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(SQ ) "matrix" recording and broadcasting is in wideuse in the U.S.A. and abroad. Nevertheless, argumentscontinue about the need for "discrete" recording andbroadcasting because of implied improvement they affordin channel separation and in artistic freedom in theuse of the four channels. Both of these inferences areadduced to be fallacious. SQ quadraphonic broadcastingin the form of SQ records, syndicated productions, andencoded four-channel sources followed by full-logic de-coding at the receiver end meets the identified needsof quadraphony. Two new advances are described in thispaper: the SQ Ghent Microphone System which, from asingle point in space is capable of picking up andtransmitting a stereo- and mono-compatible pair of sig-nals replicating the original directional performanceeither of the ambiance or surround-sound type, and acenter-back "London Box" signal processor which encodesa stereo- and mono-compatible center-back signal andallows it appropriately to be decoded in the quadrilat-eral space with an SQ decoder.

Introduction

Audio professionals are well aware of the signi-ficant advance in high-fidelity ("spatial high fideli-ty") possible through quadraphony--sound reproductionover four loudspeakers surrounding the listener. Quad-raphony allows us, for example, to portray the concertstage over the two front loudspeakers, as with stereo,with the added dimension of two back loudspeakers co-operating with the front ones to reproduce the concert-hall ambiance. Quadraphony permits us to distributespecific sources at, or in-between, the loudspeakersto recreate a performance in a cathedral where severalorgans or choirs furnish an antiphonal interlude. Quad-raphony even is able to place us on the conductor's po-dium or in the midst of a group of musicians resultingin a new and exciting surround-sound experience. And,these remarkable results can be derived from the origi-nal directional signals combined through an "encoder"according to specific matrix equations to form a stereopair which can be recorded or broadcast using existingequipments and techniques or as shown hereinafter,through the use of a special transducer array at apoint in space.1'2'3'4 This method of storing andtransmitting four-channel directional information isknown as "matrix" quadraphony.

As an alternative, the four signals to be repro-duced can be recorded on four separate channels of amultitrack magnetic tape, or distributed on modulatedcarriers inscribed on a special wideband disc (e.g."CD-4") record or transmitted over one of several pro-posed experimental quadruplex FM transmitters with mul-tiple subcarriers.5'6'7 This method of storing andtransmitting four-channel information is known as

"discrete" quadraphony.

Manuscript submitted February 4, 1977. Portions ofthis material were presented at the IEEE Broadcast Con-vention, in Washington, D.C., September 24, 1976 and atthe Third Annual Society of Broadcast Engineers NewYork Convention, Hempstead, N.Y., November 8, 1976.

SQ is a Trademark of CBS Inc.

This paper deals with recent advances in the artof quadraphonic matrix broadcasting; but since severalissues have arisen between the proponents of the dis-crete and the matrix methods, it is fitting at thistime to review them in order to bring the subject, ac-cording to our point of view, into proper focus.

The Discrete vs. Matrix Issue

Discrete quadraphony in comparison with matrixmethods requires (a) a greater area of recording medi-um, (b) more complex and costly apparatus for disc re-cording, reproduction, and broadcasting, and (c) great-er bandwidth of radio spectrum, at the same time result-ing in (a) diminution of the playing time on a disc,(b) a lower signal-to-noise ratio in recording andbroadcasting, and (c) potentially lowered listener cov-erage.5,6,8These disadvantages, it is often alleged, arecounterbalanced by(a)greater channel separation feasiblewith discrete methods and (b) the supposition that thediscrete approach leads to greater artistic freedom inthe use of the four channels. We now adduce t-hat theseallegations are without practical merit.

