PAPERS

A High-PerformanceSurround Sound Processfor HomeVideo*

STEPHEN JULSTROM

Shure Brothers Incorporated, Evanston, IL 60202, USA

Video disks, high-fidelity video cassettes, and stereo television bring to the consumera substantial library of video entertainment with high-quality stereo sound tracks. Agrowing portion of these are encoded with surround sound using a 4- 2- 4 matrix-basedmethod particularly suited to front-stage oriented film/video presentation. Home re-production is accomplished with a processor incorporating dynamic matrix modificationto stabilize and enhance directional effects and a delayed surround channel to aidforward localization of front sound.

0 INTRODUCTION two channels is used to provide a high degree of di-rectional accuracy over a wide listening area for front-

For several years now moviegoers have regularly stage sounds, particularly dialogue and effects, and a

experienced surround sound at their local theaters. The diffuse surround sound, providing both rearward di-two most commonly used theater surround sound for- rectional effects and general ambience and environment.mats were developed by Dolby Laboratories and both This encoded surround sound information is presentlyhave the trade name Dolby Stereo. (For home use the made available to the home video enthusiast throughtrade name Dolby Surround is used.) Both theater for- the two-channel sound tracks of video disks, videomats involve three full-range, behind-the-screen loud- cassettes and, soon, stereo television when encoded

speakers (left, center, and right) and a U-shaped array movies are duplicated or broadcast. The added impactof "surround" loudspeakers disposed about the rear surround sound brings to the movie theater can also be

half of the theater, receiving a common signal. Ad- enjoyed at home with the addition of a suitable decoderditional loudspeakers may also be used to augment and auxiliary equipment. In spite of the difference in

bass reproduction, the size of the listening/viewing environment, the goalsIn the 70-mm wide-screen format, four magnetic of the surround sound encoding and decoding for home

tracks are used to record the three front loudspeaker video need not differ substantially from those for the

signals and the surround loudspeaker array signal dis- theater system.cretely. In the less costly 35-mm format, practical con- An examination of the desirable goals and operationsiderations [1] limit the sound track to two optically of a surround sound decoder for home video use will

recorded channels. The four original channels are en- be aided by reviewing and analyzing the characteristicscoded into two and subsequently decoded at the theater and behavior of the theater system and its matrix. Theusing techniques related to, but differing significantly system may first be viewed as a logical outgrowth offrom, previous matrix-based quadraphony. The differ- three-loudspeaker stereo and previous ambience ex-ences arise primarily from the nature of the audio ma- traction techniques.terial, which includes not only music, but also dialogue

and sound effects, and the strong front-stage orientation, I THE CENTER LOUDSPEAKERdictated by the film action. Rather than attempting tooptimize spatial reproduction of music around a full 1.1 Some History360 ° for a few people near the center of a regular loud- In the beginning stereophonic sound did not alwaysspeaker array, the limited information capacity of the mean two-loudspeaker sound. Well-known early ex-

periments by Bell Labs [2] used three transmission* Presented at the 79th Convention of the Audio Engineering

Society, New York, 1985 October 12-16; revised 1987 May channels and three loudspeakers to achieve a stereo-26. phonic effect across a wide stage. Two channels with

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either two or three loudspeakers were also tried with significant image shift for listeners off the centerlinegood results for the centrally positioned listeners. In between the loudspeakers has also long been noted forthe last instance the center loudspeaker was derived two-loudspeaker playback of coincident microphone(not discrete), receiving an attenuated sum of the left recordings [9]. The shifting of images toward the closerand right loudspeaker signals, loudspeaker is a familiar effect. It is easily explainable

In further experimental work on stereophonic film on a qualitative level in relation to the amplitude ad-sound, Bell continued the three-channel, three-loud- vantage of the closer loudspeaker and its sound's timespeaker approach [3]. The use of at least three loud- of arrival advantage (the well-known Haas or precedencespeakers for stereophonic sound has remained the movie effect [10], [11]).

industry standard ever since its commercial introduction Even for an optimally positioned listener, the phantomin the early 1950s. Cinerama [4] used five loudspeakers center image is often not as compelling as a pure leftbehind the screen to augment its triple-width picture, or right, particularly for more widely spaced loud-The 70-mm Todd A-O format also used five behind- speakers. The phantom image may appear as an "in-the-screen loudspeakers [5], while Cinemascope stayed the-head" or "overhead" sound. As perhaps a partialwith three [6]. Maintaining at least the center loud- explanation, it has been pointed out that for a phantomspeaker has always been felt essential for adequate center image produced from equal in-phase wave frontslocalization, particularly of dialogue, over the wide from tiro loudspeakers, the particle ¥elocity and theaudience area. Initially dialogue was often recorded pressure at the listening position are not in the correctstereophonically or "panned" electronically to follow ratio compared to a real source [12]. In Fig. 1 twothe screen action. This practice is not presently followed loudspeakers flank the listener at - 45° and produceand, generally, almost all dialogue is directed to the equal, in-phase signals. Ignoring the effectofthelistenercenter [5]. on the sound field (reasonablyvalid for frequencies

below about 700 Hz), the two particle velocity vectors1.2 The Derived Center at Home add to a relative magnitude of 1.414 directed along the

The situation in the home listening environment is centerline, but the pressures add as scalars to a relativenot significantly different from that in the theater if the value of 2. This pressure is 3 dB higher than that whichdesired sound stage width is considered in relationto a real center source with a particle velocity of 1.414the typically available seating area. It may be con- at the listener's position would have given. If a centervincingly argued that the de facto standardization on is added as shown at an equal distance to the listenertwo loudspeakers for home stereo listening has been and at a level given by Eq. (1), the resultant particlemore a matter of convenience than of technical opti- velocity is of magnitude 2.828 and the pressure 3.414mization, perhaps made more palatable by the greater for a discrepancy of only 1.6 dB, more closely matchingsubjective tolerance of localization errors in music as the relative magnitudes from a real source.opposed to dialogue or special effects. In addition the The typical three-loudspeaker layout places thecritical listener can choose to sit in the room's "stereo loudspeakers in a line rather than the arc shown in Fig.seat," equidistant from the two loudspeakers. 1, and the typical listener is not on the centerline,

In the early days of home stereo, Klipsch [7] proposed negating to some degree the specifics of this example.a derived center loudspeaker to widen the listening The closer relative placement of the center loudspeakerarea and fill the hole in the middle present in many does, however, actually increase its effectiveness inearly two-channel stereo recordings. The derived center maintaining central images. Also, for specific stronglysignal C' was made equal to directional sounds, such as dialogue and sound effects,

"directional enhancement" (from the term coined byC' = 0.707(LT + RT) (1) Willcocks [13] for one such method) may be employed.

