 106, 108 and 110. The output signals from networks 104 and 110 are applied directly to the leftfront output terminal 112 and to the right-front output terminal 114, respectively. Equal portions of the output signals from networks 106 and 110 are summed in a summing junction 116, the output of which is applied to the left-back output terminal 118. and equal portions of the outputs of networks 104 and 108 are summed in a second summing network 120, the output of which is applied to the right-back output terminal 122.

A comparison will now be made of the performance of the decoder of FIG. 12 in response to signals encoded with the encoder of FIG. 7, and to composite signals encoded in accordance with the encoder of FIG. 11. Phasor groups corresponding to signals encoded with the encoder of FIG. 7 are shown in dotted lines. and the phasor groups encoded by the encoder of FIG. 11 are shown in solid lines. Phasor groups 130 and 132 portray the two input signals L and R which, upon being shifted in phase by the all-pass networks 104, I06, I08 and 110 appear as new phasor groups 134, I36, 138 and 140. The phasors in these latter four groups are labeled with a prime to differentiate them from the corresponding phasors prior to introduction of the relative phaseshifts. The signal represented by phasor group 134 appears at output terminal 112 as phasor group 142 and contains a dominant component L, together with the smaller components O.707L,,' and O.707R,,'. The phasor groups 136 and 140 after summing in junction 116 result in a signal at output terminal 118 represented by phasor group 144 containing a dominant phasor L,,' and subsidiary phasors 0.707Lf and 0.707R,'. The sum of phasors I34 and 138 appearing at the output of summing junction 120 (output terminal 122) is a composite signal represented by phasor group 146 having a dominant phasor R accompanied by subsidiary signals 0.7U7R and 0.707Lf, Finally, the phasor group 140 appears at output terminal 114 as phasor group 148, and contains a dominant signal R together with subsidiary signals U.707R,,' and (1.7()7L,,'. Thus, the decoded signals appearing at output terminals 112, 118. 122 and 114 each contains its appropriate dominant signal together with signals from two other channels reduced in amplitude by the factor 0.707. It will be noted that the two principal front channel phasors, namely L, in group 142 and R in group 148 are in phase. and that the two principal hack channel vectors, L in phasor group 144 and R,,' in group 146, are also in phase with each other. but not in phase with the L, and R phasors.

It will now be demonstrated that this favorable phase relationship is also achieved when signals encoded with the encoder of FIG. 11 are decoded in the decoder of FIG. 12. It will be remembered from the description of FIG. 11 that the encoded signals L and R are as portrayed by phasor groups 150 and 152, the former after being acted upon by ill- networks 104 and 106 appearing as phasor groups 154 and 156, respectively, and the R signal after transmission through LIJ- I'ICILWOTkS 108 and 110 appearing as phasor groups 158 and 160, respectively. Phasor group 154 appears at output terminal 112 as phasor group 162, and phasor group 160 appears at output terminal 114 as phasor group 164. These output signals contain predominant signals L, and R respectively, which are in phase with each other, and each includes subsidiary signals 0.707R and O.7U7L,,'.

The phasor groups 156 and 160 upon being summed in summing junction 116 produces at output terminal 118 the composite signal portrayed by phasor group 166, and the sum of the signals represented by phasor groups 154 and 158 appearing at the output terminal 122 of summingjunction 120 is as portrayed by phasor group 168. It will be noted that phasor groups 166 and 168 contain predominant phasors L,,' and R respectively, which are in phase with each other, and also in phase with the predominant phasors in groups 162 and 164, and each accompanied by two subsidiary signals 0.707R, and 0.707L,'. Comparison of phasor groups 162 and 142,166 with 144. 168 with 146, and 164 with 148 reveals that they contain the same respective subsidiary signals in the same magnitude and in the same intergroup phase relationships. Therefore. the respective signals are capable of properly activating the enhancing logic and control circuits described in US. Pat. No. 3,784,744.

Another advantage of the encoder of FIG. 11 will be seen from a comparison of phasor groups 144 and 146, for example, in each of which there is shown in dotted line a side signal L which results from applying equal amplitude signals to the left-front and left-back terminals of the encoder. Because ofthe angular relationship between phasors 0.707L/ and O.707L,. in phasor group 144 as compared to the quadrature relationship between the corresponding phasors in group 166, the resulting phasor L in group 144 is of greater magnitude than the corresponding phasor in group 166. The absence of exaggeration of the L, signal is of significant advantage, and by symmetry, it will be recognized that exaggeration of an R, signal which would result from application of equal amplitude signals to terminals 82 and 83 of the encoder of FIG. 1] is likewise avoided.

