SQ Quad Broadcasting by Ty chamberlain

IEEE TRANSACTIONS ON BROADCASTING, VOL. BC-23, NO. 3, SEPTEMBER 1977 ADVANCES IN QUADRAPHONIC MATRIX BROADCASTING

Benjamin B. Bauer, Fellow IEEE CBS Technology Center

Stamford, CT 06905 Abstract

TMStereo-

and mono-compatible stereo-quadraphonic ) "matrix" recording and broadcasting is in wide use in the U.S.A. and abroad. Nevertheless, arguments continue about the need for "discrete" recording and broadcasting because of implied improvement they afford in channel separation and in artistic freedom in the use of the four channels. Both of these inferences are adduced to be fallacious. SQ quadraphonic broadcasting in the form of SQ records, syndicated productions, and encoded four-channel sources followed by full-logic decoding at the receiver end meets the identified needs of quadraphony. Two new advances are described in this paper: the SQ Ghent Microphone System which, from a single point in space is capable of picking up and transmitting a stereo- and mono-compatible pair of signals replicating the original directional performance either of the ambiance or surround-sound type, and a center-back "London Box" signal processor which encodes a stereo- and mono-compatible center-back signal and allows it appropriately to be decoded in the quadrilateral space with an SQ decoder.

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Introduction

Audio professionals are well aware of the significant advance in high-fidelity ("spatial high fidelity") possible through quadraphony--sound reproduction over four loudspeakers surrounding the listener. Quadraphony allows us, for example, to portray the concert stage over the two front loudspeakers, as with stereo, with the added dimension of two back loudspeakers cooperating with the front ones to reproduce the concerthall ambiance. Quadraphony permits us to distribute specific sources at, or in-between, the loudspeakers to recreate a performance in a cathedral where several organs or choirs furnish an antiphonal interlude. Quadraphony even is able to place us on the conductor's podium or in the midst of a group of musicians resulting in a new and exciting surround-sound experience. And, these remarkable results can be derived from the original directional signals combined through an "encoder" according to specific matrix equations to form a stereo pair which can be recorded or broadcast using existing equipments and techniques or as shown hereinafter, through the use of a special transducer array at a point in space.1'2'3'4 This method of storing and transmitting four-channel directional information is known as "matrix" quadraphony. As an alternative, the four signals to be reproduced can be recorded on four separate channels of a multitrack magnetic tape, or distributed on modulated carriers inscribed on a special wideband disc (e.g. "CD-4") record or transmitted over one of several proposed experimental quadruplex FM transmitters with mulThis method of storing and tiple subcarriers.5'6'7 transmitting four-channel information is known as "discrete" quadraphony.

Manuscript submitted February 4, 1977. Portions of this material were presented at the IEEE Broadcast Convention, in Washington, D.C., September 24, 1976 and at the Third Annual Society of Broadcast Engineers New York Convention, Hempstead, N.Y., November 8, 1976. SQ is a Trademark of CBS Inc.

This paper deals with recent advances in the art of quadraphonic matrix broadcasting; but since several issues have arisen between the proponents of the discrete and the matrix methods, it is fitting at this time to review them in order to bring the subject, according to our point of view, into proper focus. The Discrete vs. Matrix Issue