This can yield discrete, single loudspeaker sources for

where, using the terminology of the remainder of this left, center, and right, and amplitude-panned (pairwisepaper, LT and RT are the total predominantly left andpredominantly right signals of the two-channel re-cording/transmission medium. The 0.707 factor was

introduced to equalize the total power from the threeloudspeakers (assuming incoherent addition) for left,

f \

center, and right sounds recorded by two widely spaced _ [I.414 _'_

microphones. This factor also turns out to be appropriate T//_ _ Ifor recordings made with the Blumleincrossed co- /incident bidirectional microphone technique [8] or

equivalent amplitude-panned mixes using standardsine-cosine panpots. This will be expanded on in Sees. Q¢3 and 4.

Although the Klipsch proposal was specifically in Fig. 1. Two loudspeakers ---45 ° off center from a listener, areference to widely spaced loudspeakers reproducing derived equidistant center loudspeaker, and their relative ye-recordings made with widely spaced microphones, locity vectors for a center signal.

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mixed) phantom sources between left and center and hanced center directionality with a mild gain-ridingbetween right and center, without directionally con- action. This boosted the center loudspeaker 2.5 dB and

fusing output from the opposite right or left loudspeaker, cut the left and right loudspeakers 2.5 dB in response(This is discussed further in Sec. 4.) These phantom to strongly centered signals (such as dialogue) [19].images suffer from the same kind of localization dif- This has since been replaced by the more elaborateficulties as with two-loudspeaker stereo, but over only directional enhancement used today [20], which alsohalf as much stage width. The result is that over a wide provides a decoded surround output.listening area dialogue can be maintained accurately

centered and sound effects can be positioned across 2 THE SURROUND LOUDSPEAKERSthe entire stage width with reasonably accurate local-

ization. Less strongly directional or more complex 2.1 Discrete Surround at the Theatersounds, such as orchestral music, which are not direc- The earliest use of surround (off-screen) loudspeakerstionally enhanced exhibit more localization variation in the movie theater was the Fantasound system, usedwith listener position, but still show significant im- only for the Walt Disney film Fantasia in 1940 andprovement over two-loudspeaker playback. 1941. The three sound tracks could be switched from

The addition of the center loudspeaker does narrow behind-the-screen loudspeakers to side or overheadthe stage width for a centered listener by roughly 25% loudspeakers [1]. Cinerama reserved two of its seven

for the nondirectionally enhanced sound. If desired, tracks for off-screen loudspeakers in various parts ofthis may be compensated for by slightly wider loud- the theater [5]. Cinemascope and Todd A-O both usedspeaker spacing without losing the benefits of the center a single track for surround information [ 1], as does the

loudspeaker. Also, in the full surround sound system present 70-mm format, with only occasional use of athe added ambience recovered in the surround loud- "split" (two-channel) surround [5].

speakers (discussed in the next section) subjectively The surround loudspeakers are generally used foroffsets the slight narrowing of the front stage, all-around environmental or ambience effects, with

The home video viewer/listener who places two similar information in the front loudspeakers. To avoidloudspeakers immediately on each side of a 23-in (0.58- localization to more closely positioned surround loud-m) monitor has no need of a center loudspeaker, but speakers, the surround track is delayed 11/2pictureneither does he enjoy a significant stereophonic effect, frames (60 ms) relative to the front tracks [5]. SpecificWhen the loudspeaker spacing is widened to obtain a surround-directed sound effects and even, rarely, dia-sound stage comparable in width to accustomed stereo logue are also recorded on the surround track. In par-listening, the center loudspeaker becomes indispensable ticular if these are important to the movie, they arefor off-center listeners, also mixed at a lower level in the front to guard against

the unpredictable nature of surround reproduction in1.3 The Derived Center at the Theater theaters [5].

The importance of the center loudspeaker was notforgotten in the evolution of the movie industry's two- 2.2 Ambience Extraction at Hometrack format. A recommendation for a derived center In the surround loudspeakers' role of creating a non-channel was included in an early demonstrated system directional ambience, an obvious analogy may be drawnof recording such tracks optically [14]. When a similar to various ambience extraction techniques suggested

system was made practical [15] through the use of noise for home playback of two-channel stereo recordings.reduction [16], the importance and benefits of a derived Madsen [21], expanding on ideas of using delayed sig-center channel were soon recognized [17]. Fig. 2, taken nals for ambience reconstruction which he credited

from Uhlig [17], shows the calculated shift of a central originally to Lauridsen [22], placed loudspeakers toimage between loudspeakers spaced 40 ft (12 m) apart the sides of the listening area and fed them delayedfor listeners 60 ft (18 m) away as they move off the front loudspeaker information. The sound from the sidecenterline. The apparent positions were calculated using ambience loudspeakers was delayed sufficiently so asdata from previous studies [2], [18]. The dashed line to arrive at the listeners' ears at least 2.5 ms later than

represents the image shift for left and right loudspeakers the front loudspeaker sound, taking into account theonly, and the solid line the considerably lessened shift propagation delay of sound in air of 0.88 ms/ft. Thiswith a derived center loudspeaker at an unspecified was to enable the Haas effect to aid instrument local-relative level. The author claimed subjective confir- ization in the front and to reduce coherence betweenmation for these curves except for an even greater image the front and side loudspeaker sounds. Madsen reportedshift for the two-loudspeaker case due to loudspeaker good subjective results in simulating hall ambience,directivity. He also reported that for listeners in the comparing favorably to some four-channel recordingscenter half of the theater the derived center loudspeaker of the time (about 1970), which used two separategave results "almost as good as a system with three channels for ambience information.discrete channels" [17]. Hafler's method [23] recognized that the difference

This work eventually resulted in the first commercial of the two stereo channels generally contains a higher

stereo optical format. The three-loudspeaker signal proportion of randomly phased, reverberant ambiencedecoding was soon embellished by a circuit which en- information than does either of the front channels or

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their sum and can make an appropriate rear loudspeaker fore undergoing a mild spectrum- and level-dependentfeed. This signal will be designated S' (for surround) downward treble shelving [24] of about 5-6 dB max-and defined as imum (modified Dolby B-type noise reduction decod-

ing). This response tailoring serves the dual purpose

S' = 0.707(LT -- RT) · (2) of quieting delay line and optical sound track noise,and reducing sibilant bleedthrough of center-encoded

