Quadraphonic Reproducing System

Abstract

Apparatus for decoding four separate channels of sound information transduced from a medium having only two separate tracks and presenting them on four loudspeakers to give the listener the illusion of sound coming from a corresponding number of separate sources. The sound information is contained in two composite signals each containing three of the four separate channels in predetermined amplitude and phase relationships, and the decoder includes a matrix of phase-shifting networks and signal-combining circuits operative to derive from the two composite signals four output signals in which different ones of said four channels are predominant.

Parent Case Text

This is a division of U.S. Pat. application Ser. No. 44,224, filed June 8, 1970.

Description

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 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 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.degree. to each other and at 45.degree. 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, U.S. Pat. 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 U.S. Pat. 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 half-power 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 U.S. Pat. application Ser. No. (44,196) filed June 8, 1970 by B. B. Bauer and Daniel W. Gravereaux now U.S. Pat. No. 3,708,631, 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 reorded 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 four-channel 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 loudpseakers were placed in the corners, the aspect of the originally recorded sound would be shifted by 45.degree., 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 oject 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.degree. 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.degree. 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. 1 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 play back 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 diagram 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;

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;

FIG. 11 is a schematic diagram of a simplified form of playback apparatus embodying the invention;

FIG. 12 is a schematic diagram of another form of playback apparatus according to the invention; and

FIG. 13 is a schematic diagram of still another alternative form of playback apparatus.

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 sets of terminals of a stereophonic cutter 10 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 0.707C, 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.degree. to the horizontal, and when an R signal is applied, the stylus motion is along the arrow R, at an angle of -45.degree. to the horizontal. Application of 0.707 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 0.707 (L + 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 = 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 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, 0.707C, 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 chracteristics of the loudspeaker 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 above-described 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 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 out-of-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 U.S. Pat. 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 applicant's (Bauer's) article entitled "Some Techniques Toward Better Stereophonic Perspective," IEEE TRANSACTIONS ON AUDIO, Vol. AU-11, 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 chanels 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.sub.1 and D.sub. 2, 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 100.degree. to 170.degree. have been successfully used, with an angle of 135.degree. 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 are 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.sub. 1 leads D.sub.2, 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 U.S. 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 (L.sub.f), left back (L.sub.b), right back (R.sub.b) and right front (R.sub.f), 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 anaylsis to follow, that the incoming signals are assumed to be of equal magnitude and referred to the same phase reference. 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 .PSI., 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 .PSI. + 45.degree., and networks 52 and 54 provide a phase shift equal to .PSI. + 90.degree..

The signals L.sub.f and R.sub.f, 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 58 to respective summing circuits 60 and 62. The "left back" signal L.sub.b 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.sub.b is similarly applied in parallel to phase shift networks 54 and 56, the output of which are respectively applied with 0.707 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.sub.T and R.sub.T 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 one 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 40 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 or those of networks to which it is applied, in the example of FIG. 7 to networks 48-58 and will become part of the composite signals L.sub.T and R.sub.T.

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.sub.f ' and R.sub.f ', respectively, both of which are shifted in phase relative to input signals L.sub.f and R.sub.f by 105 + 45.degree.. The L.sub.T signal further includes signals L.sub.b ' and R.sub.b ' at 90.degree. to each other, with the L.sub.b ' signal leading, and in a 45.degree. relationship with L.sub.f '. The C signal appears as C' in both composite signals in the same relative phase position as the signals L.sub.f ' and R.sub.f '.

