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A High-PerformanceSurround Sound Process for Home Video* STEPHEN JULSTROM

Shure Brothers Incorporated,

Evanston,

IL 60202,

USA

Video disks, high-fidelity video cassettes, and stereo television bring to the consumer a substantial library of video entertainment with high-quality stereo sound tracks. A growing portion of these are encoded with surround sound using a 4- 2- 4 matrix-based method particularly suited to front-stage oriented film/video presentation. Home reproduction is accomplished with a processor incorporating dynamic matrix modification to stabilize and enhance directional effects and a delayed surround channel to aid forward localization of front sound.

0 INTRODUCTION For several years now moviegoers have regularly experienced surround sound at their local theaters. The two most commonly used theater surround sound formats were developed by Dolby Laboratories and both have the trade name Dolby Stereo. (For home use the trade name Dolby Surround is used.) Both theater formats involve three full-range, behind-the-screen loudspeakers (left, center, and right) and a U-shaped array of "surround" loudspeakers disposed about the rear half of the theater, receiving a common signal. Additional loudspeakers may also be used to augment bass reproduction, In the 70-mm wide-screen format, four magnetic tracks are used to record the three front loudspeaker signals and the surround loudspeaker array signal discretely. In the less costly 35-mm format, practical considerations [1] limit the sound track to two optically recorded channels. The four original channels are encoded into two and subsequently decoded at the theater using techniques related to, but differing significantly from, previous matrix-based quadraphony. The differences arise primarily from the nature of the audio material, which includes not only music, but also dialogue and sound effects, and the strong front-stage orientation, dictated by the film action. Rather than attempting to optimize spatial reproduction of music around a full 360 째 for a few people near the center of a regular loudspeaker array, the limited information capacity of the * Presented at the 79th Convention of the Audio Engineering Society, New York, 1985 October 12-16; revised 1987 May 26. 536

two channels is used to provide a high degree of directional accuracy over a wide listening area for frontstage sounds, particularly dialogue and effects, and a diffuse surround sound, providing both rearward directional effects and general ambience and environment. This encoded surround sound information is presently made available to the home video enthusiast through the two-channel sound tracks of video disks, video cassettes and, soon, stereo television when encoded movies are duplicated or broadcast. The added impact surround sound brings to the movie theater can also be enjoyed at home with the addition of a suitable decoder and auxiliary equipment. In spite of the difference in the size of the listening/viewing environment, the goals of the surround sound encoding and decoding for home video need not differ substantially from those for the theater system. An examination of the desirable goals and operation of a surround sound decoder for home video use will be aided by reviewing and analyzing the characteristics and behavior of the theater system and its matrix. The system may first be viewed as a logical outgrowth of three-loudspeaker stereo and previous ambience extraction techniques. I THE CENTER

LOUDSPEAKER

1.1 Some History In the beginning stereophonic sound did not always mean two-loudspeaker sound. Well-known early experiments by Bell Labs [2] used three transmission channels and three loudspeakers to achieve a stereophonic effect across a wide stage. Two channels with J. AudioEng.Soc.,Vol.35,No.7/8,1987July/August


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SURROUND SOUNDFORHOMEVIDEO

either two or three loudspeakers were also tried with good results for the centrally positioned listeners. In the last instance the center loudspeaker was derived (not discrete), receiving an attenuated sum of the left and right loudspeaker signals, In further experimental work on stereophonic film sound, Bell continued the three-channel, three-loudspeaker approach [3]. The use of at least three loudspeakers for stereophonic sound has remained the movie industry standard ever since its commercial introduction in the early 1950s. Cinerama [4] used five loudspeakers behind the screen to augment its triple-width picture, The 70-mm Todd A-O format also used five behindthe-screen loudspeakers [5], while Cinemascope stayed with three [6]. Maintaining at least the center loudspeaker has always been felt essential for adequate localization, particularly of dialogue, over the wide audience area. Initially dialogue was often recorded stereophonically or "panned" electronically to follow the screen action. This practice is not presently followed and, generally, almost all dialogue is directed to the center [5]. 1.2 The Derived Center at Home The situation in the home listening environment is not significantly different from that in the theater if the desired sound stage width is considered in relationto the typically available seating area. It may be convincingly argued that the de facto standardization on two loudspeakers for home stereo listening has been more a matter of convenience than of technical optimization, perhaps made more palatable by the greater subjective tolerance of localization errors in music as opposed to dialogue or special effects. In addition the critical listener can choose to sit in the room's "stereo seat," equidistant from the two loudspeakers. In the early days of home stereo, Klipsch [7] proposed a derived center loudspeaker to widen the listening area and fill the hole in the middle present in many early two-channel stereo recordings. The derived center signal C' was made equal to C' = 0.707(LT + RT)