The factor of 0.707 is included for mathematical sym- dialogue due to relative phase and amplitude errors inmetry. The level of the surrounds relative to the front LT and RT. These are most likely to occur from slightlevel is somewhat at the listener's discretion, but for adjustment and positioning errors of the two recording

the system to be described in this paper it is typically light valves and the two playback photocells. The lowabout 3 dB higher than indicated in Eq. (2). Hailer frequencies are rolled off below 100 Hz for the pro-recommended placing the rear loudspeaker as far behind tection of the theater surround loudspeakers, which ofthe listeners as possible to obtain a delay effect similar necessity are much smaller than the front loudspeakers.to that suggested in Madsen [21]. Hailer also recom- During encoding, prior to being mixed into LT and RT,mended a derived center similar to that given by Eq. the surround signal is also bandpass filtered at 100 Hz(1), yielding a system with similarities to the one under and 7 kHz before undergoing an approximately eom-

discussion and to many matrix quadraphonic systems plementary spectrum- and level-dependent upward tre-of the 1970s. ble shelving.

Specific rearward directed sound may also be encoded2.3 The Derived Surround at the Theater for the surround loudspeakers in addition to ambience

When used with the two-track surround sound format, effects, but to localize these sounds unambiguously

the movie theater's rear loudspeaker array receives the rearward requires directional enhancement. The un-basic signal of Eq. (2). This can be very effective in modified characteristics of the decoding matrix provideproducing environmental and ambience effects, par- for complete separation of center encoded sounds suchticularly when a large LT--RT component is introduced as dialogue from the surrounds, but other front soundin the encoded mix. Before feeding the loudspeaker effects require directional enhancement to prevent

array, S' is delayed by 30-100 ms, dependent on the confusing surround loudspeaker output. For suchsize and geometry of the theater, to aid front localization sounds, the surround time delay alone is insufficientfor non-surround-directed sounds over the entire seating for listeners close to surround loudspeakers.area [20].

The delayed signal is low-pass filtered at 7 kHz be- 3 MATRIXING AND PANNING CONSIDERATIONS

3.1 The Decoding Matrix

The system as described so far decodes four loud-

(_) 2o _ _ _ "_ I Ji speaker signals, defined, prior to directional enhance-(_),, i~ ment,as

·' = LT (3)®(&) ' R' = RT (4)

_ '(_ ,o, ,'/"°i'/ C' = 0.707(LT + RT) (1)Ts['\ S' = 0.707(LT - RT) · (2)C) 2O _. J J I I20 15 10 $ 0 5 tO 15 20

_*_,a ,or_o_AGE Combined with complementary encoding and panningPOS_ION(FEET}around a full 360 ° listening circle, this also essentiallydefines Scheiber's first "diamond array" two-channel

LEF3' [] RIGI-__,KE._ _S_^KE. quadraphonic matrix [25]. The most obvious departuresfrom the earlier system are the movement of the sideleft and right loudspeakers to the front left and rightpositions, the rear surround loudspeaker time delay,and the spreading of the surround information aboutthe rear half of the theater. This also, of course, departsfrom the "standard" quadraphonic layout of left-front,

_(_®,,_":_'_¢A-'_J right-front, left-back, and right-back loudspeakers._ The time delay between front and surround loud-

speakers prevents any possibility of forming stable sideFig. 2. Apparent audio image position for various listenerpositions. Dashed line--two channels; solid line--two phantom images. However, even with "ideally" pannedchannels with derived center channel [17, Fig. 5]. signals between a left-front and a left-back loudspeaker,

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for example, controllable phantom side images cannot These are represented by opposite points on the X axis.be formed (see [26] among others), even for an ideally Since no relative phase shifts are introduced in the

positioned listener. If sufficiently elaborate means could encoder between its L, C, and R inputs, then signalsbe employed to form side images for a small seating which are panned (pairwise mixed) between L and C,area, this would still not represent the best use of the R and C, and L and R are represented by points on thetwo channels of information available. "in-phase" half of the circle between these encoder

Motional effects between the front loudspeakers and points. This semicircle of points, shown heavier inthe surrounds, such as "fly-by's," can be effective Fig. 3, is the pairwise pan locus (as defined by Gerzon

throughout the theater though. These require "interior" [28]) for front signals. It does not differ from an "op-pans through the theater, as do environmental sounds, timal" or "ideal" locus, as is the case in other quad-which come approximately equally from all the loud- raphonic phase-amplitude matrices in at least somespeakers. It is these interior pans which necessitate quadrants.quadrature phase shifts in the encoder. A further ex- A point on the front locus can be assigned an imageamination of the interaction of the encoding and de- localization angle ql counterclockwise from front centercoding matrices will be aided by the introduction of a for a centrally positioned listener most conveniently"flattened Scheiber sphere" geometric representation according to the stereophonic law of tangents,of two-channel phase-amplitude encoding.

3.2 The Spherical Encoding Model t_ = tan -] tan qJ0 LTT+AS pointed out by Scheiber [27] and Gerzon [28],

the relative amplitude ILTI/IRTIand phase (bLT - (8)

(bRT= (l)of Li and Ri comprise the total information [ (0_]available to encode directionality in two channels. Every -- tan- l {_tan qJ0tan k/Jcombination of these can be uniquely represented by

a point on a sphere of unity radius, and every point onthe sphere represents one such coding. This was termed where t_0 is the angle away from the centerline of thethe energy sphere by Gerzon [28] and has also been left or right loudspeaker. This gives the direction ofcalled the Scheiber sphere from its usage by Scheiber the total velocity vector from the left and right loud-[27]. The axis orientation and use of (1)will be as in speaker wave fronts (not including the center loud-Gerzon [28], but the use of 0 will not. speaker) and is in agreement with the low-frequency

(<700 Hz) localization theories for the central listener

3.2.1 Amplitude-Only Encoding of Leakey [29], Makita [30], and Gerzon [31]. t_ will

Relative amplitude-only encoding, including polarity vary roughly proportionally with 0, except for veryreversals, is represented by points on a circle of unity wide loudspeaker placements. The exact proportionalityradius in the X- Y plane, as shown in Fig. 3. The angle at qJ0 -- 45° corresponds to the directional encodings0 counterclockwise from the positive X axis determining obtained with the crossed coincident bidirectional mi-point A is given by crophone technique.