It is significant to note that the R.sub.T signal contains in addition to the signal R.sub.f ', the two signals L.sub.b ' and R.sub.b ' at 90.degree. to each other. It is important to note, however, that they are reversed in phase relative to the L.sub.T signal, with R.sub.b ' leading and signal L.sub.b ' 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.sub.f ' and R.sub.f ' 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 signal C' being in phase at both of terminals 64 and 66 will cause lateral modulation of the disc record. The fact that signal L.sub.b ' at terminal 64 leads the L.sub.b ' at terminal 66 by 90.degree. 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.sub.b ' at terminal 64 lags behind signal R.sub.b ' at terminal 66 by 90.degree. 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 clock-wise 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.sub.f ' signal causes motion at 45.degree. to the horixontal, the R.sub.f ' signal causes motion at -45.degree. 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.sub.b, corresponding to the "left back" loudspeaker signal, and the counter-clockwise circular modulation R.sub.b, corresponding to the "right back" loudspeaker signal. Since the L.sub.b and R.sub.b 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 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 above-described on a stereophonic record medium for presentation over a four-loudspeaker system. The two signals L.sub.T and R.sub.T 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 70 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 .PSI..sub.1, and network 74 providing a phase shift of .PSI..sub.1 + 90.degree.. The value of .PSI..sub.1 may be the same as the phase shift .PSI. used in the encoding apparatus of FIG. 7, or it may be different, the principal requirement being that the reference phase .PSI..sub.1 be the same in both networks 74 and 76. The phase shift networks 74 and 76 transform the input signals L.sub.T and R.sub.T into two new signals L.sub.T " and R.sub.T ", which are very nearly equal to the signals L.sub.T and R.sub.T except that they are displaced at 90.degree. with respect to each other. This is illustrated by the phasor diagrams in FIG. 9 in which the phasors of the R.sub.T " signal are in the same relative position as in the L.sub.T " signal, but the vectors of the L.sub.T " signal are all rotated 90.degree. clockwise. Since the reference phase shifts .PSI..sub.1 are the same in both networks, their relative effects have been disregarded in presenting the phasor diagrams.

The composite signal L.sub.T " is applied through an additional all-pass phase-shift network 78, which provides a reference phase shift .PSI..sub.2 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.sub.T " is L.sub.f ". Similarly, the composite signal R.sub.T " is passed through an all-pass phase-shifter 84, designed to provide a phase shift of .PSI..sub.2 + 90.degree. and then through an associated gain control amplifier 86 and a power amplifier to loudspeaker 88. This signal contains predominantly the signal R.sub.f ". A signal predominantly containing the "left back" signal L.sub.b " is derived by summing the L.sub.T " and R.sub.T " signals, each after multiplication by the factor 0.707, 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 .PSI..sub.2 + 90.degree., 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.sub.b ", the L.sub.T " and R.sub.T " signals are summed in a similar summing circuit 98 after multiplication by the factors 0.707 and -0.707, respectively. This sum (really difference) signal is translated through phase-shift network 100, which provides a reference phase shift .PSI..sub.2, 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 phase-shifters 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.sub.f ", R.sub.f ", L.sub.b " and R.sub.b ", 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 phase-shifters employed in the circuits of FIGS. 7 and 9 may be of any design that will provide the relatively constant angular differences sepcified in the drawing. Usually, the function .PSI. 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. It 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.sub.b and R.sub.b signals in the matrixor of FIG. 7. This would result in the phasor R.sub.b ' in the left channel leading the phasor R.sub.b ' in the right channel, and would cause phasor L.sub.b ' in the right channel to lead the phasor L.sub.b ' in the left channel. It will be recognized that if this were done, it would be necessary to interchange the all-pass networks 74 and 76 in order to obtain correct decoding of the four individual signals.

FIG. 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 104 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.sub.f ", R.sub.f ", L.sub.b " and R.sub.b " predominate in loudspeakers 82, 88, 96 and 104, respectively; the signals from other channels appearing in each of the main channels are about three 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 effect 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 prinicpal 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 channel separation.