In summary, the encoder of FIG. 1 I offers the following significant advantages over the encoder of FIG. 7: (l it provides encoding with four. instead of six III-[18lworks, with an attendant reduction in the cost of the encoder; (2) it produces encoded signals which upon decoding. cause the predominant signals to all be in phase; and (3) it avoids exaggeration of output signal intensity from the decoder when equal signals are applied to the side terminals of the encoder FIG. 13 illustrates a modification of the encoder of FIG. 11, differing therefrom in the manner in which the four input signals are added and phase-shifted. In this case. the full L; signal applied to terminal 170 is added in a summing junction 178 to 0.707 of the R signal applied to input terminal 174, and the full R; signal applied at input terminal 176. is added in summing junction 180 to 0.707 ofthe L signal applied at input terminal 172. The sum signals from summing junctions I78 and 180 are transmitted by respective ill-networks I82 and 184 and are added in respective summingjunctions I86 and 183 to 0.707 of signals l.,. and R,,. respectively. after being shifted in phase by (QM-90) in respective il|networks I90 and 192. The L,-and R signals appearing at output terminals 194 and 196. represented by phasor groups 198 and 200. respectnely. are similar to the corresponding phasor groups 94 and 95 in FIG. 11 except that in group 198 the 0.707R,, phasor leads the 0.707L,, phasor. whereas in group 94 the L,, phasor leads the R phasor; the positions of the L,, and R phasors in groups 200 and 95 are similarly interchanged. While this decoder provides perfectly consistent signals. it has the slight disadvantage stemming from the fact that the 0.707R,, phasor in group 198 leading the corresponding signal in phasor group 200 tends to cause this right-back signal to appear to lean slightly toward the left-front channel when the record is replayed stereophonically over two loudspeakersv By symmetry, the left-back signal likewise will tend to lean slightly to the right when the record is replayed stereophonically. Thus. while the alternative encoder of FIG. I3 produces acceptable composite signals for reproduction over four loudspeakers. it is inferior to the encoder of FIG. 11 if the record carrying the encoded sig nals is to be played over a two-channel stereophonic playback apparatus.

By seemingly slight modifications of the encoder of FIG. 13, and of the decoder shown in FIG. 12 (which is fully described in U.S. Pat. No. 3.784.744), the performance ofthe overall system can be significantly improved. particularly in its ability to resolve ambiguities in cases when the sound signals are "panned"; that is inserted into adjacent channels in an in-phase relationship. The cause of such ambiguities will become apparent from analysis of the decoded signals delivered by the decoder of FIG. 12 which. it will be remembered. contain predominant signals L L,,. R, and R respectively. together with two contaminating signals from other channels. Actually, the contaminating signals are not noticed when all four predominant signals are simultaneously present. as when four different performers produce four parts of a musical selection in all four channels. since there is sufficient mixing ofsound in the room or listening area that the presence ofthe contaminating signals in the individual channels is inconse quential. They are noticeable, however. when sound is present in only a single channel. or in at most two channels. because in these instances. when the sound should be coming from a single loudspeaker or from two loudspeakers. it is instead heard from all four. which is readily noticeable and sometimes objectionable. This situation is improved. and the realism of four channel reproduction enhanced by the logic and control systems described in the aforementioned US. Pat. No. 3.784.744 which recognize the presence of sounds in individual channels and generate signals for controlling the gain of gain control amplifiers in the individual loudspeaker circuits in response to the instantaneous presence of the predominant signals. Thus. if a signal appears only in the left-front channel. for example. (and which. because of the protocol of the decoder. also appears at reduced level in both of the back chan nels) the logic functions to enhance the gain of the front loudspeaker amplifiers and to turn down the gain ofthe back loudspeaker amplifiers thereby to cause the sound to appear to originate at the left-front loudspeaker only. The logic and control circuitry operates similarly with respect to the other three loudspeakers with the consequence that when artists are performing in concert in all four channels the gains of the respective amplifiers are increased and decreased to instantaneously enhance the channel or channels in which signals are predominant at a particular instant to give a highly realistic replication of the original four channel program.