The addition of the center loudspeaker narrows the

/L--\ soundstage somewhat from that determined by the val-

0 = 2 tan-] _RTT) -- 90° ues given by Eq. (8). However, the full stage widthcan be restored and the localizations returned to those

(5) of Eq. (8) for specific sources with the use of. appropriate

Lw - RT) directional enhancement circuitry. The localizations

\

= 2tan -1 LT + RT ' will be more accurate than with the use ofleft and right

Point A's X and Y coordinates arex¢

X A = cos 0 (6)

YA ---- sin0 . (7) A e

Also shown are the four primary encoding/decoding Lpoints for the matrix under discussion, although thesemay be different for other matrices. LT only and RTonly are encodings intended for the left and right loud-speaker positions, respectively, and are represented byopposite circle points on the Y axis. Equal in-phase Lwand RT encoding is intended for the center position,

and equal-magnitude opposite relative polarity LT and Fig. 3. Amplitude-only encoding circle. 0 = 2 tan-_(LT/RT)RTencoding is intended for the surround loudspeakers. - 90°.

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loudspeakers only, due to the closer spacing of the this, the representational model will be expanded toimage-producing loudspeakers, include the relative LT, RT phase shift qb.

The magnitude of the response of each of the decoded

outputs to an encoded direction represented by point 3.2.2 Phase-Ampfitude EncodingA is A Z axis is added in Fig. 4, and the plane containing

the circle of Fig. 3 is rotated about the Y axis by an

(?) angle qbas shown. To locate point B on the surface ofIAI = cos (9) a sphere, point A' is first found from 0',

where A0 is the angular difference between the encoding O' = 2 tan-l[ILz[h 90 °and decoding directions. Each decoded output (before _IRTI/

l

directionalenhancement)containsnotonlysignalsfrom (5a)

its intended direction but also signals from each adjacent i[[LTI -- [R_loudspeaker's direction attenuated only 3 dB. Complete = 2 tan- _l_-_x[ + [RT[J 'isolation is obtained only from signals encoded at the

point that is diametrically opposed to the decoding point. Then the plane of the circle is rotated by the angle qbA signal encoded at left center, 0 = 45 °, for example, to move A' to B. LT leading RT is represented by pointsappears at equal levels in L' and C' and at equal levels, in the upper hemisphere (intersecting the positive Z7.7 dB lower, in R' and S'. These relationships can, axis) and LT lagging RT by points in the lower hemi-of course, be deduced from Eqs. (1)-(4) and their sphere.

complementary encoding equations, but the graphic Such a representation returns many useful insightsrepresentation gives a more intuitive understanding. as discussed in Scheiber [27] and Gerzon [28]. For

Mono reproduction is equivalent to decoding at C', instance, Eq. (9) 'remains valid if A0 is reinterpretedmaking the relative reproduced level

as a spherical angle. An encoding where (ILTI/IRT[)=1 and qb = -90 °, for example, determines a point at

(0)}AMi = COS _ . (10) the bottom of the sphere forming equal 90 ° angles withall four decoding Points, resulting in equal output fromL', C', R', and S'. Eq. (11) also holds if X8 is substituted

This can be stated equivalently as the relative power for XA.level being proportional to the distance from S' of theX coordinate of point A; 3.3 Flattening the Sphere

While useful, the sphere is sometimes difficult to

_ represent and visualize in two dimensions.Two-di-iAMI2 X^ + 1 (11) mensional projections have been introduced [32], [33],2

which clarify some encoding characteristics at the ex-

pense of others. The two-dimensional representationPlayed back in mono, then, center-encoded signals areused here is simply a vertical projection as if lookingboosted 3 dB relative to left- and right-encoded ones,

as with conventional stereo recording. Signals mixed downward on the sphere. This is obtained by settingonly to surround, being pure difference information, the Z coordinate of point B, Za = 0, resulting in thedo not appear in mono reproduction, a characteristic projection of B to B', as shown in Fig. 4. As justifi-in common with the center-back direction of most cation, it is noted that all four decoding points are in

quadraphonic matrices. In this instance, it is a necessary

compromise because of the importance of isolating

center dialogue from the surround loudspeakers. In Zpractice, critical sounds are mixed at least slightly for-ward of the surround direction, which can still result

in good rearward localization with the aid of directionalenhancement.

If no relative phase shifts were introduced in the B ,,--encoder between the front directions and the surrounds,

then the pan loci for front-to-surround pans, includingcenter-to-surround, would be confined to points on thecircle of Fig. 3. Panning from center to surround wouldbe confined to traversing the circle through either theleft or the right points. There are no points on the circlewhich result in approximately equal output from allfour decoded outputs and which, therefore, would allow

symmetrical panning through the "interior" of the Fig. 4. Forming the phase-amplitude sphere and projectingtheater. To examine how encoder phase shifts allow it to the X-Y plane (after [28, Fig. 2]).

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the X-Y plane and remain unmoved, and that only the phase shifts. Relative values ofL', C', R', S', LT, andlower hemisphere of phase encoding is used in the sys- RT may be calculated from XB, and YB'with the addedtem under discussion, so no information is lost in this relationship.instance.

The specification of point B' is shown also in the IL'[2 + IR'I2 ---[c'l2 + Is'l2 . (16)projected view of Fig. 5. Throughout the phase rotationand vertical projection, the Y coordinate remains, The point (X, Y) = (0, 0) represents ILTI/IRTI-'-1,

4) = _+90°, and gives Im'l= Ic'l= Ig'l= Is'l.PointsYB' = sin 0' . (7a) farther from the center are decoded less equally among

the outputs with points on the perimeter representingAlso, the strongestdirectionalencoding.Eq. (11)stillholds

with XB, substituted for XA.

YB' = sin 2ta - 90° 3.4 The Encoding Locus(12) Fig. 6 is a simplified block diagram of the Dolby

Stereo/Dolby Surround matrix encoder. Not shown are

[. _i[IRT['_] the surround input frequency response processing blocks= cos 2 tan [[_) described in Sec. 2 and front signal path phase-com-pensating networks which match the phase shifts which

and the processingintroducesin the surroundpath. The 'desired 90 ° phase shift of surround relative to front

ILTI2 - IRTI2 can only be introduced by the use of all-pass networksYv = ILTI2 + [Rx12 in both the front and the surround paths with 90° relative

(13) phase shift across the audio frequency range [34], as

IL, ]2 _ [R'I2 shown.