These observations provide the basis of the further improvement and simplification of the decoder system shown in schematic form in FIG. 11. In this system, the two composite signals L.sub.T and R.sub.T, which, again, may be the outputs of a stereophonic pickup or a magnetic head amplifier, are applied to respective all-pass phase-shifting networks 114 and 116 having the porperties described earlier, the latter exhibiting a reference phase shift .PSI..sub.3 and network 114 providing a phase shift .PSI..sub.3 + 90.degree.. The outputs of these phase-shift networks are applied to respective amplifiers 118 and 120, which may be the output amplifiers of a conventional stereophonic phonograph. In place of the two loudspeakers normally used with the stereophonic phonograph, there are provided four loudspeakers 112, 124, 126 and 128. Loudspeaker 122 is connected directly across amplifier 118 (with return through ground) and consequently presents the L.sub.f " signal. Similarly, the output of amplifier 120 directly drives loudspeaker 124 which, therefore, reproduces the output signal R.sub.f ". These two loudspeakers are normally placed at the front of the listening room in the left and right corners, respectively. Loudspeaker 128 is connected across the output terminals of amplifiers 118 and 120, both of which are above ground, with the result that this loudspeaker receives the difference signal output of the two amplifiers. Accordingly, loudspeaker 128 corresponds to the loudspeaker 96 in the system of FIG. 9, and is therefore placed in the back right corner of the listening room or area. A sum signal representing L.sub.b " is conveniently obtained by a transformer 130, the primary of which is connected between the output terminal of amplifier 118 and ground so as to receive the full voltage output of the amplifier. The secondary winding of the transformer is connected between the output terminal of amplifier 120 and the loudspeaker 126, the second terminal of this loudspeaker being connected to ground. With this connection, the output voltages of the two amplifiers are added, and the transformer affords the additional advantage of isolating the output of one of the amplifiers. Consequently, the loudspeaker 126 receives the sum signal and thus corresponds to loudspeaker 96 in FIG. 9 and is placed in the back left corner of the listening area. Loudspeakers 126 and 128 have coil impedances approximately twice that of loudspeakers 122 and 124 to correspondingly reduce their outputs so that the output sounds from the four loudspeakers are essentially balanced. While this circuit does not provide all of the refinements of the FIG. 9 system, it offers a simplified, low cost system capable of providing excellent illusion of four-channel stereophonic reproduction.

Another decoder for reproducing four signals processed for recording by the system of FIG. 7 is illustrated in FIG. 12. The output signals, L.sub.T and R.sub.T, from a suitable pickup device are applied to terminals 132 and 134 and thence through respective all-pass phase shift networks 136 and 138. All-pass network 136, as in the FIG. 11 embodiment, introduces a phase shift of .PSI..sub.3 + 90.degree. and network 138 introduces the reference phase shift .PSI..sub.3, thereby to introduce a relative phase shift of 90.degree. between the two signals. The outputs of the two phase-shift networks, L.sub.T ' and R.sub.T ' are amplified, if necessary, by amplifier 140 and 142, and the outputs thereof intermixed in a resistive network consisting of three resistors 144, 146 and 148. Resistors 144 and 146 each have a value of 1/3R, and resistor 148 has a value of 2/3R, where R is any value chosen to match the impedance of amplifiers 140 and 142. For example, if these amplifiers have a low impedance, of the order of 1 ohm or less, R may have a value of 100 ohms or more. If R is chosen to be 1,000 ohms, for example, resistors 144 and 146 would each have a value of 333 ohms and resistor 148 would have a value of 666 ohms. It is important, also, that the value of R be chosen to have a low impedance compared to the input impedance of amplifiers 150 and 152.

The purpose of the resistive network is to cause, in response to application of a signal L.sub.T to terminal 132, the appearance at the input of amplifier 150 of potential L.sub.T ", and at the same time a signal -9.5 db relative to L.sub.T " at the input of amplifier 152. Conversely, with a signal R.sub.T applied to terminal 134, a corresponding signal R.sub.T " appears at the input of amplifier 152, the resistive network insuring that another signal -9.5 db relative to R.sub.T " appears at the input of amplifier 150.