The channel separation argument has to be weighedin the light of the needs and capabilities. The needfactor, of course, is subjective and is based on theartistic content of the program. It has been notedthat relatively little separation is needed, say, for achoir in a highly reverberant church; a great deal isdesired for a musical "ping-pong game." With stereo-phonic pickups and disc records, over the years a 20 dBminimum overall left-right channel separation guidelinehas emerged. A more stringent 29.7 dB FCC test is usedto verify FM stereo transmitter performance, which ispropitious, as the various separation dilutants in asystem are cumulative.9 Our own experience has con-firmed that a 20 dB separation minimum also is desirablewhen the concert stage is defined by the two front chan-nels in quadraphony in order to preserve the artisticintegrity of orchestral music. For man, front sourcesare dominant and the auditory location tends to be es-tablished by the transient sounds of first arrival.1'10Slightly (but not much) lower overall separation figuresappear to be acceptable for the remaining three quad-rants which usually carry the reverberant sounds butwhich also are often used for antiphonal or solo soundsin surround-sound performances.

The above criteria appear not to conflict with1975 large scale tests sponsored by the National Quad-raphonic Radio Committee (NQRC) in which 918 musicaltests were administered to 184 naive auditors in twodifferent cities (San Francisco and Syracuse), to as-sess their preference among (a) a fully discrete(4-4-4) system, (b) a semi-discrete (4-3-4) system with10 dB interchannel separation, and (c) a "BMX" matrix(4-2-4) system (without logic) with 3 dB interchannelseparation.6'11 No preferences appear to have beenestablished between the first two systems (49% vs. 51%and 57% vs. 43% preference votes), while a degradationthreshold was perceived between the first two systemsand the third (77% vs. 23% and 74% vs. 26%). We be-lieve that trained listeners might have been more dis-criminating.

*

Because approximately 50% of the listeners appeared tohave noticed the difference, while the 50% who did notnotice the difference simply guessed.

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As to capabilities, it is known that well-designedfour-channel tape machines can have an interchannelseparation in excess of 40 dB. But, importantly, wehave to focus on the disc medium which is responsiblefor the majority of all programs broadcast, at leastin the U.S.A. A discrete carrier disc, per se, has aleft-right baseband interchannel separation equal tothat of the stereodisc, but its front-back basebandcrosstalk is 100%, equivalent to 0 dB channel separa-tion.5 With a suitable pickup and demodulator (de-pending upon the smoothness of the pickup response andthe tracking between the compressor in the recordingstudio and the expander in the demodulator), front-backseparation of a discrete disc reported in one study hasbeen established to average 14 dB with a range from 5to 30 dB at 2 kHz.8 Similarly, the SQ encode/decodesystem theoretically produces infinite interchannelseparation for the front and the back pairs of channelsbut, in practice, this figure is limited by the designof the decoder and by the pickup separation and re-sponse. The front-back interchannel separation pro-vided by the decoding matrix is 3 dB.1 By using acrosstalk cancellation logic decoder, 30-40 dB front-back separation figures have been obtained in prototypedecoders. However, with a commercially available full-logic "power transfer" unit, crosstalk of the order of16-20 dB at 2 kHz has been reported.8

An argument has been made that in a discrete sys-tem the separation properties are maintained regard-less of the program content, while in a matrix system,the separation figures are defined for individual sig-nals only. This would appear to be a cogent argumentexcept for the fact that the average human ear (likethe 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 froma practical point of view, especially since the recordproducer is the final arbiter of the system performance.

Thus, it can be said without equivocation that us-ing modern equipment, the discrete vs. matrix channelseparation issue is, for all intents and purposes, anon-issue. Separation-wise, either approach--discreteor SQ matrix with full-logic--meets the realistic needsof practical quadraphony, however the latter offers -

(a) higher fidelity, (b) greater economy, and (c) supe-rior compatibility with existing broadcasting and soundreproducing apparatus.

The Artistic Freedom Issue

The second argument--that of greater artistic free-dom--stems from the belief that signals can be appliedto the four independent channels by the producer as thespirit moves him while with a matrix system he is cir-cumscribed by the characteristics of the matrix. Wenow show that taking the practical recording and trans-mission processes into account, both systems are sub-ject to quite similar operating constraints.

