United States Patent 1 1 Bauer 1 Nov. 6, 1973 STEREO-QUADRAPHONIC DISC RECORD  Inventor: Benjamin B. Bauer, Stamford, Conn.
 Assignee: Columbia Broadcasting System, Inc.,
New York, NY.
221 Filed: Sept.30, 1971 211 Appl.No.: 185,204
Related U.S. Application Data  Division of Ser. No. 44,244, June 8, 1970, abandoned.
52 us. c1.' ..179 100.4 ST, 179/1004 (3, 274142 R 51 1m. (:1. c1111 3/74 58 Field of Search 179/1001 B, 100.4 R,
179/1004 sr, 100.4 c; 274/42 R  References Cited OTHER PUBLICATIONS Bauer, Some Techniques Toward Better Stereophonic Perspective, May-June 63, IEEE Transactions on Audio, Vol. AU-l 1, No. 3.
Primary Examiner-Raymond F. Cardillo, Jr. Att0rney-Spencer E. Olson et al.
 ABSTRACT A stereophonic disc record for a four channel sound system has recorded in the groove thereof, as +45 and 45 modulations, signals intended for presentation at the left front and right front corners, respectively, of a listening area, together with modulations adapted to cause a playback stylus to describe a circular motion in one direction representative of audio signals intended for presentation as one of the back corners of a listening area, and modulations adapted to cause a transducing stylus to describe a circular motion in the opposite direction representative of signals intended for presentation at the other back corner of the listening area. The record is reproducible on monophonic or stereophonic playback systems, the former reproducing all of the channels with approximately equal balance, and the latter distributing the four separate channels to the two loudspeakers in a manner to give a balanced and symmetrical reproduction. With an appropriate decoder (also disclosed) the signals transduced from the record may be separated into four signals for presentation on respective loudspeakers positioned at the corners of a listening area to give the illusion of four separate sources of sound.
4 Claims, 10 Drawing Figures PATENTEDNBV sum 3.770.901- SHEET 16F 3 c 0 A vi E .707 INVENTOR.
. 5H? 0 BENJAMIN B. BAUER .707 BY 24M 32 5' @1494;
' I ATTORNEY PAIENTEmuv- 61975 3.770.901 SHEET 20F 3 INVENTOR. BENJAMIN B. BAUER ATTORNEY STEREO-QUADRAPHONIC DISC RECORD This is a division of application Ser. No. 44,244, filed June 8, 1970, now abandoned.
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 due to the well known phenomenon that the quality of music reproduction is enhanced when the number of reproduction channels increases. In the early days of the phonograph, only single channel or monophonic recording was used, and as early as 40 or 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 not considered sufficient 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. Oftentimes, to obtain special effects, a fourth channel may be applied to the tracks in phase opposition, in a manner exemplified by test records Models STR 110, 111 and 120 produced and distributed by CBS Laboratories, a Division of the assignee of this invention. 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 illusion of the third channel being located between the other two. The fourth, or vertical, channel when reproduced on a conventional two-loudspeaker stereophonic phonograph gives the illusion of unlocalized sound. 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 co-pending application of William S. Bachman, Ser. No 40,510 filed May 26, 1970, and assigned to the assignee of the present invention, now abandoned in favor of continuation-in-part application Ser. No. 164,675, filed July 21, 1971, 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 representing 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 halfpower signals from its adjacent channels. This system provides a pseudo-four-channel effect, but does not give a complete illusion 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 co-pending application Ser. No. 44,196 filed June 8, 1970, now US. Pat. No. 3,708,631 by B. B. Bauer and Daniel W. Gravereaux and assigned to the assignee of the present invention, 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) information originating from the back 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 undesirable 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) in 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 summetry with 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 method and apparatus for transferring a program recorded on four channels, edited for presentation on four loudspeakers placed in the corners of a room, into a two-channel program suitable for presentation over two loudspeakers placed in adjacent corners of a listening area in a manner to obtain an artistically acceptable result; that is, so that the four separate channels will be distributed over the two loudspeakers in a manner to create the illusion of the original four-loudspeaker presentation in a symmetrical and balanced manner to give a pleasant reproduction.