The above-described methods of encoding four signals into two and decoding them back into four works very well in the majority of circumstances. one exception, however, being when the sounds are panned. and then only in two specific instances: namely. when the sound is panned exactly between the two front channels by application of equal amplitude signals to the L, and R, encoder terminals, or when it is panned precisely between the two back channels L,, and R,,. It can be shown that in these two circumstances the modula tion produced on the stereophonic disc is the same when the two front channels are panned as it is when the two back channels are panned. Consequently. the logic and control system used with the decoder is unable to distinguish whether such panned sound signal belongs to the front channels or to the back channels. resulting in an ambiguity. In accordance with another aspect of this invention. this ambiguity is resolved by another embodiment of the encoder and modification of the decoder so to provide a significant improvement in performance of the system.

The alternative encoder. illustrated in FIG. 14, has four input terminals 210. 212, 214 and 216 to which the four signals L,. L,,. R,, and R depicted as in-phase signals of equal amplitude. are respectively appliedv The total L signal is added in a summing junction 218 to 0.707 of the R,, signal. the output of the summing junction being applied to a phase-shifting network 220 which introduces a reference phase-shift 111. The full R signal at terminal 216 is added in summing network 222 to 0.707 of the L signal appearing at input terminal 212. and the output is passed through the ill-network 224, which also provides the reference phaseshift 11/. The L and R,, signals are also applied to respective il1networks 226 and 228, each of which provides a phase-shift of Uri-). The full signal appearing at the output of network 220 is added in a summing circuit 230 to 0.707 of the signal appearing at the output of network 226 to produce at its output terminal 232 a composite signal designated L Similarly. the full signal from network 224 is added in summing junction 234 to 0.707 of the signal from network 228, the latter in this case being in the positive sense. The signal appearing at the output terminal 236 is the composite signal designated R As in the case of the other encoders. the signal L and R may be transmitted by FM multiplex radio, or they may be recorded on any twochannel medium such as a two-track tape or stereophonic record for later reproduction.

The significance of the modifications to the encoder of FIG. 13 to provide the encoder of FIG. 14 tnamely. the reversal of the phase of the 0.707 terminals of the two summing circuits in the upper half of the diagram) will be appreciated from an analysis of the phasor rela' tionship of the L and R composite signals portrayed as phasor groups 238 and 240. respectively. It will be observed that phasor group 238 consists of the signal L;(which although shown in the same phase relationship as the input signal L, has a tfl-asa-function-offrequency angle difference between them), a signal 0.707R in a negative sense with respect to its corresponding input phasor, and a 0.707L,, signal which lags phasor 0.707R,, by 90 because of the action of network 226. Phasor group 240 consists of the original signal R, in the same relative phase position as its corresponding input signal, a signal 0.707L in phase with the R; signal, and a 0.707R signal lagging the .707L,, signal by 90 due to the action of ill-network 228. As has been pointed out hereinabove, in the interest of providing better realism of image placement when the record is played in conventional stereophonic mode over two loudspeakers, it is preferable to arrange the phasor 0.707L in phasor group 240 to lag behind the corresponding phasor in phasor group 238, and conversely, to arrange the phasor 0.707R,, in phasor group 238 to lag behind the corresponding phasor in group 240. Alternatively, the positions of phasor groups having its component phasors oriented as depicted at 238 and 240 can be interchanged and still enjoy the benefits of the invention. For example, simply by reversing the order in which the input signals are applied to the en coder-that is, R; to terminal 210, R,, to terminal 212, L b to terminal 214 and L, to terminal 2I6-the composite signal R having the components shown in pa rentheses in phasor group 238 will appear at output ter minal 232 and the composite signal L having the components shown in parenthesis in phasor group 240 will appear at output terminal 236.