Consider the steps followed by the producer of afour-channel master tape intended for a discrete recordor broadcast. As a first step, he adjusts the signalsto obtain the desired reproduction of the original per-formance, and then proceeds to monitor the results on .aquadraphonic loudspeaker system. As a next step, how-ever, he must verify the compatibility of the stereo-phonic and the monophonic performance of the four-chan-nel program. According to conventional wisdom, for dis-crete encoding the summed left and right pairs of chan-nels produce the stereophonic program and the sum ofall four signals produces the monophonic program.

At this point, a moment's reflection shows thatthe above requirements severely limit the producer'sfreedom. For example, if the unbridled artistic spir-it were to move him to apply the solo signals antiphaseto the side pairs of channels, they would disappearcompletely from the steophonic and monophonic programs-resulting in a catastrophic failure of the final pro-

duct. Few producers, of course, would be so motivated.But, as a practical fact, we have had to cope with onewell-known producer who interprets the original perform-ance by placing the soloists on the diagonal cornersfor antiphonal response. In the discrete mode this pro-vides an interesting sonic interplay; but for the stereolistener it results in an immobile center signal repro-duction. When encoded with the forward-oriented SQ en-coder, however, an antiphonic reproduction is heard inboth the stereophonic and the decoded quadraphonicmodes.*

By the same token, with the discrete system, theproducer who elects to place four soloists in the cor-ners of the quadrantn ends up with the signals concen-trated on the sides, with a "hole in the middle" in thestereo mode. In contrast, with SQ, the corner quadra-phonic arrangement results in four distinct sources an-

propriately distributed over the stereophonic field.12Of course, any matrix system also has its prohibi-

tions. With SQ, as with the discrete system, applica-tion of equal antiphase signals to adjacent channelsis not recommended. The position of instruments insurround-sound orchestral recording should be such asto promote a pleasing quad/stereo fold.12 And becausethe center-back signal in SQ usually is recorded anti-phase, 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 dimin-ished in the monophonic mode. ** Center-top signalswith SQ must be applied by establishing a 10-15 ms de-lay between the front and the back pairs of channels(except when panning along the diagonals, where thedelay apparatus is not required).

The above and similar experiences have demonstratedthat the SQ matrix system skillfully handled provides atleast as much artistic freedom to a knowledgeable pro-ducer as does a discrete system. A novice will commitblunders with any system; an experienced producer learnsthe rules of the road and avoids the pitfalls.

A corollary is that a four-channel master tapemade for a given system of quadraphony is not necessari.ly optimum for another system, but alas, this prime ruleoften is ignored by the inexperiencedi

SQ Broadcasting

Broadcasting practices vary widely in differentcountries. The "needle time" (allowed use of phono-graph records) in some instances is strictly limited."Live" broadcasts, both of the orchestral and dramaticvariety are common. In the U.S.A., the vast majority ofbroadcasters use disc records. But, the matrix broad-casting principles in all instances are similar. Wediscuss them in this order:(l)broadcasting using SQ rec-ords, (2) broadcasting using syndicated services,(3)broadcasting of recorded four-channel material, (4) SQsynthesis of stereo records, and (5) live SQ quadraphon-ic broadcasting.

1. Broadcasting Using SQ Records

An FM stereo station becomes a mono- and stereo-compatible quadraphonic station simply by placing an SQquadraphonic record on the turntable. No other changesare needed. The pickup, stylus, and all the electronic

Nevertheless, with some other matrix systems, an immo-bile signal is produced in both the stereo and the de-coded quad modes.***Nevertheless, if a producer insists on placing a de-codable center-back signal in an SQ-encoded program,he can accomplish this with the aid of a center-back-signal "London Box" described elsewhere in this paper.