Discrete quadraphony in comparison with matrix methods requires (a) a greater area of recording medium, (b) more complex and costly apparatus for disc recording, reproduction, and broadcasting, and (c) greater bandwidth of radio spectrum, at the same time resulting in (a) diminution of the playing time on a disc, (b) a lower signal-to-noise ratio in recording and broadcasting, and (c) potentially lowered listener coverage.5,6,8These disadvantages, it is often alleged, are counterbalanced by(a)greater channel separation feasible with discrete methods and (b) the supposition that the discrete approach leads to greater artistic freedom in the use of the four channels. We now adduce t-hat these allegations are without practical merit. The channel separation argument has to be weighed in the light of the needs and capabilities. The need factor, of course, is subjective and is based on the artistic content of the program. It has been noted that relatively little separation is needed, say, for a choir in a highly reverberant church; a great deal is desired for a musical "ping-pong game." With stereophonic pickups and disc records, over the years a 20 dB minimum overall left-right channel separation guideline has emerged. A more stringent 29.7 dB FCC test is used to verify FM stereo transmitter performance, which is propitious, as the various separation dilutants in a system are cumulative.9 Our own experience has confirmed that a 20 dB separation minimum also is desirable when the concert stage is defined by the two front channels in quadraphony in order to preserve the artistic integrity of orchestral music. For man, front sources are dominant and the auditory location tends to be established by the transient sounds of first arrival.1'10 Slightly (but not much) lower overall separation figures appear to be acceptable for the remaining three quadrants which usually carry the reverberant sounds but which also are often used for antiphonal or solo sounds in surround-sound performances. The above criteria appear not to conflict with 1975 large scale tests sponsored by the National Quadraphonic Radio Committee (NQRC) in which 918 musical tests were administered to 184 naive auditors in two different cities (San Francisco and Syracuse), to assess their preference among (a) a fully discrete (4-4-4) system, (b) a semi-discrete (4-3-4) system with 10 dB interchannel separation, and (c) a "BMX" matrix (4-2-4) system (without logic) with 3 dB interchannel No preferences appear to have been separation.6'11 established between the first two systems (49% vs. 51% and 57% vs. 43% preference votes), while a degradation threshold was perceived between the first two systems and the third (77% vs. 23% and 74% vs. 26%). We believe that trained listeners might have been more dis-

criminating. *

Because approximately 50% of the listeners appeared to have noticed the difference, while the 50% who did not notice the difference simply guessed.

duct. Few producers, of course, would be so motivated. But, as a practical fact, we have had to cope with one well-known producer who interprets the original performance by placing the soloists on the diagonal corners for antiphonal response. In the discrete mode this provides an interesting sonic interplay; but for the stereo listener it results in an immobile center signal reproduction. When encoded with the forward-oriented SQ encoder, however, an antiphonic reproduction is heard in both the stereophonic and the decoded quadraphonic

As to capabilities, it is known that well-designed four-channel tape machines can have an interchannel separation in excess of 40 dB. But, importantly, we have to focus on the disc medium which is responsible for the majority of all programs broadcast, at least in the U.S.A. A discrete carrier disc, per se, has a left-right baseband interchannel separation equal to that of the stereodisc, but its front-back baseband crosstalk is 100%, equivalent to 0 dB channel separation.5 With a suitable pickup and demodulator (depending upon the smoothness of the pickup response and the tracking between the compressor in the recording studio and the expander in the demodulator), front-back separation of a discrete disc reported in one study has been established to average 14 dB with a range from 5 Similarly, the SQ encode/decode to 30 dB at 2 kHz.8 system theoretically produces infinite interchannel separation for the front and the back pairs of channels but, in practice, this figure is limited by the design of the decoder and by the pickup separation and response. The front-back interchannel separation proBy using a vided by the decoding matrix is 3 dB.1 crosstalk cancellation logic decoder, 30-40 dB frontback separation figures have been obtained in prototype decoders. However, with a commercially available fulllogic "power transfer" unit, crosstalk of the order of 16-20 dB at 2 kHz has been reported.8