The output terminals of amplifiers 150 and 152 are connected to four loudspeakers 154, 156, 158 and 160 in the manner illustrated. Specifically, loudspeaker 158 is connected directly across the output terminals of amplifiers 150 and 152 and consequently directly reproduces the difference signal of these two amplifiers. Loudspeaker 154 has one of its terminals connected to the output terminal of amplifier 150, and loudspeaker 156 has its corresponding terminal connected to the output terminal of amplifier 152. The remaining terminals of loudspeakers 154 and 156 are connected together and to one terminal of loudspeaker 160, the other terminal of which is connected to ground and therefore to the return circuit of amplifiers 150 and 152. It is evident, therefore, that loudspeaker 160 carries the sum of the two currents produced by amplifiers 150 and 152 and consequently reproduces the sum of the two signals L.sub.T and R.sub.T. Each of loudspeakers 154 and 156 largely carries the current corresponding to its respective channel L.sub.T and R.sub.T ; however, loudspeaker 160 adds a shunt impedance which permits a small amount of each channel L.sub.T and R.sub.T to act negatively upon the loudspeaker in the other channel. The resistive network consisting of resistors 144, 146, and 148 causes a small portion of the opposite channel voltage to be fed into the opposite amplifier, thereby cancelling the adverse effect of this reverse current and providing the desired L.sub.T to appear at loudspeaker 154 only, and the current from signal R.sub.T to appear in loudspeaker 156 only.

For appropriate action of the circuit, it is necessary that the impedances of the voice coils of the four loudspeakers be properly interrelated. If loudspeakers 154 and 156 are chosen to have an impedance Z.sub.1, which may conveniently be 8 ohms, then loudspeaker 160 should have an impedance of Z1/2 or 4 ohms, while loudspeaker 158 should have an impedance of 2Z.sub.1, or 16 ohms. While these values of impedance may be adjusted for optimum performance in known manner, the values given are favored in that they tend to equalize the powers of all of the channels of the composite signals L.sub.T and R.sub.T.

The embodiments in FIGS. 9, 11 and 12 are adapted to the use of four loudspeakers carrying the full frequency range of interest. For reasons of size and economy, it is sometimes desired to use a single "woofer" loudspeaker to reproduce the low frequency sounds of all the channels (below approximately 200 Hz), but to retain separate and independent loudspeakers for the higher frequency sounds. This objective is achieved in another alternative embodiment of decoder apparatus which is illustrated in FIG. 13. In this embodiment, the so-called "mixed lows" principle is employed, namely, that low frequency signals (i.e., below approximately 200 Hz) are considerably less influential in conveying the effect of direction than signals above this frequency. Consequently, the low frequency response of all of the channels in the composite signal may be mixed together and applied to a single loudspeaker, with the balance of the signals reproduced on four additional loudspeakers, each containing the predominant signal of a given channel.

To this end, the signals L.sub.T and R.sub.T are applied to input terminals 132 and 134, and as in the previous embodiment, are passed through respective all-pass phase-shift networks 136 and 138, respectively, which provide phase shifts of .PSI..sub.3 + 90.degree. and .PSI..sub.3. The present embodiment differs, however, in that one of the signals, for example R.sub.T, is inverted (i.e., shifted 180.degree.). This can be accomplished by inverting one of the generating elements of the pickup, for example, so that the composite signal R.sub.T is applied to therminal 134 in the negative sense, or, alternatively, by utilizing an inverting element in the decoding circuit itself. For example, amplifier 142' may be designed to invert the phase of the signal applied thereto. As in the embodiment of FIG. 12, the outputs of amplifiers 140 and 142' are interconnected by a resistive mixing network consisting of resistors 144, 148 and 146, which in turn, is connected to the input terminals of amplifiers 150 and 152. Of significance to the utilization of the inversion of one of the input signals, R.sub.T for example, a sum signal--instead of the difference signal in the embodiment of FIG. 12--appears at the outputs of amplifiers 150 and 152 which, therefore, contain all, or most of the low frequency or bass response. These low frequency signals are isolated and reproduced by a special low frequency loudspeaker 162 connected between the output terminals of amplifiers 150 and 152, preferably in series with an inductor 164 of a value which prevents all high frequency response above some cutoff point, say 100 Hz, from reaching loudspeaker 132. The loudspeakers 154, 156, 158 and 160 for presenting the balance of the signals are connected in a manner similar to the connections of FIG. 12, the principal difference being that the signals from amplifiers 150 and 152 are connected through respective capacitors 166 and 168 of equal value chosen to prevent all signal energy below the cutoff point from reaching the higher frequency loudspeakers. By reason of inversion of one of the signals, it will be noted that loudspeaker 158, connected between the output terminals of the two amplifiers now carries the sum signal and should, therefore, be installed in the left back corner of the listening area. Similarly, loudspeaker 160 carries the difference signal and should, accordingly, be placed in the right back corner of the room. As in the embodiment of FIG. 12, loudspeaker 154 carries the left front signal and loudspeaker 156 carries the right front signal; accordingly, they should be placed, as before, in the front left and front right corners of the room, respectively. Also as a consequence of inversion of the R.sub.T signal, the "hot" terminal of loudspeaker 156 should be reversed, as indicated by the coding dot at the right-hand side of this loudspeaker.