Referring now to FIG. 16, the effect of panning is to divide the signal (as by means of two coupled attenuators) between two channel inputs. At the mid-point of the panned operation, the signal becomes divided evenly between the front channels L, and R,, or between the back channels L or R this condition will now be examined. The phasor groups 238 and 240 from FIG. 14 are repeated here as phasor groups 250 and 252, respectively, and the panned center signals have been added. The front center signal, C,, is placed in the proportion 0.707C, and in-phase in the phasor groups 250 and 252, appearing as phasors 254 and 256. Since these phasors are equal and in-phase the signals L and R combine to produce a horizontal motion of the cutter stylus; accordingly, the center front signal C, appears as a horizontal arrow 246 in FIG. 15. From the discussion thus far, it is seen that FIG. I5 depicts the left front channel phasor, Ly, the center channel phasor, C;, and the right-front channel phasor, R in a relationship which those skilled in the art will recognize as portraying the modulation of a conventional stereophonic record.

Reverting now to FIG. 16, it will be noted that the center-back signal, C,,, is divided in the proportion 0.707 in the left back and right back channels, and since these two phasors already appear as a 0.707 fraction, the corresponding fraction of the C signal is 0.5 in phase with the 0.707L,, phasor and 0.5 in phase with the 0.707R phasors in both phasor groups. With this convention in mind, it is seen that the two phasors in each group add to the larger phasors 0.707C,, in each of the phasor groups 250 and 252; however, it should also be observed that the phasor 0.707C,, in phasor group 250 is out-of-phase with the corresponding phasor in group 252. This is an important quality of the encoder of FIG. I4 because now the center back signal,

C,,, is of an entirely different character than the center front signal C,. It will be recognized that the signal, C having an out-of-phase relationship in the two channels will result in a vertical modulation of the record groove, which is depicted by the arrow 248 in FIG. 15. It will be realized that any signal recorded in this man' ner cannot be reproduced by a monophonic phonograph pick-up, nor by the monophonic section of an FM multiplex transmitting station; consequently, when using the encoder of FIG. I4 the centerback location should preferably be used for occasional sounds such as reverberation, motion during panning, etc., and not for the placement of an important artist, because he would not be heard when the signal is broadcast over AM radio or over monophonic FM radio. Such signals would, however, be fully audible with stereophonic or quadraphonic modes or reproduction, and all other locations of the artist would be transmitted satisfactorily.

Another significant feature of the FIG. 14 encoder is illustrated by the phasor groups 256 and 258 in FIG. 17, the former depicting the situation which results when the phasor groups 250 and 252 of FIG. 16 are added and the latter depicting the situation when the composite signal R (phasor group 252) is subtracted from L (phasor group 250). It will be noted that when L and R are added the phasors L], L,,, R,, and R, all have an amplitude equal to unity, whereas the front center signal, Cy, is augmented by a factor l.4l4, which is exactly what happens when a stereophonic record is played over a monophonic player. The back center signal, C, is cancelled, however, because of the aforementioned out-of-phase relationship. When the phasor groups are subtracted, the phasors L L R,, and R, again all appear with unity amplitude, but this time the center back signal, C is augmented by the factor I.4l4 while the center front signal, C is canceled. The relationship portrayed by phasor groups 256 and 258 are extremely important since they indicate that if only a center front signal is present (i.e., no center back signal) the phasor group 256 will be greater than group 258, and, conversely, if there is only a center back signal but no center front signal, the phasor group 258 will be larger. This interesting property is used to advantage to enhance the operation of the decoder to be utilized with the encoder of FIG. 14, all as fully described in Us. Pat. No. 3,821,471.

Additional features and attributes of the abovedescribed encoders will now be described in connection with FIG. I8 which illustrates the encoder of FIG. 14 modified as described above to produce the composite signals L R having the components shown in parentheses in phasor groups 238 and 240 of FIG. 14. More specifically, in the encoder of FIG. 18 the full L, signal is added in a summing junction 326 to 0.707 of the R,, signal, the output signal from the summing junction 326 being applied to an all-pass phase-shifting network 328 which introduces a reference phase-shift III which varies as a function of frequency. The full R signal at terminal 316 is added in a second summing junction 330 to 0.707 of the L,, signal appearing at input terminal 3I2, and the sum signal is passed through a second Ill-network 332 which also provides the reference phase-shift ill. The L,, and R,, signals are applied to respective Ill- networks 334 and 336, each of which pro vides a phase shift of (lb-) and wherein the IIJ-fLIIIC- tions are essentially the same. The full signal appearing at the output terminal of network 328 is added in a

Claims (33)

1. Apparatus for transforming a multi-channel program including at least first, second, third and fourth program signals into two composite signals, said apparatus comprising: first, second, third and fourth input terminals to which said first, second, third and fourth signals to the extent they are present are respectively applied, first and second output terminals, 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 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 one of said output terminals and said second and third signals, respectively, at the other of said output terminals over the frequency range of interest.