modes.*

By the same token, with the discrete system, the producer who elects to place four soloists in the corners of the quadrantn ends up with the signals concentrated on the sides, with a "hole in the middle" in the stereo mode. In contrast, with SQ, the corner quadraphonic arrangement results in four distinct sources anpropriately distributed over the stereophonic field.12 Of course, any matrix system also has its prohibitions. With SQ, as with the discrete system, application of equal antiphase signals to adjacent channels is not recommended. The position of instruments in surround-sound orchestral recording should be such as to promote a pleasing quad/stereo fold.12 And because the center-back signal in SQ usually is recorded antiphase, and thus is not heard by the monophonic listener, this position generally is reserved for reverberation, with the beneficial effect that it tends to be diminished in the monophonic mode. ** Center-top signals with SQ must be applied by establishing a 10-15 ms delay between the front and the back pairs of channels (except when panning along the diagonals, where the delay apparatus is not required). The above and similar experiences have demonstrated that the SQ matrix system skillfully handled provides at least as much artistic freedom to a knowledgeable producer as does a discrete system. A novice will commit blunders with any system; an experienced producer learns the rules of the road and avoids the pitfalls. A corollary is that a four-channel master tape made for a given system of quadraphony is not necessari. ly optimum for another system, but alas, this prime rule often is ignored by the inexperiencedi

An argument has been made that in a discrete system the separation properties are maintained regardless of the program content, while in a matrix system, the separation figures are defined for individual signals only. This would appear to be a cogent argument except for the fact that the average human ear (like the eye) finds it difficult to focus on more than one (usually the predominant) sensory stimulus at a time, and thus the effect, if it exists, matters little from a practical point of view, especially since the record producer is the final arbiter of the system performance. Thus, it can be said without equivocation that using modern equipment, the discrete vs. matrix channel separation issue is, for all intents and purposes, a non-issue. Separation-wise, either approach--discrete or SQ matrix with full-logic--meets the realistic needs of practical quadraphony, however the latter offers (a) higher fidelity, (b) greater economy, and (c) superior compatibility with existing broadcasting and sound reproducing apparatus.

SQ Broadcasting

The Artistic Freedom Issue

The second argument--that of greater artistic freedom--stems from the belief that signals can be applied to the four independent channels by the producer as the spirit moves him while with a matrix system he is circumscribed by the characteristics of the matrix. We now show that taking the practical recording and transmission processes into account, both systems are subject to quite similar operating constraints. Consider the steps followed by the producer of a four-channel master tape intended for a discrete record or broadcast. As a first step, he adjusts the signals to obtain the desired reproduction of the original performance, and then proceeds to monitor the results on .a quadraphonic loudspeaker system. As a next step, however, he must verify the compatibility of the stereophonic and the monophonic performance of the four-channel program. According to conventional wisdom, for discrete encoding the summed left and right pairs of channels produce the stereophonic program and the sum of all four signals produces the monophonic program. At this point, a moment's reflection shows that the above requirements severely limit the producer's freedom. For example, if the unbridled artistic spirit were to move him to apply the solo signals antiphase to the side pairs of channels, they would disappear completely from the steophonic and monophonic programsresulting in a catastrophic failure of the final pro-

Broadcasting practices vary widely in different countries. The "needle time" (allowed use of phonograph records) in some instances is strictly limited. "Live" broadcasts, both of the orchestral and dramatic variety are common. In the U.S.A., the vast majority of broadcasters use disc records. But, the matrix broadcasting principles in all instances are similar. We discuss them in this order:(l)broadcasting using SQ records, (2) broadcasting using syndicated services,(3) broadcasting of recorded four-channel material, (4) SQ synthesis of stereo records, and (5) live SQ quadraphonic broadcasting. 1. Broadcasting Using SQ Records An FM stereo station becomes a mono- and stereocompatible quadraphonic station simply by placing an SQ quadraphonic record on the turntable. No other changes are needed. The pickup, stylus, and all the electronic

Nevertheless, with some other matrix systems, an immobile signal is produced in both the stereo and the decoded quad modes. **

*Nevertheless, if a producer insists on placing a decodable center-back signal in an SQ-encoded program, he can accomplish this with the aid of a center-backsignal "London Box" described elsewhere in this paper.

produced for the stereophonic or monophonic listener albeit unless deliberately encoded via the later-described "London Box" they will be decoded as if they were front signals. Also, when LT and RT are added for monophonic reception, the four corner signals are heard at full strength. This ability to retain full front-channel separation of the concert stage and simultaneously to deliver equal strength of the corner signals to the monophonic listener is a characteristic unique to SQ and which no other quadraphonic matrix system thus far devised is able to provide.12