- [m,[2 + Ig, I2' The unfortunate frequency-dependent phase shiftintroduced by the all-pass networks, and any consequent

The X coordinate is given by degradation of sound quality, has been the subject ofcontroversy since such networks were introduced to

XB, = cos 0' cos qb . (6a) the technology of quadraphonics. At present, a probableconsensus may be drawn from a recent study [35] on

Also by analogy to Eqs. (12) and (13), the audibility of midrange phase distortions, particularly

900]A'

[= cos 2 tan i_ I (14)y_

= c°s[2tan-l(LL;- Rx"_l+RTIJJ

and

lc'12- Is'12 Fig. 5. Specification of phase-amplitude encoding point B'x_,- ic,r + is'l2 in the X- Y plane.

(15)ILT + RTl2 -- ILT -- RTl2

= ILT+ RTl2 + ILT-- RTl2 ' L'4 ') [--7-3---------,_O

Thus phase-amplitude encoding of LT and RT is rep- __ .707 I_ .707_ = LTresented by points on or within the circle of Fig. 5, /

with the only loss of information being that leading c_ s----_ ¢_ '?_L*O

and lagging phase relationshipsof the same amount -'--"Tare representedidentically.Points on the perimeter of +

the circle represent amplitude encoding only, including "a.opolarity inversions, and points in the interior of thecircle represent encodings with other than 0 ° or 180° Fig. 6. Simplified encoder block diagram.

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with reference to those resulting from loudspeaker surround pans obtained by sweeping the first (front-S)crossovers (which are not dissimilar to those from the pan pot in Fig. 7 through its range with fixed settingsall-pass networks). Such distortions are clearly audible of the second (front) pan pot. Fully symmetrical panningwith appropriate test signals auditioned on headphones capability through the interior of the theater is obtainedor anechoically, but with normal music over loud- at the inconsequential loss of L to S and R to S perimeterspeakers in a typical room, they are either inaudible panning capability.or extremely subtle in their effect. It is also possible, An interesting similarity exists between Fig. 8 andif desired, to bypass the phase-shifting circuitry by Gerzon's Fig. 11 [28], which is used to help explainfeeding signals which are not to be panned between his "two-loudspeaker stereo localization theorem."front and surround directly to LT and RT. According to low-frequency (<700 Hz) theories of

Ignoring the phase shift common to all inputs, the sound localization such as those of Leakey [29], Makitaencoder is characterized by the encoding equations [30], and Gerzon [31], the front-to-surround pan loci

shown also represent loci of constant two-loudspeakerLT = L + 0.707C - j0.707S (17) playback localization for a centrally positioned listener.

For a sound panned from a front point to the surround

RT ----R + 0.707C + j0.707S (18) point, the two-loudspeaker listener hears approximatelyconstant localization with increasing "phasiness" and

where the j coefficient is used to denote an idealized image vagueness.frequency-independent 90 ° phase shift. The resultingfront pan locus and the surround point are as shown in 3.5 Comparison with Other MatricesFig. 3. However, following the guidelines of Gerzon The graphic representation can also be used to aid[28] for determining pairwise pan loci in the spherical comparison with other matrices. Fig. 9 shows the pair-model, the pairwise pan locus from any front direction wise pan locus of the QS [36] matrix between adjacentto the surround point will depart from the front semi- loudspeaker positions. Its four primary encoding/de-circle at a right angle, following a vertical semicircular coding points correspond to the conventional quadra-path on the lower half of the sphere of Fig. 4 on its phonic loudspeaker layout and are located at 0 = _+45 °,way to the surround point. This projects onto the X- _+135 ° in the X-Y plane. Again, the back points areYplane as the chord connecting the two perimeter points, given a 90° phase relationship to the front points, but

A dual pan-pot arrangement such as shown in Fig. with qbpositive instead of negative. This leads to vertical7 is capable of reaching all encodable positions of the semicircular front-to-back pan loci through the uppermatrix. Fig. 8 shows several representative front-to- hemisphere, projecting to the chords of Fig. 9.

With the addition of a center-front loudspeaker atthe usual decoding point, Fig. 9 represents a plausibletheater system with interior pan capability, includingFRONT-S PAN FRONT PAN

'"'"'"_V_2_/L?{FRON_ the center of the circle, and a perhaps useful 3 dB of

left-back to right-back separation. However, left-frontto right-front separation is also only 3 dB, severely

TO limiting the front soundstage width in the absence ofI._' ENCODER directional enhancement. Also, separation from center

front to either left back or right back is only 8.3 dB,s resulting in severe back-loudspeaker dialogue leakage.

--xAAP _ ) As noted in Willcocks [33], full front-stage width

Fig. 7. Dual pan-pot arrangement capable of reaching all may be restored by introducing -7.7 dB of opposite-encodable positions, polarity crosstalk between the QS-encoded LT and RT,

x x

L

Y y

Fig. 8. Representative pan loci for encoder of Fig. 6 with Fig. 9. Pairwise pan locus of QS matrix between adjacentpan pots of Fig. 7 loudspeaker positions.

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giving the diagram of Fig. 10. (Compare Figs. 9 and center-front dialogue is still a heavy handicap for sub-10 to the spherical representations in Figs. 13 and 14 sequent directional enhancement to overcome. Theof [33].) Left back and right back move to 0 = _160.5 °, difficulties of applying other matrices to film/videoreducing their separation to a negligible 0.5 dB. Center- surround sound should not be surprising since this wasfront to left-back and right-back separation improves not their original intended use.to 15 dB. Complete center-to-back separation is Both Scheiber [27] and Gerzon [38] have discussedachieved if left back and right back are combined to a the possibility of using the third axis of the Scheiber

single point at 0 = 180 °, as in Fig. 8. sphere (in this case the Z axis of Fig. 4) to encode aThe optimal or "position"-encoded pan locus for the degree of height information. In the matrix of Fig. 8,

SQ matrix [37] is shown in Fig. 11. (Note that the side as defined by encoding Eqs. (17) and (18), the lower

pans shown projected on the Y axis are not obtained hemispherepointsarealreadyusedforinteriorpanning,by pairwise mixlng.) The front pan locus is the same but the upper hemisphere remains undefined. It is plaus-as Fig. 8, as is the center-back point. The center-front ible that the top of the sphere (ILTI/IgTI= 1, qb= 90o)and center-back positions are not normally used for could be designated as overhead or ceiling, but theloudspeaker-feed decoding. The left-back and right- difficulty of its being separated by only 3 dB from theback points represent [LTI/IRTI-- i and qb = 90° and perimeter directions, including center dialogue, would-90 °, respectively, placing them at the top and bottom have to be overcome.of the spherical representation.