Another object of the invention is to provide a method and apparatus for recording four channels of information on a two-track medium in which the two additional channels are recorded with types of modulation that will admit of subsequent decoding into four separate signals corresponding to the four signals originally encoded.
A further object of the invention is to provide a method for recording four channels of information on a two-track stereophonic disc record which does not require significant addition of vertical modulation.
A still further object of the invention is to provide a method of recording which enables addition of a common central signal which appears superimposed centrally upon the four main signals.
Still another object of the invention is to provide signal processing apparatus for converting the signals recorded on the two-track medium into four separate signals respectively predominantly containing the four original signals used in recording the record.
Yet another object of the invention is to provide, in combination with the above-mentioned signal processing circuit, a system for accentuating the dominance of any particular channel in response to signals contained in that channel to thereby provide a more realistic illusion of four separate independent sources of sound.
Briefly, the foregoing objects are obtained by recording on a two-track disc record or two-track magnetic tape the four channels of information intended to be identified with four loudspeaker sources by means of a combination of phase shift networks and adding circuits in such a manner that the following relationships are obtained. The signal which for convenience may be identified as the front left signal is recorded in the left channel, and the signal identified with the right front" loudspeaker is recorded on the right" channel. The signal identified with the left back" loudspeaker is recorded on both the left and the right channels at 90 out-of-phase with each other, and similarly, the signal identified with the right back" loudspeaker is also recorded on both the left" and the right channels, also at 90 with respect to each other. However, the signal corresponding to the left back loudspeaker is recorded with the left" channel component leading, and that identified with the right back loudspeaker is recorded with the right" channel component leading. Thus, a stereophonic disc record recorded in accordance with the invention basically contains four different signals, a left front signal appearing as modulation of the left wall of the record groove, a right front signal appearing as modulation of the right groove wall, a left back signal manifested as groove modulation resulting from a clockwise circular motion of the cutting stylus, and a right back" signal manifested by groove modulation resulting from a counterclockwise motion of the cutting stylus. The circular motion is effectively obtained by the quadrature motion of the two groove walls, and as the stylus moves along with respect to the groove, the circular motion causes a helix to be generated.
In another aspect of the invention, a signal intended to appear in reproduction as a phantom between any two loudspeakers may be applied equally to any of the two channels, or, if desired, to all of the channels together. Thus, if a center signal is used, the record made in accordance with the principles of this invention may contain five distinct signals.
Another important aspect of the invention resides in the signal processing, or dematrixing apparatus, which contains two all-pass phase shift networks for changing the relative phases of the signals transduced from the left and the right channels to permit their being reconstituted into four separate signals for presentation on four separate loudspeaker systems. Such signal processing apparatus may include the decoder control logic and gain control amplifiers described in the aforementioned Bauer et al application for further emphasizing the individual signals as they predominate in any one of the loudspeaker circuits.
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 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 stereophonic playback system for providing the illusion of a third channel;
FIG. 5 is a schematic diagram of the system described in the aforementioned Bauer et al application 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 a combiner or matrixor according to the invention for combining independent signals intended for ultimate display on four separate loudspeakers;
FIG. 8 is a vector digaram useful in explaining the operation of the circuit of FIG. 7;
FIG. 9 is a schematic diagram of one form of playback apparatus embodying the invention; and
FIG. 10 is a plan view of a listening area illustrating the location of four loudspeakers therein and the phasor diagrams of the signals appearing on the four loudspeakers.