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adjustments remain unaltered. The program encoded onthe front channels of the record continues to span thefull stereo space and any back channel signals are"tucked" in between. A stereophonic or monophonic lis-tener is not aware of any change. But, a listenerequipped with a decoder and two additional amplifiersand loudspeakers receives the full quadraphonic programcontained on the record. A number of record producerscurrently issue single-inventory stereo/quadraphonicSQ records which are matketed as ordinary stereophonicproduct with a note on the jacket about their SQ-decod-ing capabilities.

Some SQ programs are available on two-track tapein place of records. But, it is important that thetape mechanism be capable of good interchannel align-ment--for the sake of both mono compatibility of con-ventional stereo material and proper reception by theradio listener of SQ programs.

2. SQ Broadcasting Using Syndicated Services

A number of syndicators offer SQ-encoded recordsand tapes to the broadcaster. Among these are "BBCPresents," "The King Biscuit Flower Hour," "Boston Sym-phony Orchestra," "Cleveland Symphony Orchestra," andothers. As with all SQ records, these are indistin-guishable from stereophonic and monophonic programs forthe regular broadcast audience. The listeners equippedwith decoders, however, are able to hear these record-ings quadraphonically.

3. SQ Broadcasting of Recorded Four-Channel Material

Discrete four-channel prerecorded material isscarce. That which is available to the broadcaster isto be found in three formats: reel-to-reel tapes, four-channel cartridges, and decoded CD-4 records. To broad-cast these, an SQ encoder is required.4 The most popu-larly used SQ-encoding mode for broadcasting is the so-called "SQ forward-oriented" type. The circuit diagramof this encoder, and the resulting encoded signal phas-ors are shown in Fig. 1. Four input terminals are pro-vided, 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 encodedoutput signals, LT (left total) and RT (right total).It is noted that the front channels, LF and RF, arefully independent, as with stereo. Therefore, any SQencoder can be permanently connected to the audio con-sole, if desired, its LF and RF terminals taking theplace ordinarily occupied by the L and R input terminalsof the console.

It is important to note that the back-channel sig-nals LB and RB both appear at the output terminals inequal magnitudes, but with an in-quadrature phase re-lationship. LB leads at the LT terminal ahd RF leadsat the RT terminal. The audible signals correspondingto these back-channel phasors are properly positionedin the stereo field, slightly displaced from the center,and somewhat distant from the listener, adding depth tothe 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 pro-posed an optional 57 kHz tone to be emitted by the sta-tion which could be used to switch the decoder withinthe receiver when SQ material is broadcast; albeit manyusers leave their SQ decoders in the receiver "on" allthe time for listening to both stereo and quadraphonicrecords.

produced for the stereophonic or monophonic listener al-beit unless deliberately encoded via the later-described"London Box" they will be decoded as if they were frontsignals. Also, when LT and RT are added for monophonicreception, the four corner signals are heard at fullstrength.

This ability to retain full front-channel separa-tion of the concert stage and simultaneously to deliverequal strength of the corner signals to the monophoniclistener is a characteristic unique to SQ and which noother quadraphonic matrix system thus far devised isable to provide.12

-~~ -F

r T | n FMId 1.V( l Fii RI~~~~~~~~~~~~~~~~~TR

LB RBSQ FORWARD-ORIENlED ENCODER

LOD 0

LT RT

LF

.n Rn

L1 FIT

0 1.0B

LT RI

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LI RT LI RT

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Fig. 1. The SQ "Forward-Oriented" Encoder and theCorresponding Output Phasors vs Source Azimuth

4. SQ Synthesis of Stereo Records

SQ synthesis is a procedure of re-encoding stereorecords for surround-sound reproduction and is the mostfrequently used supplemental method of quadraphonicbroadcasting in the U.S.A. Through SQ synthesis manystations extend the usefulness of their libraries ofstereophonic records by broadcasting them in a semi-sur-round-sound format which is decodable by their quadra-phonic audience without significantly affecting the re-

ception of their stereophonic and monophonic listeners.SQ synthesis has been described elsewhere and need bementioned only briefly here: the left and right outputsof a stereo record are simply connected to the leftfront and back and to the right front and back termi-nals, respectively, of a forward-oriented SQ encoder;the overall level is readjusted to retain the propermodulation values; and the encoded signal is then trans-mitted as a conventional stereo program.