Interior sounds intended in the theater system to come 3.6 Representation of Average Directionalityapproximately equally from all loudspeakers would, Gerzon [28] introduces the idea of a point interiorif played back through the decoding matrix implied by to the sphere representing the average directionalityFig. 11, appear most strongly in the right back and of a multiplicity of simultaneous, independent signals.most weakly in the left back. Center-front dialogue That point is given as the center of "mass" of all theand center-back sounds (encoded identically by the individual signal points weighted by the relative energy

matrices of both Figs. 8 and 11) decode equally in all levels of their respective signals. The orientation offour loudspeaker points of Fig. 11, leading to diffuse that point with respect to the center of the sphere "rep-localization. This could be improved with the addition resents essentially all the information" [28] that isof center-front and center-back loudspeakers, but the available to control directional enhancement circuitry.mere 3-dB separation of left-back and right-back from These points inside the sphere can be projected onto

the X-Y plane as the surface points were by setting

x Z -- 0. The X and Y coordinates are still given by Eqs.(12)-(15), and Eq. (16) is still valid if the IIsigns arereinterpreted as rms averages. Thus a point in the interiorof the circle can have either of two related meanings.It can represent the directionality of an individual en-coded signal, or the average directionality of a complex

LF .F sound field made up of a plurality of independentsources. In this second context, the point will moveabout the interior of the circle dependent on the natureof the sound encoded in LT and RT and the averagingtime considered. The point will generally spend moretime in the front half of the circle than in the rear half,

indicating more sum than difference information. It

Fig. 10. Pairwise pan locus of QS matrix after -7.7-dB will approach the perimeter only in response to aopposite-polarity blending of Lt and R-r. strongly predominant directional sound.

x 4 DIRECTIONALENHANCEMENT(CF)

4.1 The Need and the Limitations

The basic matrix of Fig. 8 as described providescomplete center-to-surround isolation, symmetricalinterior panning, a wide front stage, and appropriate

LF R8 .F ambience extraction from stereophonic music. Withy _

just two channels of information, however, it is not by

itself capable of stable image localizations over a widelistening area. By Eq. (9), center dialogue appears at-tenuated only 3 dB in the adjacent left and right loud-speakers, allowing sideways image shifts for off-center

(em listeners (although not as much as without the centerFig. 11. Pan locus of"position" encoded SQ matrix, loudspeaker). Far left and right sound effects appear

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at an approximately equal level in the surround loud-speakers, considering the typical 3-dB surround level 4.2 Circuit Considerationsplayback advantage. Surround sound effect localization The means used to effect the directional enhancementdifficulty caused by leakage to the left and right loud- have evolved in sophistication since the early days ofspeakers is exacerbated by the surround time delay, quadraphonics. The earliest circuits concentrated on

Ideally, sounds directed at each of the loudspeaker making a dominant sound at a loudspeaker positiondirections should appear only at their intended loud- more "discrete" by attenuating the neighboring loud-speaker outputs. Signals panned between L and C, and speakers and slightly boosting the desired loudspeakerbetween R and C, should appear only at those pairs of and its diagonal opposite to compensate the overallloudspeaker outputs, and at the appropriate relative power level [25]. The loudspeaker level changes couldlevels. The ideal directional enhancement circuit would be quite audible, particularly for an off-center listener,transform the basic decoded outputs L', C', R', and S' so subsequent developments have tended toward variousinto enhanced outputs L", C", R", and S" with these cross-coupling and leakage cancellation schemes. Thesecharacteristics for all encoded directions perfectly and all effectively vary the decoding matrix dynamicallysimultaneously. This, of course, is impossible, to cancel unwanted components of a predominant di-

A well-designed directional enhancement circuit can rectional signal from undesired loudspeaker outputs.provide such a transformation for only one, or at most The later versions of these perform at least approximatetwo, diametrically opposite (on the circle) directions directional enhancement for all encoded directions, notat a time. In addition, the average sound field direc- just the loudspeaker locations (see [ 13], for example).tionality point defined in the previous section, which Various circuit topologies may be employed to obtainis used to control the enhancement, can only indicate the appropriate matrix modifications, but their resultsone direction at any given instant. These considerations must, of necessity, be similar. For a decoded loud-establish firm limits on what enhancement circuitry speaker output not to contain a given encoded directioncan accomplish. Within these limits, however, careful it must, by Eq. (9), decode at the diametrically oppositedesign monitored by careful listening can result in cir- point on the circle of Fig. 8. When the left directioncuitry enabling excellent directional acuity for pre- is enhanced, for example, the other three loudspeakerdominant sounds over a wide listening area, with min- outputs must decode temporarily at the diametricallyimal audible side effects. An important part of achieving opposite right position (perhaps with differing signalthis is to design the enhancement circuitry to limit its polarities) if they are to produce output that does not

action essentially to strongly predominant directional contain L. This is not necessarily as audibly discon-sounds such as sound effects and dialogue, and not certing as might be imagined since the shift of theoverreact to complex sound fields which cannot be reproduced sound field should normally be masked byproperly enhanced, the onset of the strongly predominant directional sound

It is also important that the overall subjective result that triggered it. It is evident, then, that the audibleobtained with such circuitry not differ substantially differences among various directional enhancementfrom that obtained with present theater system en- circuits will not be due so much to the matrix modifi-hancement circuitry. Monitoring is normally done cation topology chosen as to the directional sensingthrough this system when the movie sound track is characteristics, time constants, matrix modifier controloriginally mixed to confirm the result obtained after signal processing, and degree of enhancement.processing through the matrix and enhancement circuitry[1]. 4.3 Power Compensation

The front soundstage of the theater matrix is identical A level boost is normally given to the enhanced di-to conventional stereo, so the directional enhancement rection's output or outputs to maintain constant total

circuitry will act similarly on such material. This may power for the predominant sound when its unwantedor may not give desirable results if the material was leakage is canceled. For a primary direction such asnot intended for such playback. For example, a close center, a boost of 3 dB is appropriate if the 3-dB downmultimiked string orchestra may have an overexuberant leakages at left and right are canceled. For the L to Cviolinist who leans into his microphone and becomes and R to C pan positions, the appropriate power corn-momentarily directionally predominant, causing the pensation is not so obvious. There are two possibilitiesreproduced sound field to momentarily focus unnaturally based on discrete L-C-R panning or the nearly equiv-on him. In general, the best results are obtained when alent L-R panning.the enhancement circuitry is used with material mixed Fig. 12(a) shows the relative levels of LT and RT forfor it, such as surround sound movie sound tracks. For the front pan locus when panned by a standard sine-such material, decidedly inferior results are obtained cosine pan pot applied between the L and R inputs ofwhen the enhancement is not employed. The enhance- the encoder of Fig. 6. When decoded through eitherment circuitry should find useful application, however, two (left and right) or four loudspeakers, the total power

with upcoming stereo television productions in main- level shown in Fig. 12(b) remains uniform with thetaining dialogue solidly on screen, panned effects ac- pan position (before the application of directional en-curatelypositioned, and even enhancing surround effects hancement). Fig. 13(a) shows two sine-cosine panswhich may ultimately be specifically encoded, between L and C, and between R and C, as would be