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. 14. The currently provided left (L), right (R) and center (C) signals are applied to the two sets of terminals of a stereophonic cutter having a cutting stylus 12 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 applied through a signal splitter 16 of known configuration resulting in application of portions thereof, equivalent to .707C, to each of the L and R lines in an additive manner. 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 .7 07 parts of C to each of the L and R lines in an additive manner causes motion of the stylus along the arrow C, equal in magnitude to .707 (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, be first 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 .707C and R .707C, resulting in a 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 modulation 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, .707C, appear on 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, and 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 ofL and R are .707 ofC. 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 perceive 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 .707 times the amount of D contained in the left and right channels subtracted from each other; i.e., .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. 164,675 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 is preferably applied in the manner suggested in applicants (Bauers) article entitled Some Techniques Toward Better Stereophonic Perspective, IEEE TRANSACTIONS ON AUDIO, Vol. AU-l I, No. 3, May-June, 1963. 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. 1, 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 of the frequencies of the D signal, but displaced in phase with respect to each other. Relative phase displacements in the range of 1 to 170 have been successfully used, with an angle of 135 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 channels of information recorded as described above is enhanced by the control system described in the aforesaid Bauer and Gravereaux US. Pat. No. 3,708,631 which derives signals from the left and right terminals of the 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 loudness of the respective components in a manner to give a substantially perfect illusion of four separate independent sources of sound.
DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to FIG. 7 there is shown in schematic form 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 transmission on a two-track medium, such as a stereophonic disc record or a two-track tape. The matrixor includes four input terminals 40, 42, 44 and 46 intended to receive four separate signals which, for convenience, will be designated left front (Ly), 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 ref-' erence. The combining matrix includes six all-pass phase- shift networks 48, 50, 52, 54, 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 ili, which is a function of frequency, the two phase shifters 50 and 56 introducing only the reference phase shift. Phase shifters 48 and 58 provide a phase shift equal to if; 45, and networks 52 and 54 provide a phase shift equal to 1: 90.
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 associated all- pass networks 48 and 48 to respective summing circuits 60 and 62. The "left back signal L, is applied in parallel to phase shift networks 50 and 52, the output of the former being applied to summing circuit 60 with attenuation corresponding to the multiplicand 0.707, and the output of network 52 is applied to summing circuit 62 with the same attenuation. The right back" signal R, is similarly applied in parallel to phase shift networks 54 and 56, the output of which are respectively applied with .707 attenuation to summing circuits 60 and 62. The summing 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 terminals of a stereophonic 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 matrixing 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 system, a center signal designated by the arrow labeled C, may be applied equally and in phase to terminals 46 and 46, or to the terminals 42 and 44, or to all four terminals simultaneously, as indicated by the curved arrows. 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 L and R The nature of the composite signals appearing at terminals 64 and 66 will be seen from the vector 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 if: 45. The L, signal further includes signals L',, and R,, at to each other, with the L',, signal leading, and in a 45 relationship with L',. The C signal appears as C in both composite signals in the same relative phase position as the signals L, and R',.
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 is important to note, however, that they are reversed in phase relative to the L signal, with R,, leading and signal 11;, 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.
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 of terminals 64 and 66 will cause lateral modulation of the disc record. The fact that signal L at terminal 64 leads the L',, at terminal 66 by 90 will cause modulation of the record groove in a clocksie 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 being recorded, in the manner of a 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, signalcauses motion at 45 to the horixontal, 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 the 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 capability with a monophonic phonograph player. An important advantage of the just-described method of combining the input signals to form the two composite signals is that the stereophonic record or tape recorded in this manner can be replayed over any stereophonic or monophonic player with full and complete reproduction of all of the sounds recorded on the record.
It will be recognized that all of the modulations need not be present throughout the record groove; that is, only portions of the length of the groove may have modulations therein representative of a left back signal, for example, as would cause the reproducing stylus to describe a circular motion in the clockwise direction, and there may be other portions containing only modultaions representative of right back signal information as would cause the stylus to describe a circular motion in the counterclockwise direction. The locations along the length of the groove at which such portions would appear would depend on the nature of the recorded selection, and should it happen that left back and right back signal information is recorded on the same portion of the groove, the motion of the stylus would be a complex motion representing the resultant of the superposition of the two modulations. If it happens that a given selection results in the simultaneous recording on a given portion of the groove of left and/or right front signals in addition to left and/or right back signals, the stylus motion would again be complex, representing the resultant of the components of motion contributed by the simultaneously appearing signals. The portions of the groove in which the various modulations appear are of a length depending upon the time the signal or signals associated with that portion are to be reproduced on their corresponding loudspeaker, and in the case of signals which impart circular motion to the stylus the portion is sufficiently long to encompass at least one full cycle of the signal, the minimum required to produce the circular motion; in the claims the term recognizable portion is used to define this minimum length.