2. Apparatus in accordance with claim 1 wherein said first-mentioned means is operative to transfer said first and fourth signals to said first and second output terminals, respectively, with the same relative phase relationship they exhibit at their respective input terminals.

3. Apparatus in accordance with claim 1 wherein said differential phase-shift angle has a value of substantially 90* and said second signal at one of said output terminals leads said second signal at the other of said output terminals and said third signal at said one output terminal lags said third signal at said other output terminal.

4. Apparatus in accordance with claim 1 wherein the proportions of said second and third signals transferred to said output terminals are substantially 3db down in amplitude from the proportions of said first and fourth signals transferred to said output terminals.

5. Apparatus in accordance with claim 1 wherein said second signal at one of said output terminals leads said second signal at the other of said output terminals and said third signal at said one output terminal lags said third signal at said other output terminal.

6. Apparatus for transforming a multi-channel program including two or more signals identified as Lf, Rf, Lb and Rb into two composite signals, said apparatus comprising: first, second, third and fourth input terminals to which said Lf, Lb, Rb and Rf signals to the extent they are present are respectively applied, first and second output terminals, means connected between said first and fourth input terminals and said first and second output terminals, respectively, operative to transfer said Lf signal to said first output terminal and to transfer said Rf signal to said second output terminal, and means connected between both said second and third input terminals and both said first and second output terminals operative to transfer reduced amplitude proportions of said Lb and Rb signals to both of said output terminals with a 90* relative phase shift angle and to cause said Lb signal at one of said output terminals to lead said Lb signal at the other of said output terminals and said Rb signal at said one output terminal to lag said Rb signal at said other output terminal.

7. Apparatus according to claim 6 wherein said means connected between said input terminals and said output terminals includes first and second summing circuits each having first, second and third output terminals and an output terminal, the output terminal of said first and second summinG circuits being connected to said first and second output terminals, respectively, each of said summing circuits being operative to produce at its output terminal a composite signal including predetermined proportions of signals applied to its first, second and third input terminals, first and second all-pass phase-shifting networks each operative to shift the relative phase of signals applied thereto by an angle differing from a reference phase angle by 45*, said first and second phase-shifting networks being respectively connected between said first input terminal and the second input terminal of said first summing circuit and between said fourth input terminal and the second input terminal of said second summing circuit, third and fourth all-pass phase-shifting networks each operative to shift the relative phase of signals applied thereto by an angle differing from the aforesaid reference phase angle by 90*, said third and fourth phase-shifting networks being respectively connected between said second input terminal and the first input terminal of said second summing circuit and between said third input terminal and the first input terminal of said first summing circuit, and fifth and sixth all-pass phase-shifting networks each operative to shift the relative phase of signals applied thereto by the aforesaid reference phase angle, said fifth and sixth phase-shifting networks being respectively connected between said second input terminal and the third input terminal of said first summing circuit and between said third input terminal and the third input terminal of said second summing circuit.

8. Apparatus according to claim 7 wherein said first and second summing circuits are each operative to produce at its output terminal a composite signal including a first signal corresponding in amplitude to a signal applied to its second input terminal and second and third signals each having an amplitude corresponding to 0.707 of the amplitude of signals applied to its first and third input terminals.

9. Apparatus for transforming a multi-channel program including two or more signals identified as Lf, Rf, Lb and Rb intended for reproduction by loudspeakers positioned at the left front, right front, left back and right back corners, respectively, of a listening area into two composite signals, said apparatus comprising, first, second, third and fourth input terminals to which said Lf, Lb, Rb and Rf signals to the extent they are present are respectively applied, first and second output terminals, 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 Lf and Rf signals to said first and second output terminals, respectively, with the same relative phase relationship they exhibit at their respective input terminals, 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 Lb and Rb signals to both of said output terminals with a substantially 90* phase-shift angle between said Lb signals and between said Rb signals at said output terminals and to cause the Lb signal at one of said output terminals to lead the Lb signal at the other of said output terminals and to cause the Rb signal at said one output terminal to lag the Rb signal at said other output terminal.