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done in the discrete four-channel format. Fig. 13(b) in Sec. 2.2), which do not receive directional enhance-

shows that, reproduced over three loudspeakers fed ment. The delay adjustment range (in this case, 16-discretely, the total power remains constant. Fig. 14(a) 36 ms) enables adaptation to differing seating andshows the results in LT and RT if L, C, and R of Fig. loudspeaker layouts and personal taste.13(a) are fed to the inputs of the encoder of Fig. 6, aswould be the case when encoding from a four-channel 5.1 The Center Loudspeaker Revisited

master. In Fig. 14(b) the total power, when reproduced The processor will often be incorporated into an ex-over two or four loudspeakers, exhibits a 2.3-dB peak isting two-loudspeaker stereo system. For cost reasons,at left center and right center as compared to the L, C, the center loudspeaker may not be incorporated initially.and R positions. [The amplitude curve shown is given While its desirability has been thoroughly justified,by (1 + Isin(20)l/V_)_.] The angular locations re- acceptable results can be obtained without it for centrallysuiting from this panning are still correct, positioned listeners with narrow left-right loudspeaker

The directional enhancement circuitry may equalize spacing. A switchable option must be provided to varythe panning level for enhanced sounds for either the the characteristics of the directional enhancement cir-curve in Fig. 12(b) or that in Fig. 14(b), but not both. cuitry so that center sounds are not enhanced to a non-Equalizing for Fig. 12(b) has the advantage that the existent loudspeaker. The enhancement circuitry is stillpower level for an enhanced direction remains the same essential in separating off-center front sound effectsafter enhancement as before, possibly minimizing from the surround loudspeakers and surround-directed"pumping" effects. However, left to right sound effect sound effects from the front loudspeakers.pans which were smooth in the discrete format take on It will also be found that for reasons of cost, acs-the uneven character of Fig. 14(b). thetics, or practicality, the center loudspeaker may not

In the home decoder discussed in the next section, match the left and right loudspeakers. To the extentthe curve of Fig. 14(b) is compensated, giving left-to- that the side and center loudspeakers do not match inright sound effect pans as smooth as the original discrete amplitude and phase response, image stability will sufferpan. This means that sound effects panned according for other than enhanced loudspeaker-directed sounds.to Fig. 12(a) exhibit 2.3-dB level drops at left center The results should be acceptable, though, if the am-and right center, if enhanced. There is also a 2.3-dB plitude responses are in general agreement and the phasedrop in the overall power level of a left-center or right- responses are matching at least up through the midfre-center sound when it is enhanced as compared to before quencies.enhancement. In practice, the directional enhancement

is applied sufficiently quickly that the preenhancement OdB----.-_ _- OdB

levelis not discerned.Theenhancementpowereom- _3d8_LT___ TOTAL POWER

pensation curve chosen represents a compromise infavor of fidelity to sound effects as encoded from adiscrete master.

L C R L C R

5 THE HOME ENVIRONMENT Fig. 12. Front sine-cosine panning from left to right. (a) LTand RTrelative levels. (b) Total relative power over two or

The discussion of the previous sections has not made four loudspeakers.distinctions between the motion picture theater and thehome video environment. While the scale is smaller

in the home, the positioning of the loudspeakers and .... TOTALPOWERthe sound field in relation to a typical seating area aresimilar to those in the theater. Even for small-screenviewers there is still a desire for a normal stereo sound-

stage width. The benefitof a center loudspeakerwith L C R L C R

directional enhancement in keeping dialogue on screen Fig. 13. Front sine-cosine panning between discrete frontand center screen is even more apparent for these view- channels. (a) L, C, and R relative levels. (b) Total relativeers. poweroverthreeloudspeakers.

Adapting the theater system to the home does suggestsome special considerations. These have been taken

into account in the design of a new home video surround ods - _ f ,sound processor [39] (Fig. 15). The unit incorporates TOTALPOWERdirectional enhancement circuitry [40] capable of ac- -3dB....

curately enhancing strongly predominant sounds from --_T--//__any encoded perimeter direction, and an adjustable / 'xwide-dynamic-rangedigital delay [41] for processing L C , L C athe surround loudspeaker signals. The surround loud- Fig. 14. Front sine-cosine panning between L, C, and Rspeaker time delay is needed to aid forward localization encoded into LT and RT. (a) LT and RT relative levels. (b)of less strongly predominant front'sounds (as discussed Total relative power over two or four loudspeakers.

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If the center loudspeaker is smaller than the side spectral balance of naturally occurring hall ambience.loudspeakers, it will probably have less bass response.Since bass frequencies are often mixed to the center 5.4 Other Useful Featuresposition, activation of center enhancement (in response In addition to video surround sound processing cir-to dialogue, for example) could remove bass "leakage" cuitry, other features enhance such a unit's usefulness.from the side loudspeakers and direct the energy to the Switching for conventional mono and stereo playbackcenter loudspeaker with weak bass response. This could is, of course, necessary. With the requisite loudspeakerscause a drop in bass level and audible bass modulation, and amplifiers already in place, it is logical to provideTo avoid this problem, center-directed low bass fre- surround sound synthesis capability from mono andquencies should not receive directional enhancement, conventional stereo signals, which still make up theThis does not introduce localization problems since bulk of available source material.the low bass frequencies do not contribute significantly Remote volume control is a worthwhile convenience,to perceived directionality in the home environment in as is remote surround balance control to adapt to dif-the presence of other, stronger directional cues. fering recorded mixes and individual taste. Left-to-

right output balance adjustment is not needed or ap-5.2 Subwoofers propriate once the system has been set up properly for

This property of the low bass frequencies suggests the given listening area. A test disk or tape is helpfulthe use of a common subwoofer, allowing smaller in performing this setup.loudspeakers to be used for left and right also. The An input balance control should be adjusted to a