It is recognized that other types of modulation may occur during the recording process. Reverberant components, for example, often occur in an out-of-phase mode distributed. between the input terminals. It is not intended to restrict the signal to the five distinct types recited above; other forms of modulation in addition to these basic five produce results which are of intermediate nature, which may be analyzed in terms of these basic ones.
Referring now to FIG. 9, there is illustrated one .form of apparatus for decoding the four individual signals from the two composite signals recorded as abovedescribed on a stereophonic record medium for presentation over a four-loudspeaker system. The two signals L, and R are derived from the record medium by a suitable transducer (e.g., a conventional stereophonic pickup in the case of a disc record) and are applied to terminals and 72, respectively of the decoder apparatus. These signals are first translated through respective all-pass phase- shift networks 74 and 76, the latter providing a reference phase shift of I111, and network 74 providing a phase shift of [I]! 90. The value of 41 may be the same as the phase shift ill used in the encoding apparatus of FIG. 7, or it may be different, the principal requirement being that the reference phase [Ill be the same in both networks 74 and 76. The phase shift networks 74 and 76 transform the input signals L and R into two new signals L" T and R",, which are very nearly equal to the signals L and R except that they are displaced at 90 with respect to each other. This is illustrated by the phasor diagrams in FIG. 9 in which the phasors of the R signal are in the same relative position as in the L", signal, but the vectors of the L" signal are all rotated 90 clockwise. Since the reference phase shifts ip, are the same in both networks, their relative effects have been disregarded in presenting the phasor diagrams.
The composite signal L" is applied through an additional all-pass phase-shift network 78, which provides a reference phase shift 41, without changing the amplitude of the signal, and thence through a gain control amplifier 80 (whose gain may be controlled by applying a control signal to the amplifier) and a suitable power amplifier to a loudspeaker 82. In the normal operating mode the predominant signal in L is L",. Similarly, the composite signal R is passed through an all-pass phase-shifter 84, designed to provide a phase shift of 41 and then through an associated gain control amplifier 86 and a power amplifier to loudspeaker 88. This signal contains predominantly the signal R",. A signal predominantly containing the left back signal L", is derived by summing the L and R" signals, each after multiplication by the factor .707, in a summing circuit 90 of known configuration. The output of summing circuit 90 is applied to all-pass phase-shifter 92 which introduces a relative phase shift of 1p, 90, and thence through gain control amplifier 94, and a suitable power amplifier, to loudspeaker 96. To obtain the fourth signal, predominantly containing the right back signal, R",,, the L and R" signals are summed in a similar summing citcuit 98 after multiplication by the factors .707 and .707, respectively. This sum (really difference) signal is translated through phase-shift network 100, which provides a reference phase shift i11 and is then applied to gain control amplifier 102 and thence through a power amplifier to loudspeaker 104.
The gains of amplifiers 80, 94, 102 and 86 are controlled by a control and switching logic 106 in response to signals derived from the output terminals of phaseshifters 74 and 76 in the manner described in the aforesaid U.S. Pat. No. 3,708,631. Briefly, the control and switching logic 106 is operative to recognize the channel or channels having the dominant signal among L,, R",, L", and R",,, and applies control signals to the gain control amplifiers which increase the gain of the channel containing the instantaneously dominant signal and to reduce the gain of the other channels to give a substantially perfect illusion of four separate independent sources of sound. As the sound diminishes in the channel first identified and another sound appears on a different channel, the logic circuit functions to rapidly attenuate the gain in the first channel and to increase the gain in a different channel.