The program encoded on adjustments remain unaltered. the front channels of the record continues to span the full stereo space and any back channel signals are "tucked" in between. A stereophonic or monophonic lisBut, a listener tener is not aware of any change. equipped with a decoder and two additional amplifiers and loudspeakers receives the full quadraphonic program contained on the record. A number of record producers currently issue single-inventory stereo/quadraphonic SQ records which are matketed as ordinary stereophonic product with a note on the jacket about their SQ-decoding capabilities. Some SQ programs are available on two-track tape in place of records. But, it is important that the tape mechanism be capable of good interchannel alignment--for the sake of both mono compatibility of conventional stereo material and proper reception by the radio listener of SQ programs.

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A number of syndicators offer SQ-encoded records and tapes to the broadcaster. Among these are "BBC Presents," "The King Biscuit Flower Hour," "Boston Symphony Orchestra," "Cleveland Symphony Orchestra," and others. As with all SQ records, these are indistinguishable from stereophonic and monophonic programs for the regular broadcast audience. The listeners equipped with decoders, however, are able to hear these record-

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SQ Broadcasting of Recorded Four-Channel Material

Discrete four-channel prerecorded material is scarce. That which is available to the broadcaster is to be found in three formats: reel-to-reel tapes, fourchannel cartridges, and decoded CD-4 records. To broadcast these, an SQ encoder is required.4 The most popularly used SQ-encoding mode for broadcasting is the socalled "SQ forward-oriented" type. The circuit diagram of this encoder, and the resulting encoded signal phasors are shown in Fig. 1. Four input terminals are provided, namely, LF (left front), RF (right front), LB (left back), RB (right back). These are combined with "psi-type" phase shift networks to produce two encoded output signals, LT (left total) and RT (right total). It is noted that the front channels, LF and RF, are fully independent, as with stereo. Therefore, any SQ encoder can be permanently connected to the audio console, if desired, its LF and RF terminals taking the place ordinarily occupied by the L and R input terminals of the console. It is important to note that the back-channel signals LB and RB both appear at the output terminals in equal magnitudes, but with an in-quadrature phase relationship. LB leads at the LT terminal ahd RF leads at the RT terminal. The audible signals corresponding to these back-channel phasors are properly positioned in the stereo field, slightly displaced from the center, and somewhat distant from the listener, adding depth to the reception but otherwise retaining full fidelity. Any center-back signals casually present are fully re*

In a recent petition to the FCC, (RM 2742), CBS proposed an optional 57 kHz tone to be emitted by the station which could be used to switch the decoder within the receiver when SQ material is broadcast; albeit many users leave their SQ decoders in the receiver "on" all the time for listening to both stereo and quadraphonic records. **

Matrix systems other than SQ cause a serious dilution of front-channel separation and often cause monophonic incompatibility. (See Ref. 12.)

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SQ Synthesis of Stereo Records

SQ synthesis is a procedure of re-encoding stereo records for surround-sound reproduction and is the most frequently used supplemental method of quadraphonic broadcasting in the U.S.A. Through SQ synthesis many stations extend the usefulness of their libraries of stereophonic records by broadcasting them in a semi-surround-sound format which is decodable by their quadraphonic audience without significantly affecting the reception of their stereophonic and monophonic listeners. SQ synthesis has been described elsewhere and need be mentioned only briefly here: the left and right outputs of a stereo record are simply connected to the left front and back and to the right front and back terminals, respectively, of a forward-oriented SQ encoder; the overall level is readjusted to retain the proper modulation values; and the encoded signal is then transmitted as a conventional stereo program. Neither the tonal balance, nor the monophonic reception, nor the sum/difference power ratio of the transmitter are affected by SQ synthesis, although the channel separation for the stereophonic listener is diminished to a measured 7.7 dB and a perceived 14 dB.* *