While the choice of values for capacitors 166 and 168 and inductor 164 for separating and directing the low and high frequency signals into their respective looudspeakers is subject to considerable latitude, it is preferable that capacitors 166 and 168 have a reactance equal to the impedance Z.sub.1 at the cutoff frequency, with the inductance 164 chosen to have a reactance equal to twice the impedance Z.sub.1 at the same frequency. It will be understood, however, that any other appropriate means for dividing the low frequency and high frequency for reproduction over their respective loudspeakers may be employed without departing from the spirit of the invention.

It is also recognized that the choice of resistors 144, 146 and 148 in the values of 1/3R, 1/3R and 2/3R respectively is subject to considerable latitude and is related to the impedance values of the loudspeakers 154, 156, and 158 and 160.

An even simplier system for decoding signals processed for a recording by the system of FIG. 7 is realized by omitting the all-pass phase-shift networks 136 and 138 and applying the signals L.sub.T and R.sub.T directly to the input terminals 170 and 172 of amplifiers 140 and 142', respectively. For best performance in this case, however, loudspeaker 158 which carries the sum signal preferably is placed in the general front area of the listening room, loudspeaker 154 which carries the left front signal should be placed toward the front left region of the listening area, and loudspeaker 156 which carries the right front signal should be placed toward the front right part of the listening area. Loudspeaker 160 which carries the difference signal should be placed in the generally rearward portion of the listening area. The loudspeaker 162 carrying the mixed lows may be placed anywhere in the listening area and yet give pleasant results. These rules it will be recognized are general rules, and each listener may arrange the loudspeakers to positions to produce results pleasant to him.

It will, of course, be recognized that the system of FIG. 13, with the phase shift networks 136 and 138 omitted, is operative to reproduce the four principal signals on respective loudspeakers without the refinement of the "mixed lows" feature. That is, the circuit of FIG. 3 may be further simplified by omitting capacitors 166 and 168, inductor 164 and special loudspeaker 162. When this is done, loudspeakers 154, 156, 158 and 160 carry the predominant left front, right front, center front and back information, respectively, throughout the full frequency range they are capable of reproducing.

Claims

I claim:

1. Apparatus for reproducing on four loudspeakers four audio information signals respectively designated L.sub.f, R.sub.f, L.sub.b and R.sub.b contained in first and second composite signals respectively containing dominant signals L.sub.f and R.sub.f and each including sub-dominant signal components L.sub.b and R.sub.b and wherein said signal components L.sub.b and R.sub.b in one of said composite signals are in substantially quadrature relationship with said signal components L.sub.b and R.sub.b, respectively, in the other of said composite signals, with the L.sub.b component leading the R.sub.b component in one of said composite signals and with the L.sub.b component lagging the R.sub.b component in the other of said composite signals, said apparatus comprising:

first and second input circuits for respectively receiving said first and second composite signals, said input circuits including means for shifting the phase of one of said composite signals relative to the other by substantially 90.degree. to thereby position said L.sub.b and R.sub.b signals components in one of said composite signals either in phase coincidence or in phase opposition with the L.sub.b and R.sub.b signal components, respectively, in the other of said composite signals,