: processor of Fig. 15 includes a subwoofer output with given source to balance incoming LT and RT levels fora fixed second-order Butterworth low-pass characteristic optimum directional decoding. An input level controlat 80 Hz. This balances well with many small, sealed and level indicator is needed to approximately calibrateloudspeaker systems with second-order rolloffs below the signal level through the decoding circuitry. Finally,about 65-100 Hz. The low-bass power-handling re- a visual sound-direction display (at the right end of thequirement of the main loudspeakers may be reduced panel in Fig. 15) is an aid in system setup and in con-in a more elaborate system by the use of additional firming proper system operation.high- and low-pass filtering to create higher order While different approaches may be taken in the de-crossover networks, velopment of a home processing unit, the end result

The level of the subwoofer required to balance the should be a system that combines all the importantmain front loudspeakers depends on whether one, two, elements needed for an accurate reproduction of cinemaor three front loudspeakers are operating, and should surround sound. In so doing, a standard is created forshift with changes in the operating mode (mono, stereo, the future in providing not just a vague all-around sound,or surround). When listening to movie sound tracks, but the specific, accurate effect intended by the movie-though, some listeners will find it appropriate to use makers.the subwoofer not as a "high-fidelity" bass extension,

but rather as a visceral special effect. 6 SUMMARY

5.3 The Surround Loudspeakers Revisited Sound tracks encoded with the surround sound of

In the theater numerous small surround loudspeakers the movie theater are now available for home audio/are used to diffuse the rear sounds and provide even video playback. The directional information is encodedcoverage. In the home, two surround loudspeakers are by a phase-amplitude matrix method similar to four-more practical. In the processor of Fig. 15 an image- loudspeaker quadraphony, but with its roots in well-spreading technique is employed to diffuse the rear grounded three-loudspeaker stereo and ambience ex-image and discourage localization at the closer surround traction techniques. A new two-dimensional represen-loudspeaker. This does not reduce the ability of the tation of two-channel phase-amplitude encoding aidsdirectional enhancement circuitry to direct intended in understanding and analyzing this and comparable

sounds solidly rearward, matrices. The surround information can be accuratelyThe surround loudspeakers need not have extended recovered for home reproduction with a processor

bass response, since they receive signals intended in incorporating a wide-dynamic-range delay for the sur-the theater system for loudspeakers with limited bass round loudspeakers and directional enhancement cir-response. Also, as noted in Sec. 2, the surround en- cuitry to stabilize the localizations of strongly predom-coding and decoding treble response is limited to 7 inant sounds over a wide listening area.kHz, and some mild noise reduction is employed. Bothare done to overcome practical limitations of the opticalsound track-based theater system. It may be argued _that these limitations do not necessarily apply to the

emerging home video formats, and some extension ofsurround treble response for these formats may be jus-tified in the future. Some degree of treble rolloff isoften found to be appropriate in any case to match the Fig. 15. Home video surround sound processor.

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7 ACKNOWLEDGMENT Noise Reduction System," SMPTE J., vol. 84, p. 720(1975 Sept.).

Of many people associated with the home surround [17] R. Uhlig, "Two- and Three-Channel Stereo-sound project, the author wishes in particular to ac- phonic Photographic Soundtracks for Theaters and

knowledge the contributions of William Bevan, Mark Television," $MPTE J., vol. 83, p. 729 (1974 Sept.).Gilbert, Paul Jenrick, and Robert Schulein, and their [18] H. F. Olson, "Stereophonic Sound Reproductiondedication to the concept of home video surround sound, in the Home," J. Audio Eng. Soc., vol. 6, p. 80 (1958An additional note of thanks goes to Dolby Laboratories Apr.).for their helpful input. [19] D. Robinson, "CP100 Cinema Processor--A

New Audio Control Center for the Movie Theaters,"

8 REFERENCES presented at the 54th Convention of the Audio Engi-neering Society, J. Audio Eng. Soc. (Abstracts), vol.

[1] L. Blake, "Mixing Dolby Stereo Film Sound," 24, p. 492 (1976 July/Aug.), preprint 1112.Record. Eng./Produc., vol. 12, p. 68 (1981 Feb.). [20] D. Robinson, "CP200--A Comprehensive

[2] J. Steinberg and W. Snow, "Auditory Per- Cinema Theater Audio Processor," $MPTE J., vol.spective--Physical Factors," Elec. Eng., vol. 53, p. 90, p. 778 (1981 Sept.).64 (1934Jan.). [21] E. R. Madsen, "Extraction of AmbienceIn-

[3] H. Fletcher, "The Stereophonic Sound Film formation from Ordinary Recordings," J. Audio Eng.System--General Theory," J. Acoust. Soc. Am., vol. Soc., vol. 18, pp. 490-496 (1970 Oct.).13, p. 89, (1941 Oct.). [22] H. Lauridsen, "Experiments Concerning Dif-

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[7] P. W. Klipsch, "Stereophonic Sound with Two [26] P. Ratliff, "Properties of Hearing Related toTracks, Three Channels by Means of a Phantom Circuit Quadraphonic Reproduction," BBC Research Dept.,(2PH3)," J. Audio Eng. Soc., vol. 6, p. 118 (1958 Rep. BBC RD 1974/38, 1974 Nov.Apr.). [27] P. Scheiber, "Analyzing Phase-Amplitude Ma-

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"On the Audibility of Midrange Phase Distortion in 350 (1973 June).Audio Systems," J. Audio Eng. Soc., vol. 30, pp. [38] M. A. Gerzon, "Periphony: With-Height Sound580-595 (1982 Sept.). Reproduction," J. Audio Eng. Soc., vol. 21, pp. 2-

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dio Eng. Soc., vol. 20, pp. 167-173 (1972 Apr.). Sheet 27A8083, 1985[37] B. B. Bauer, R. G. Allen, G. A. Budelman, [40] S. Julstrom, "Directional Enhancement Circuit,"

and D. W. Gravereaux, "Quadraphonic Matrix Per- patent applied for.spective--Advances in SQ Encoding and Decoding [41] M. Gilbert and S. Julstrom, "Delta ModulationTechnology," J. Audio Eng. Soc., vol. 21, p. 342- Encoding/Decoding Circuitry," patent applied for.

THE AUTHOR

Stephen Julstrom studied electrical engineering at where he is now a senior staff engineer. In addition tothe University of Iowa where he was later employed surround sound system development, his duties havein the recording studios of the School of Music. Since included work on automatic microphone systems, mi-1981 he has been employed in the electronics devel- crophone preamplifiers, and teleconferencing systems.opment department at Shure Brothers Incorporated, He holds one patent in automatic microphone control.

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