Because of the in-quadrature relationship of the transferred signals. (See, for example, Ref. 14.)

ventionally used, and, furthermore, provides an output which can be recorded directly on a stereo record. Also, the Ghent System allows the performers in a dramatic production to move around the single centrally-placed microphone, resulting in a reproduction at proper level and angular orientation when an SQ quadraphonic decoder is used, at the same time retaining full stereophonic and monophonic compatibility. (Such a production is facilitated by drawing a polar coordinate system on the floor to guide the actors to the proper distance and

SQ synthesis should not be used with SQ-encoded records as it will result in "double encoding," which will adversely affect the intended direction of the encoded sounds.

Live SQ Quadraphonic Broadcasting

Experience and ingenuity are indispensable ingredients for successful "live" quadraphonic sound broadcasting (which often is taped for broadcasting at a later time). An example of sound pickup for live broadcast purposes is shown in Fig. 2 which is a plan view of the Tanglewood Music Shed (Lenox, Mass.U.S.A.), where a group of four "main" ambiance microphones are used, receiving the advancing sound wave front of the orchestra. The microphones, LF, RF, LB, RB, are connected to the corresponding input terminals of a forward-oriented SQ encoder. Proximate microphones on the stage cover the soloists and some instrumental groups. They are "panned" in between the LF and RF channels. The soloist usually is "placed" in CF. An operator in the radio control room monitors the sound balance during the performance. In this arrangement, the proximate microphones provide a sharp, well-defined group of front images for the stereo listener and for the front loudspeakers in quadraphony. The ambiance microphones receive the sounds of the various instruments in random phase relationships, with suitable delay between the front and back pair, resulting in a surround-sound decoding, replicative of the concert-hall experience.

angular position.) The Ghent Microphone System is illustrated schematically in Fig. 3. It consists of four limayon transducers placed coaxially in a single envelope. (The polar patterns are drawn away from the center for the sake of clarity.) The two front units, designated Ll and R1, have their maximum sensitivity at +650, respectively. The two back units, designated L2 and R2, are oriented at +1650, respectively. The polar sensitivity patterns of the four microphones are defined by the limayon equation F (9) = .3 + .7 cos (e). It is demonstrable, although beyond the scope of this paper, that this configuration may be obtained from any fourlimayon microphone system by proper matrixing.13

STAGE

Fig. 3. Schematic Principle of the Ghent Microphone System

Fig. 2. Quadraphonic Broadcasting Microphone Arrangement, Tanglewood Music Shed, Lenox, Massachusetts, U. S. A.

The output of the microphone cluster is combined in a special encoder outlined in the broken-line rectangle in Fig. 3. It may be shown that the two output signals, LT and RT, provide a forward-oriented SQ code with the front quadrant encompassing a total azimuth of +5째00, and the side quadrants each sustaining an arc of 800. At the points designated LF, RF, LB,and RB, as well as in the CF (center front) position, an ideal SQ forward-oriented code is obtained; in all other positions, the code is nearly ideal. By placing the Ghent Microphone System in front of the orchestra stage such that the front +500 '"quadrant" covers the stage, the rest of the hall will be picked up by the remaining microphone perimeter, resulting in a conventional ambiance-type recording. For surroundsound recording, the Ghent microphone is placed at the center of a group of performers. Conventional performance is obtained on mono or stereo players. On quadraphonic reproduction, the orchestra unfolds into a circular arc. Other applications will be readily envi-

With the more avant-garde surround-sound performances, proximate microphones are placed near the performers and connected directly to the appropriate encoder inputs. The

Ghent

Microphone System

A microphone system newly developed at the CBS Technology Center is capable of providing either ambiance or surround-sound SQ encoded pickup from a single location. It offers the very signiflcant advantage of precise, fully compatible, quadraphonic orchestral pickup for "live'' radio broadcasting or recording without requiring the veritable forest of microphones con*