Neither the tonal balance, nor the monophonic re-

ception, nor the sum/difference power ratio of the

transmitter are affected by SQ synthesis, although thechannel separation for the stereophonic listener is di-minished to a measured 7.7 dB and a perceived 14 dB.*

**

Matrix systems other than SQ cause a serious dilutionof front-channel separation and often cause monophonicincompatibility. (See Ref. 12.)

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

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SQ synthesis should not be used with SQ-encodedrecords as it will result in "double encoding," whichwill adversely affect the intended direction of the en-coded sounds.

5. Live SQ Quadraphonic Broadcasting

Experience and ingenuity are indispensable ingre-dients for successful "live" quadraphonic sound broad-casting (which often is taped for broadcasting at alater time). An example of sound pickup for livebroadcast purposes is shown in Fig. 2 which is a planview of the Tanglewood Music Shed (Lenox, Mass.U.S.A.),where a group of four "main" ambiance microphones areused, receiving the advancing sound wave front of theorchestra. The microphones, LF, RF, LB, RB, are con-nected to the corresponding input terminals of a for-ward-oriented SQ encoder. Proximate microphones on thestage 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 inthe radio control room monitors the sound balance dur-ing the performance.

In this arrangement, the proximate microphonesprovide a sharp, well-defined group of front imagesfor the stereo listener and for the front loudspeakersin quadraphony. The ambiance microphones receive thesounds of the various instruments in random phase re-lationships, with suitable delay between the front andback pair, resulting in a surround-sound decoding, rep-licative of the concert-hall experience.

STAGE

ventionally used, and, furthermore, provides an outputwhich can be recorded directly on a stereo record. Also,the Ghent System allows the performers in a dramaticproduction to move around the single centrally-placedmicrophone, resulting in a reproduction at proper leveland angular orientation when an SQ quadraphonic decoderis used, at the same time retaining full stereophonicand monophonic compatibility. (Such a production isfacilitated by drawing a polar coordinate system on thefloor to guide the actors to the proper distance andangular position.)

The Ghent Microphone System is illustrated sche-matically in Fig. 3. It consists of four limayon trans-ducers placed coaxially in a single envelope. (The po-lar patterns are drawn away from the center for thesake of clarity.) The two front units, designated Lland R1, have their maximum sensitivity at +650, respec-tively. The two back units, designated L2 and R2, areoriented at +1650, respectively. The polar sensitivitypatterns of the four microphones are defined by thelimayon equation F (9) = .3 + .7 cos (e). It is demon-strable, although beyond the scope of this paper, thatthis configuration may be obtained from any four-limayon microphone system by proper matrixing.13

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

With the more avant-garde surround-sound perform-ances, proximate microphones are placed near the per-formers and connected directly to the appropriate en-coder inputs.

The SQ Ghent Microphone System

A microphone system newly developed at the CBSTechnology Center is capable of providing either ambi-ance or surround-sound SQ encoded pickup from a singlelocation. It offers the very signiflcant advantage ofprecise, fully compatible, quadraphonic orchestralpickup for "live'' radio broadcasting or recording with-out requiring the veritable forest of microphones con-

*

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

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Fig. 3. Schematic Principle of the Ghent Microphone System

The output of the microphone cluster is combinedin a special encoder outlined in the broken-line rect-angle in Fig. 3. It may be shown that the two outputsignals, LT and RT, provide a forward-oriented SQ codewith the front quadrant encompassing a total azimuth of+5°00, and the side quadrants each sustaining an arc of800. At the points designated LF, RF, LB,and RB, aswell as in the CF (center front) position, an ideal SQforward-oriented code is obtained; in all other posi-tions, the code is nearly ideal.