first and second amplifiers each having first and second input terminals and an output terminal, the second input terminal of each of which is connected to a point of reference potential,

means for coupling the relatively phase-shifted composite signals from said first and second input circuits to the first input terminal of said first and second amplifiers, respectively,

first, second, third and fourth output circuits each including a respective one of said loudspeakers for respectively displaying composite signals containing said L.sub.f, R.sub.f, L.sub.b and R.sub.b signals are predominant components, each of said loudspeakers having first and second terminals,

means connecting the output terminals of said first and second amplifiers to one terminal of said first and second loudspeakers, respectively,

means connecting said third loudspeaker in circuit between the output terminals of said first and second amplifiers and operative to couple to said third loudspeaker a signal proportional to the sum of said relatively phase-shifted first and second composite signals and containing said L.sub.b signal as its predominant component, and

means connecting said fourth loudspeaker in circuit between the output terminals of said first and second amplifiers and operative to couple to said fourth loudspeaker a signal proportional to the difference of said relatively phase-shifted first and second composite signals and containing said R.sub.b signal as its predominant component.

2. Apparatus according to claim 1 wherein said first and second loudspeakers are respectively connected between the output terminals of said first and second amplifiers and said point of reference potential, said fourth loudspeaker is connected between the output terminals of said first and second amplifiers, said third loudspeaker has one of its terminals connected to said point of reference potential, and including a transformer having a first winding connected between the output terminals of said first amplifier and said point of reference potential and a second winding coupled in adding relationship to said first winding and connected between the output terminal of said second amplifier and the other terminal of said third loudspeaker.

3. Apparatus according to claim 2 wherein said first and second loudspeakers have substantially equal impedances of value Z and said third and fourth loudspeakers have substantially equal impedances of approximately 2Z.

4. Apparatus according to claim 1 wherein a resistive network is connected to the first input terminal of said first and second amplifiers and operative to apply a portion of said relatively phase-shifted first and second composite signals to the first input terminals of said second and first amplifiers, respectively, said first and second loudspeakers are connected in series between the output terminal of said first amplifier and the output terminal of said second amplifier, said fourth loudspeaker is connected between the output terminal of said first amplifier and the output terminal of said second amplifier, and said third loudspeaker has one of its terminals connected to the junction between said first and second loudspeakers and its other terminal connected to said point of reference potential.

5. Apparatus according to claim 4 wherein said first and second loudspeakers have substantially equal impedances of a value Z, said third loudspeaker has an impedance of approximately 2Z, and said fourth loudspeaker has an impedance of approximately Z/2.

6. Apparatus according to claim 1 further including means connected between said second input circuit and said second amplifier for inverting said phase-shifted second composite signal before application to the first input terminal of said second amplifier, and a fifth loudspeaker connected in series with an inductor between the output terminals of said first and second amplifiers, said series circuit having a reactance to accept signals of frequencies below a predetermined cutoff frequency and to reject frequencies above said cutoff frequency whereby said fifth loudspeaker displays the "mixed lows" contained in the sum of said first and second composite signals, and wherein said first and second loudspeakers respectively have one of its terminals capacitively connected to the output terminals of said first and second amplifiers and their other terminals connected together, said fourth loudspeaker is connected between the junction of said first and second loudspeakers and said point of reference potential, and said third loudspeaker is connected between the said one terminals of said first and second loudspeakers.

7. Apparatus according to claim 6 wherein said first and second loudspeakers have substantially equal impedances of value Z, said third and fifth loudspeakers have substantially equal impedances of approximately 2Z, and said fourth loudspeaker has an impedance of approximately Z/2.

8. Apparatus according to claim 7 further including a resistive network connected to the input terminals of said first and second amplifiers operative to apply a portion of said relatively phase-shifted first composite signal also to the input terminal of said second amplifier and to apply a portion of said inverted relatively phase-shifted second composite signal also to the input terminal of said first amplifier.