10. Apparatus in accordance with claim 9 wherein the proportion of said Lf and Rf signals transferred to said output terminals is related to the proportion of said Lb and Rb signals transferred to said output terminals by substantially the ratio 1:0.707.

11. Apparatus in accoRdance with claim 10 wherein substantially the full Lf and Rf signals applied to said first and fourth input terminals are transferred to said first and second output terminals, respectively.

12. Apparatus for transforming a multi-channel program including up to four audio information signals identified as Lf, Lb, Rb and Rf into two composite signals, said apparatus comprising: first, second, third and fourth input terminals to which the Lf, Lb, Rb and Rf signals, to the extent they are present, are respectively applied, first and second output terminals, a first summing circuit connected to add the Lf signal from the first input terminal to a reduced amplitude proportion of the Rb signal from the third input terminal, a second summing circuit connected to add the Rf signal from the fourth input terminal to a reduced amplitude proportion of the Lb signal from the second input terminal, a third summing network connected to add the signal from the first summing network to a reduced amplitude proportion of the Lb signal from the second input terminal and first phase-shifting means operative to cause the signal received by said third summing network from the second input terminal to be substantially in phase quadrature with the signal received from said first summing network, a fourth summing network connected to add the signal from the second summing network to a reduced amplitude proportion of the Rb signal from the third input terminal and second phase-shifting means operative to cause the signal received by said fourth summing network from the third input terminal to be substantially in phase quadrature with the signal received from said second summing network, and means connecting the output terminals of said third and fourth summing networks to said first and second output terminals, respectively.

13. Apparatus in accordance with claim 12 wherein said first and second summing circuits are operative to add the signals from the first and third input terminals and from the fourth and second input terminals, respectively, substantially in the ratio 1: 0.707.

14. Apparatus in accordance with claim 13 wherein said third and fourth summing networks are operative to add the signals from the first summing network and the second input terminal and the signals from the second summing network and the third input terminal, respectively, substantially in the ratio 1:0.707.

15. Apparatus in accordance with claim 13 wherein said first summing network is operative to add the signals from the first and third input terminals substantially in the ratio 1:-0.707, and the second summing network is operative to add the signals from the fourth and second input terminals substantially in the ratio 1:0.707.

16. Apparatus in accordance with claim 15 wherein said third summing network is operative to add the signals from the first summing network and the second input terminal substantially in the ratio 1:-0.707 and the fourth summing network is operative to add the signals from the second summing network and the third input terminal substantially in the ratio 1:0.707.

17. Apparatus for transforming a multi-channel program including up to four audio information signals identified as Lf, Rf, Lb and Rb into two composite signals, said apparatus comprising: first, second, third and fourth input terminals to which said Lf, Lb, Rb and Rf signals to the extent they are present are respectively applied, first and second output terminals, and signal transfer means connected in circuit between said input terminals and said output terminals for transferring signals from said input terminals to said output terminals, said signal transfer means including first, second, third and fourth summing circuitS each having first and second input terminals and an output terminal, means connecting said third and first input terminals to the first and second terminals, respectively, of said first summing circuit, said first summing circuit being operative to add unequal portions of said Lf and Rb signals, means connecting said second and fourth input terminals to the first and second input terminals, respectively, of said second summing circuit, said second summing circuit being operative to add unequal portions of said Lb and Rf signals, first and second like all-pass phase-shifting networks connected between the output terminal of said first summing circuit and the first input terminal of said third summing network and between the output terminal of said second summing circuit and the first input terminal of said fourth summing circuit, respectively, third and fourth like all-pass phase-shifting networks connected between said second and third input terminals and the second input terminal of said third and fourth summing circuits, respectively, said third and fourth phase-shifting networks being operative to cause a phase-shift to signals applied thereto differing by 90* to the phase-shift introduced by said first and second phase-shifting networks, said third and fourth summing circuits being operative to add unequal portions of signals applied to their first and second input terminals, and means connecting the output terminals of said third and fourth summing circuits to said first and second output terminals, respectively.