By placing the Ghent Microphone System in front ofthe orchestra stage such that the front +500 '"quadrant"covers the stage, the rest of the hall will be pickedup by the remaining microphone perimeter, resulting ina conventional ambiance-type recording. For surround-sound recording, the Ghent microphone is placed at thecenter of a group of performers. Conventional perform-ance is obtained on mono or stereo players. On quadra-phonic reproduction, the orchestra unfolds into a cir-cular arc. Other applications will be readily envi-sioned by the recording engineer.

A photograph of the Ghent Microphone System andits special encoder is shown in Fig. 4 and a diagramdepicting the deployment for pickup of the BBC Orches-tra in Royal Albert Hall, London, on September 9, 1976,is shown in Fig. 5. ~

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

One important observation has been made in usingthe Ghent Microphone System for recording and broad-casting: Since it is not possible for antiphase sig-nals to occur, the vertical modulation problems in re-

cording and sub-carrier overloading problems in FMstereo broadcasting are less likely to take place thanwith conventional stereo sound pickup, increasing per-formance reliability.

The Center Back Issue--The "London Box"

Throughout this paper we have dealt with the SQforward-oriented encoder, and for good reasons: thisencoder, in common with all SQ encoders, provides thefull-channel separation for the front quadrant, withthe center-front signals precisely in phase, as withconventional stereo; it encodes the side signals with aproper auditory perspective; and it reproduces the fourcardinal corner signals at identical levels in thequadraphonic, stereophonic, and monophonic modes. Italso satisfactorily encodes diagonally-split signals.Further, the forward-oriented encoder transmits any

center-back signals as in-phase signals, and thus thebroadcaster is able to SQ-encode any four-channel mate-rial of unknown origin without fear that center-backsignals, should they be contained therein, will becomeinaudible to the monophonic listeners. But, precisely

because of this last characteristic, the forward-ori-ented encoder requires the use of an attachment if itis desired to ensure that center-back signals will bedecoded appropriately in the back of a quadraphonic ar-ray.

This latter occurrence, in reality, is quite infre-quent. We are not aware of any important classical com-positions in which soloists must be placed in the deadback of the audience. Furthermore, extensive discus-sions with several musicologists have revealed that thecenter-back requirement does not exist in serious musi-cal works. It is recognized, however, that occasionsdo arise when a center-back signal is desired in quadra-phonic reproduction. In one recent example, a producerwished to portray an automobile race around the quadran-gle. In another, it was desired to record church musicwith a soloist in the back center. Here, the backchoir could readily be divided into two groups appliedto the left back and the right back channels, respec-tively, but the soloist had to be decoded in the centerback and also had to remain mono compatible.

Fig. 5. Position of the Quadraphonic Ghent Microphoneas used in Royal Albert Hall, London, England

This problem was solved with*a new device we callthe 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 fil-ters would suffice) covering the full audio range of20-20,000 Hz in 10 octaves. The odd octave bands of one

of the filters are adjusted for a relative voltagetransmission of 0.95 (cos 180) and the eveh bands of thesame bank are adjusted for transmission of -0.31(sin-180). These filter bands are summed and connectedto, say, the left-back-channel input. Similarly, theeven bands of the second bank are adjusted to a voltagetransmission of 0.95 and the odd bands are adjusted to-0.31, and all these bands are connected to the right-

*

So named since the author developed this solution dur-ing a return trip from London.

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back-channel input. Because of the antiphase connec-tion of the corresponding filters, upon decoding twosets of alternate octave-band signals are reproduced inphase in the back quadrant, audibly fusing into a sin-gle sharp image. And since the sets are quasi-symmetri-cal, they present a centered, somewhat spread, image inthe stereophonic mode.