(1)

where, using the terminology of the remainder of paper, LT and RT are the total predominantly left predominantly right signals of the two-channel cording/transmission medium. The 0.707 factor

this and rewas

introduced to equalize the total power from the three loudspeakers (assuming incoherent addition) for left, center, and right sounds recorded by two widely spaced microphones. This factor alsothe turns out to be crossed appropriate for recordings made with Blumlein coincident bidirectional microphone technique [8] or equivalent amplitude-panned mixes using standard sine-cosine panpots. This will be expanded on in Sees. 3 and 4. Although

the Klipsch

proposal

was specifically

reference to widely spaced loudspeakers reproducing recordings made with widely spaced microphones, J.AudioEng.Soc.,Vol.35,No.7/8,1987July/August

in

significant image shift for listeners off the centerline between the loudspeakers has also long been noted for two-loudspeaker playback of coincident microphone recordings [9]. The shifting of images toward the closer loudspeaker is a familiar effect. It is easily explainable on a qualitative level in relation to the amplitude advantage of the closer loudspeaker and its sound's time of arrival advantage (the well-known Haas or precedence effect [ 10], [11]). Even for an optimally positioned listener, the phantom center image is often not as compelling as a pure left or right, particularly for more widely spaced loudspeakers. The phantom image may appear as an "inthe-head" or "overhead" sound. As perhaps a partial explanation, it has been pointed out that for a phantom center image produced from equal in-phase wave fronts from tiro loudspeakers, the particle ¥elocity and the pressure at the listening position are not in the correct ratio compared to a real source [12]. In Fig. 1 two loudspeakers flank the listener at - 45 ° and produce equal, in-phase signals. Ignoring the effectofthelistener on the sound field (reasonablyvalid for frequencies below about 700 Hz), the two particle velocity vectors add to a relative magnitude of 1.414 directed along the centerline, but the pressures add as scalars to a relative value of 2. This pressure is 3 dB higher than that which a real center source with a particle velocity of 1.414 at the listener's position would have given. If a center is added as shown at an equal distance to the listener and at a level given by Eq. (1), the resultant particle velocity is of magnitude 2.828 and the pressure 3.414 for a discrepancy of only 1.6 dB, more closely matching the relative magnitudes from a real source. The typical three-loudspeaker layout places the loudspeakers in a line rather than the arc shown in Fig. 1, and the typical listener is not on the centerline, negating to some degree the specifics of this example. The closer relative placement of the center loudspeaker does, however, actually increase its effectiveness in maintaining central images. Also, for specific strongly directional sounds, such as dialogue and sound effects, "directional enhancement" (from the term coined by Willcocks [13] for one such method) may be employed. This can yield discrete, single loudspeaker sources for left, center, and right, and amplitude-panned (pairwise

f

\

_

[I.414

T//_

_

_'_ /I

Q¢ Fig. 1. Two loudspeakers ---45 ° off center from a listener, a derived equidistant center loudspeaker, and their relative yelocity vectors for a center signal. 537


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mixed) phantom sources between left and center and between right and center, without directionally confusing output from the opposite right or left loudspeaker, (This is discussed further in Sec. 4.) These phantom images suffer from the same kind of localization difficulties as with two-loudspeaker stereo, but over only half as much stage width. The result is that over a wide listening area dialogue can be maintained accurately

hanced center directionality with a mild gain-riding action. This boosted the center loudspeaker 2.5 dB and cut the left and right loudspeakers 2.5 dB in response to strongly centered signals (such as dialogue) [19]. This has since been replaced by the more elaborate directional enhancement used today [20], which also provides a decoded surround output.

centered and sound effects can be positioned across the entire stage width with reasonably accurate localization. Less strongly directional or more complex sounds, such as orchestral music, which are not directionally enhanced exhibit more localization variation with listener position, but still show significant improvement over two-loudspeaker playback. The addition of the center loudspeaker does narrow the stage width for a centered listener by roughly 25% for the nondirectionally enhanced sound. If desired, this may be compensated for by slightly wider loudspeaker spacing without losing the benefits of the center loudspeaker. Also, in the full surround sound system the added ambience recovered in the surround loudspeakers (discussed in the next section) subjectively offsets the slight narrowing of the front stage, The home video viewer/listener who places two loudspeakers immediately on each side of a 23-in (0.58m) monitor has no need of a center loudspeaker, but neither does he enjoy a significant stereophonic effect, When the loudspeaker spacing is widened to obtain a sound stage comparable in width to accustomed stereo listening, the center loudspeaker becomes indispensable for off-center listeners,