It should be mentioned that the all-pass phaseshifters employed in the circuits of FIGS. 7 and 9 may be of any design that will provide the relatively constant angular differences specified in the drawing. Usually, the function it; should be as small as possible, consistent with the provision of the necessary differential phase shift. The phase-shift function should be smooth, without any rapid changes of phase angle as a function of frequency, as such rapid changes might cause slight changes in the timbre of the sound being recorded and- /or reproduced. lt should also be noted that the sense of rotation of the respective helical modulations can be reversed in the recording process by inverting the relative phase shifts of the L and R signals in the matrixor of HG. 7. This would result in the phasor R',, in the left channel leading the phasor R, in the right channel, and would cause phasor L, in the right channel to lead the phasor L, in the left channel. It will be recognized that if this were done, it would be necessary to interchange the all- phase networks 74 and 76 in order to obtain correct decoding of the four individual signals.
H6. 10 illustrates the location in a listening room or area of loudspeakers 82, 96, 104 and 88 for optimum display of the signals decoded by the system of FIG. 9. Specifically, loudspeakers 82, 88, 96 and I04 are placed in positions corresponding to the left front," right front, left back and right back," respectively. Phasor diagrams of the signals appearing on each of these loudspeakers are presented adjacent their respective loudspeaker. it will be observed that the signals L", 12",, L", and RQ, predominate in loudspeakers 82, 88, 96 and 104, 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.
A significant improvement in the direction of eliminating the efiect on the listener of the lower level signals from other channels appearing in the main channel is obtained by the action of the control and switching logic 106 (FIG. 9). As was mentioned earlier, a given loudspeaker is turned on or momentarily increased in signal intensity, by its associated gain control amplifier when the principal signal appears in that channel, resulting in a positive gain control signal being applied to the amplifier, while the gain control amplifiers in the other channels are momentarily reduced in gain to attenuate or turn off" the signals applied to the other loudspeakers. This action greatly enhances the illusion of four independent channels. It is to be understood, however, that it has been observed that the objects of the invention are partly achieved without the use of the gain control amplifier and associated control and switching logic. In other words, the system of FIG. 9 with control amplifiers 80, 94, 102 and 86 and their associated control and switching logic 106 removed, is operative to display the four separate signals on four separate loudspeakers with a high degree of realism.
Furthermore, it has been observed that phase shifting networks 78, 92, 100 and 84, while contributing to the audible channel separation of the four-loudspeaker system, can be dispensed with and still obtain acceptable chanel separation.
I. A stereophonic disc record having a single spiral V-shaped groove adapted to receive a stylus for travel therein as the record is rotated with respect to the stylus, the two side walls of sais groove each being at an angle of substantially 45 with respect to the record surface and at substantially to each other,
at least a recognizable portion of said groove having modulated deformation means on the walls thereof representative of a first audio signal for causing said stylus to describe, as said portion of the goove moves past the stylus, a circular motion in a clockwise direction in the plane perpendicular to the direction of motion between the record and stylus, and
at least a recognizable portion of said groove having modulated deformation means on the walls thereof representative of a second audio signal for causing said stylus to describe, as said portion of the groove moves past the stylus, a circular motion in a counterclockwise direction in said plane.
2. A stereophonic disc record according to claim 1 wherein at least a recognizable portion of said groove has modulated deformation means on at leas one of the walls thereof representative of a third audio signal for causing said stylus to move in a direction perpendicular to said at least one wall.
3. A stereophonic disc record according to claim 1 wherein at least one wall of at least one of said portions of said groove has further modulated deformation means thereon representative of a third audio signal for imparting to said stylus a component of motion in a direction perpendicular to said at least one wall.
4. A stereophonic disc record according to claim 1 wherein at least a recognizable portion of said groove has modulated deformation means on at least one of the walls thereof representative of a third directional audio signal for causing said stylus to move, as said portion of the groove moves past the stylus, in a direction perpendicular to said at least one wall,
at least a recognizable portion of said groove has modulated deformation means on the other wall thereof representative of a fourth audio signal for causing said stylus to move, as said portion of the groove moves past the stylus, in a direction perpendicular to said other wall, and
wherein said first and second audio signals are intended for reproduction at the left back and right back corners, respectively, of a listening area, and said third and fourth audio signals are intended for reproduction at the left front and right front corners, respectively, of a listening area.