Fg. 6. SQ Quadraphonic Sound Transmission I ncludingCenter-Back Solo

The stated constants used in the London Box fil-ters can be altered to obtain the optimum result for a

given set of conditions. In the example given, the mon-

ophonic transmission mode has a relative output of

0.95 - 0.31 = 0.64, corresponding to a signal reductionof 3.9 dB. With a basic or position encoder, the con-

nection is reversed. If the even and odd octave bandsare simply connected to the respective back-channel in-puts of the encoder without the corresponding negativefractional opposite inputs, no loss will occur in themonophonic mode, but the back signal both in the decod-ed and stereophonic transmission modes will becomespread, covering a wide segment of the back quadrant.The experienced producer will use his judgment in mak-ing the most beneficial adjustments of the filter con-

stants for the particular selection being recorded.

Conclusion

This paper has attempted to dispose of some stilt-ed ideas about matrix vs. discrete quadraphonic systems.Both systems are shown to be operationally equivalentbut the matrix technique has the advantage of (a) econo-

my of the recording medium, (b) conservation of broad-casting spectrum, and (c) prudence of equipment cost.With this understanding, the broadcaster can now aug-

ment his concern for the public interest, convenience,

and necessity by offering a quadraphonic SQ servicewhich brings satisfaction to the significant and grow-

ing segment of quadraphonic listeners without disadvan-taging existing stereophonic and monophonic audiences.

While quadraphonic broadcasting using the SQ sys-

tem is simply accomplished by using SQ-encoded recordedprogram in place of stereo material, we have reviewedsome of the recent advances in live quadraphonic matrixbroadcasting. The SQ Ghent Microphone System, a new de-velopment, has been described which permits live, com-

patible stereophonic/quadraphonic broadcasting to beachieved in a more reliabie manner and at lower instal-lation cost than has been possible with the conventionalmultimicrophone techniques. A new center-back-signalLondon Box circuit has been devised to serve those in-

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frequent events which necessitate placing a decodablemono-compatible signal in the center back of a quadra-phonic reproduction system. These innovations shouldbe of significant additional assistance to the broad-cast engineer in the implementation of SQ broadcastingsystems.

References

1. B.B.Bauer, D.W.Gravereaux,A.J.Gust,"A CompatibleStereo-Quadraphonic (SQ) Record System," J. AudioEng. Soc., vol. 19, pp. 638-646, (Sept. 1971).

2. B.B.Bauer, G.A.Budelman, D.W.Gravereaux,"RecordingTechniques for SQ Matrix Quadraphonic Discs,"J. Audio Eng. Soc., vol. 21, pp. 19-26, (Jan./Feb.1973).

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

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

5. T. Inoue, N. Takahashi, I. Owaki, "A Discrete Four-Channel Disc and its Reproducing System (CD-4 Sys-tem)," J. Audio Eng. Soc., vol. 19, pp.576-583,(July/August 1971).

6. Report of the National Quadraphonic Radio Committeeto the Federal Comm. Commission (Nov. 1975). Pub-lished by Electronic Industry Association.

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

8. "High Fidelity Compares Columbia's and RCA's Four-Channel Disc Systems," High Fidelity and MusicalAmerica, vol. 24, pp. 35-44, (January 1974).

9. Federal Communications Commission, Rules and Regu-lations, Vol. III, Part 73.222(m).

10. N.V. Franssen, "Some Considerations on the Mechan-ism of Directional Hearing" (a doctoral thesis),Technical University of Delft, Netherlands, 6 July1960.

11. D.H. Cooper, T. Shiga, "Discrete-Matrix Multi-Chan-nel Stereo," J. Audio Eng. Soc., vol. 20, pp.346-360, (June 1972).

12. B.B. Bauer, "Quadraphony, Spatial High Fidelity andCompatibility," (companion paper in this issue).

13. B.B. Bauer, L.A. Abbagnaro, D.W. Gravereaux andT.J. Marshall, "The Ghent Microphone System for SQQuadraphonic Recording and Broadcasting," presentedat the 55th Convention of the Audio Engineering So-ciety, New York City, Nov. 1, 1976.

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