The name credits the Belgian city in which it was invented by the author.13

sioned by the recording engineer.

because of this last characteristic, the forward-oriented encoder requires the use of an attachment if it is desired to ensure that center-back signals will be decoded appropriately in the back of a quadraphonic array. This latter occurrence, in reality, is quite infrequent. We are not aware of any important classical compositions in which soloists must be placed in the dead back of the audience. Furthermore, extensive discussions with several musicologists have revealed that the center-back requirement does not exist in serious musical works. It is recognized, however, that occasions do arise when a center-back signal is desired in quadraphonic reproduction. In one recent example, a producer wished to portray an automobile race around the quadrangle. In another, it was desired to record church music with a soloist in the back center. Here, the back choir could readily be divided into two groups applied to the left back and the right back channels, respectively, but the soloist had to be decoded in the center back and also had to remain mono compatible.

A photograph of the Ghent Microphone System and its special encoder is shown in Fig. 4 and a diagram depicting the deployment for pickup of the BBC Orchestra in Royal Albert Hall, London, on September 9, 1976, is shown in Fig. 5. ~

Fig. 4. The Neumann QM-69 Microphone and the Special Microphone Encoder Used in the Ghent System

One important observation has been made in using the Ghent Microphone System for recording and broadcasting: Since it is not possible for antiphase signals to occur, the vertical modulation problems in recording and sub-carrier overloading problems in FM stereo broadcasting are less likely to take place than with conventional stereo sound pickup, increasing performance reliability.

Fig. 5. Position of the Quadraphonic Ghent as

Microphone used in Royal Albert Hall, London, England

This problem was solved with *a new device we call the center-back-signal London Box shown in Fig. 6. Two banks of octave band filters are provided (however, with some circuit modifications, a single bank of filters would suffice) covering the full audio range of 20-20,000 Hz in 10 octaves. The odd octave bands of one of the filters are adjusted for a relative voltage transmission of 0.95 (cos 180) and the eveh bands of the same bank are adjusted for transmission of -0.31 (sin-180). These filter bands are summed and connected to, say, the left-back-channel input. Similarly, the even bands of the second bank are adjusted to a voltage transmission of 0.95 and the odd bands are adjusted to -0.31, and all these bands are connected to the right-

The Center Back Issue--The "London Box"

Throughout this paper we have dealt with the SQ forward-oriented encoder, and for good reasons: this encoder, in common with all SQ encoders, provides the full-channel separation for the front quadrant, with the center-front signals precisely in phase, as with conventional stereo; it encodes the side signals with a proper auditory perspective; and it reproduces the four cardinal corner signals at identical levels in the quadraphonic, stereophonic, and monophonic modes. It also satisfactorily encodes diagonally-split signals. Further, the forward-oriented encoder transmits any center-back signals as in-phase signals, and thus the broadcaster is able to SQ-encode any four-channel material of unknown origin without fear that center-back signals, should they be contained therein, will become inaudible to the monophonic listeners. But, precisely

So named since the author developed this solution dura return trip from London.

ing

back-channel input. Because of the antiphase connection of the corresponding filters, upon decoding two

frequent events which necessitate placing a decodable mono-compatible signal in the center back of a quadra-

sets of alternate octave-band signals are reproduced in phase in the back quadrant, audibly fusing into a single sharp image. And since the sets are quasi-symmetrical, they present a centered, somewhat spread, image in the stereophonic mode.

phonic reproduction system. These innovations should be of significant additional assistance to the broadcast engineer in the implementation of SQ broadcasting systems. References

B.B.Bauer, D.W.Gravereaux,A.J.Gust,"A Compatible Stereo-Quadraphonic (SQ) Record System," J. Audio Eng. Soc., vol. 19, pp. 638-646, (Sept. 1971).

B.B.Bauer, G.A.Budelman, D.W.Gravereaux,"Recording Techniques for SQ Matrix Quadraphonic Discs," J. Audio Eng. Soc., vol. 21, pp. 19-26, (Jan./Feb.