18. Apparatus according to claim 12 wherein said first and second phase-shifting networks are operative to shift the relative phase of signals applied thereto by a predetermined reference angle and said third and fourth phase-shifting networks are operative to shift the relative phase of signals applied thereto by an angle differing from said reference angle by 90*.

19. Apparatus according to claim 18 wherein said first and second summing circuits are each operative to add 0.707 of a signal applied to its first input terminal to 1.00 of a signal applied to its second input terminal.

20. Apparatus according to claim 19 wherein said third and fourth summing circuits are each operative to add 1.00 of a signal applied to its first input terminal to 0.707 of a signal applied to its second input terminal, whereby in the composite signal appearing at said first output terminal said Lf signal is in phase with the 0.707 Rb signal component thereof, and in the composite signal appearing at said second output terminal said Rf signal is in phase with the 0.707 Lb signal component thereof.

21. Apparatus according to claim 18 wherein said first summing circuit is operative to add -0.707 of a signal applied to its first input terminal 1.00 of a signal applied to its second input terminal, said second summing circuit is operative to add 0.707 of a signal applied to its first input terminal to 1.00 of a signal applied to its second input termrinal, said third summing circuit is operative to add 1.00 of a signal applied to its first input terminal to -0.707 of a signal applied to its second input terminal, and said fourth summing circuit is operative to add 1.00 of a signal applied to its first input terminal to 0.707 of a signal applied to its second input terminal, whereby in the composite signal appearing at said first output terminal said Lf signal is in phase opposition with the 0.707 Rb signal component thereof, and in the composite signal appearing at said second output terminal said Rf signal is in phase with the 0.707 Lb signal component thereof.

22. Apparatus according to claim 17 wherein said third and fourth phase-shifting networks are each operative to shift the relative phase of signals applied Thereto by a predetermined reference angle and said first and second phase-shifting networks are each operative to shift the relative phase of signals applied thereto by an angle differing from said reference angle by 90*.

23. Apparatus according to claim 22 wherein each of said first and second summing circuits is operative to add 0.707 of a signal applied to its first input terminal to 1.00 of a signal applied to its second input terminal, and wherein each of said third and fourth summing circuits is operative to add 1.00 of a signal applied to its first input terminal to 0.707 of a signal applied to its second input terminal.

24. A method for transforming a multi-channel program including at least first, second, third and fourth audio information signals, to the extent they are present, into two composite signals suitable for recording or transmitting on only first and second channels, comprising the steps of: transferring substantially equal predetermined-amplitude proportions of said first and fourth signals to said first and second channels, respectively, and transferring substantially equal predetermined smaller amplitude proportions of both said second and third signals to both said first and second channels with a substantially constant differential phase-shift angle between said second signal in said first channel and said second signal in said second channel, and between said third signal in said first channel and said third signal in said second channel, over the audio frequency range of interest.

25. A method for transforming a multi-channel program including up to four audio information signals identified as Lf, Lb, Rb and Rf, if present, into first and second composite signals identified as LT and RT, respectively, suitable for recording or transmitting on first and second channels, respectively, comprising the steps of: transferring substantially equal amplitude proportions of said Lf and Rf signals to said first and second channels, respectively, and transferring substantially equal smaller amplitude proportions of both said Lb and Rb signals to both said first and second channels with a substantially constant differential phase-shift angle of substantially 90* between said Lb and Rb signals in said first channel and said Lb and Rb signals, respectively, in said second channel over the audio frequency range of interest.

26. A method according to claim 25 wherein the transferred proportion of said Lf and Rf signals is related to the transferred proportion of said Lb and Rb signals by substantially the ratio 1:0.707.

27. A method for transforming a multi-channel program including up to four audio information signals identified as Lf, Lb, Rb and Rf, to the extent they are present, into first and second composite signals identified as LT and RT, respectively, suitable for recording on first and second channels, respectively, comprising the steps of: adding the Lf signal to a reduced amplitude proportion of the Rb signal to produce a first sum signal, adding the Rf signal to a reduced amplitude proportion of the Lb signal to produce a second sum signal, adding said first sum signal to a reduced amplitude proportion of the Lb signal shifted in phase relative to said first sum signal by substantially 90* to produce a first composite signal LT containing the Lf signal and substantially equal reduced amplitude proportions of the Lb and Rb signals, and adding said second sum signal to a reduced amplitude proportion of the Rb signal shifted in phase relative to said second sum signal by substantially 90* to produce a second composite signal RT contAining the Rf signal and substantially equal reduced amplitude proportions of the Lb and Rb signals.