1973).

Fg. 6. SQ Quadraphonic Sound Transmission I ncluding Center-Back Solo

B.B.Bauer, R.G.Allen, G.A.Budelman, D.W.Gravereaux, "Quadraphonic Matrix Perspective--Advances in SQ Encoding and Decoding Technology," J.Audio Eng.Soc., vol. 21, pp. 342-350, (June 1973).

B.B.Bauer, "A New Encoder for SQ Matrix Broadcasts," BME, vol. 11, pp. 76-79, (March 1975).

T. Inoue, N. Takahashi, I. Owaki, "A Discrete FourChannel Disc and its Reproducing System (CD-4 System)," J. Audio Eng. Soc., vol. 19, pp.576-583,

(July/August 1971).

6. The stated constants used in the London Box filters can be altered to obtain the optimum result for a given set of conditions. In the example given, the monophonic transmission mode has a relative output of 0.95 0.31 = 0.64, corresponding to a signal reduction of 3.9 dB. With a basic or position encoder, the connection is reversed. If the even and odd octave bands are simply connected to the respective back-channel inputs of the encoder without the corresponding negative fractional opposite inputs, no loss will occur in the monophonic mode, but the back signal both in the decoded and stereophonic transmission modes will become spread, covering a wide segment of the back quadrant. The experienced producer will use his judgment in making the most beneficial adjustments of the filter constants for the particular selection being recorded.

Report of the National Quadraphonic Radio Committee (Nov. 1975). Published by Electronic Industry Association.

to the Federal Comm. Commission

B.B.'Bauer, U.S. Patents 3,937,896 and 3,940,559, "Compatible Four-Channel Radlo Broadcast and Receiving System."

"High Fidelity Compares Columbia's and RCA's FourChannel Disc Systems," High Fidelity and Musical America, vol. 24, pp. 35-44, (January 1974).

Federal Communications Commission, Rules and Regulations, Vol. III, Part 73.222(m).

10. N.V. Franssen, "Some Considerations on the Mechanism of Directional Hearing" (a doctoral thesis), Technical University of Delft, Netherlands, 6 July 1960. 11. D.H. Cooper, T. Shiga, "Discrete-Matrix Multi-Channel Stereo," J. Audio Eng. Soc., vol. 20, pp.346360, (June 1972).

Conclusion This paper has attempted to dispose of some stilted ideas about matrix vs. discrete quadraphonic systems. Both systems are shown to be operationally equivalent but the matrix technique has the advantage of (a) economy of the recording medium, (b) conservation of broadcasting spectrum, and (c) prudence of equipment cost. With this understanding, the broadcaster can now augment his concern for the public interest, convenience, and necessity by offering a quadraphonic SQ service which brings satisfaction to the significant and growing segment of quadraphonic listeners without disadvan-

12. B.B. Bauer, "Quadraphony, Spatial High Fidelity and Compatibility," (companion paper in this issue). 13. B.B. Bauer, L.A. Abbagnaro, D.W. Gravereaux and T.J. Marshall, "The Ghent Microphone System for SQ Quadraphonic Recording and Broadcasting," presented at the 55th Convention of the Audio Engineering Society, New York City, Nov. 1, 1976.

14. Y. Makita, "On the Directional Localization of Sound in the Stereophonic Sound Field," EBU, Rev., pt. A, no. 73, pp. 102, (June 1962).

taging existing stereophonic and monophonic audiences. While quadraphonic broadcasting using the SQ system is simply accomplished by using SQ-encoded recorded program in place of stereo material, we have reviewed some of the recent advances in live quadraphonic matrix broadcasting. The SQ Ghent Microphone System, a new development, has been described which permits live, compatible stereophonic/quadraphonic broadcasting to be achieved in a more reliabie manner and at lower installation cost than has been possible with the conventional multimicrophone techniques. A new center-back-signal London Box circuit has been devised to serve those in-