28. The method according to claim 27 wherein said reduced amplitude Lb and Rb signals are relatively phase-shifted to cause the Lb signals in the two composite signals to be substantially in phase quadrature and the Rb signals in the two composite signals to also be substantially in phase quadrature and the Lb and Rb signals in one of the two composite signals to lead and lag, respectively, the Lb and Rb signals in the other composite signal, said Lf signal to be either in phase or in phase opposition with the Rb signal in said first composite signal and the Rf signal to be either in phase or in phase opposition with the Lb signal in said second composite signal, and at least one of the Lf and Rf signals in said composite signals to be in phase with the signal with which it is either in phase or phase opposition.

29. The method according to claim 28 wherein the Lf signal in said first composite signal is in phase with the Rb signal in said first composite signal.

30. The method according to claim 28 wherein the Lf signal in said first composite signal is in phase opposition with the Rb signal in said first composite signal.

31. The method according to claim 30 wherein the Rf signal in said second composite signal is in phase with the Lb signal in said second composite signal.

32. Apparatus for transforming a multi-channel program including up to four audio information signals identified as Lf, Lb, Rb and Rf into two composite signals suitable for recording or transmitting on respective separate channels, said apparatus comprising; first, second, third and fourth input terminals to which the Lf, Lb, Rb and Rf signals to the extent they are present are respectively applied, first and second output terminals, signal-transfer means including a plurality of summing networks and a plurality of phase-shifting networks connected in circuit between said second and third input terminals and said output terminals, said signal-transfer means being operative to transfer substantially equal portions of the Lb and Rb signals to both of the output terminals with the Lb signals at the two output terminals substantially in phase quadrature and with the Rb signals at the two output terminals also substantially in phase quadrature and the Lb and Rb signals at one of the output terminals leading and lagging, respectively, the Lb and Rb signals at the other output terminal, and means directing connecting said first and fourth input terminals to said first and second output terminals, respectively, for transferring the Lf and Rf signals to the first and second output terminals, respectively, with the same relative phase relationship that they exhibit at their respective input terminals, said phase-shifting networks being operative to shift the Lb and Rb signals transferred therethrough by phase-shift angles which include a reference angle which varies as a function of frequency, thereby causing said Lf and Rf signals to be equally displaced relative to their associated quadrature-related Lb and Rb signals by said reference angle.

33. Apparatus for transforming a multi-channel program including up to four audio information signals indentified as Lf, Lb, Rb and Rf into two composite signals suitable for recording or transmission on respective separate channels, said apparatus comprising, in combination: first, second third and fourth input terminals to which the Lf, Lb, Rb and Rf signals to the extent they are present are respectively applied, first and second output terminals, and signal-transfer means connected between said input terminals and said output terminals for combining the signals applied to the input terminals with predetermined amplitude and phase relationships and transferring them to the output terminals, said signal-transfer means including first, second, third and fourth phase-shifting networks each of which is operative to shift the phase of signals applied thereto by a frequency-dependent reference angle, and said second and third phase-shifting networks each being operative to introduce an additional phase shift of substantially 90*, means including first summing means for coupling the sum of said Lf signal and a portion of said Lb signal through said second phase-shifting network to said first output terminal, means including second summing means for coupling the sum of said Rf signal and a portion of said Rb signal through said third phase-shifting network to said second output terminal, means including said fourth phase-shifting network for coupling a portion of said Lb signal to said second output terminal, and means including said first phase-shifting network for coupling a portion of said Rb signal to said first output terminal, whereby said signal-transfer means is operative to transfer substantially equal portions of the Lb and Rb signals to both of the output terminals with the Lb signals at the two output terminals substantially in phase quadrature and with the Rb signals at the two output terminals also substantially in phase quadrature, and with the Lb and Rb signals at one of the output terminals leading and lagging, respectively, the Lb and Rb signals at the other output terminals, and to transfer the Lf signal to the first output terminal in phase with the Lb signal at said first terminal and to transfer the Rf signal to the second output terminal in phase with the Rb signal at said second output terminal and in anti-phase relationship with the Lf signal at said first output terminal.

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