2 THE SURROUND

1.3 The Derived Center at the Theater The importance of the center loudspeaker was not forgotten in the evolution of the movie industry's twotrack format. A recommendation for a derived center channel was included in an early demonstrated system of recording such tracks optically [14]. When a similar system was made practical [ 15] through the use of noise reduction [16], the importance and benefits of a derived center channel were soon recognized [17]. Fig. 2, taken from Uhlig [ 17], shows the calculated shift of a central image between loudspeakers spaced 40 ft (12 m) apart for listeners 60 ft (18 m) away as they move off the centerline. The apparent positions were calculated using data from previous studies [2], [18]. The dashed line represents the image shift for left and right loudspeakers only, and the solid line the considerably lessened shift with a derived center loudspeaker at an unspecified relative level. The author claimed subjective confirmation for these curves except for an even greater image shift for the two-loudspeaker case due to loudspeaker directivity. He also reported that for listeners in the center half of the theater the derived center loudspeaker gave results "almost as good as a system with three discrete channels" [17]. This work eventually resulted in the first commercial stereo optical format. The three-loudspeaker signal decoding was soon embellished by a circuit which en538

LOUDSPEAKERS

2.1 Discrete Surround at the Theater The earliest use of surround (off-screen) loudspeakers in the movie theater was the Fantasound system, used only for the Walt Disney film Fantasia in 1940 and 1941. The three sound tracks could be switched from behind-the-screen loudspeakers to side or overhead loudspeakers [1]. Cinerama reserved two of its seven tracks for off-screen loudspeakers in various parts of the theater [5]. Cinemascope and Todd A-O both used a single track for surround information [ 1], as does the present 70-mm format, with only occasional use of a "split" (two-channel) surround [5]. The surround loudspeakers are generally used for all-around environmental or ambience effects, with similar information in the front loudspeakers. To avoid localization to more closely positioned surround loudspeakers, the surround track is delayed 11/2 picture frames (60 ms) relative to the front tracks [5]. Specific surround-directed sound effects and even, rarely, dialogue are also recorded on the surround track. In particular if these are important to the movie, they are also mixed at a lower level in the front to guard against the unpredictable nature of surround reproduction in theaters [5]. 2.2 Ambience Extraction at Home In the surround loudspeakers' role of creating a nondirectional ambience, an obvious analogy may be drawn to various ambience extraction techniques suggested for home playback of two-channel stereo recordings. Madsen [21], expanding on ideas of using delayed signals for ambience reconstruction which he credited originally to Lauridsen [22], placed loudspeakers to the sides of the listening area and fed them delayed front loudspeaker information. The sound from the side ambience loudspeakers was delayed sufficiently so as to arrive at the listeners' ears at least 2.5 ms later than the front loudspeaker sound, taking into account the propagation delay of sound in air of 0.88 ms/ft. This was to enable the Haas effect to aid instrument localization in the front and to reduce coherence between the front and side loudspeaker sounds. Madsen reported good subjective results in simulating hall ambience, comparing favorably to some four-channel recordings of the time (about 1970), which used two separate channels for ambience information. Hafler's method [23] recognized that the difference of the two stereo channels generally contains a higher proportion of randomly phased, reverberant ambience information than does either of the front channels or J. Audio Eng. Soc., Vol. 35, No. 7/8, 1987 July/August


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their sum and can make an appropriate rear loudspeaker feed. This signal will be designated S' (for surround) and defined as S' = 0.707(LT -- RT) ·

(2)

The factor of 0.707 is included for mathematical symmetry. The level of the surrounds relative to the front level is somewhat at the listener's discretion, but for the system to be described in this paper it is typically about 3 dB higher than indicated in Eq. (2). Hailer recommended placing the rear loudspeaker as far behind the listeners as possible to obtain a delay effect similar to that suggested in Madsen [21]. Hailer also recommended a derived center similar to that given by Eq. (1), yielding a system with similarities to the one under discussion and to many matrix quadraphonic systems of the 1970s. 2.3 The Derived Surround at the Theater When used with the two-track surround sound format, the movie theater's rear loudspeaker array receives the basic signal of Eq. (2). This can be very effective in producing environmental and ambience effects, particularly when a large LT--RT component is introduced in the encoded mix. Before feeding the loudspeaker array, S' is delayed by 30-100 ms, dependent on the size and geometry of the theater, to aid front localization for non-surround-directed sounds over the entire seating area [20]. The delayed signal is low-pass filtered at 7 kHz be-

fore undergoing a mild spectrum- and level-dependent downward treble shelving [24] of about 5-6 dB maximum (modified Dolby B-type noise reduction decoding). This response tailoring serves the dual purpose of quieting delay line and optical sound track noise, and reducing sibilant bleedthrough of center-encoded dialogue due to relative phase and amplitude errors in LT and RT. These are most likely to occur from slight adjustment and positioning errors of the two recording light valves and the two playback photocells. The low frequencies are rolled off below 100 Hz for the protection of the theater surround loudspeakers, which of necessity are much smaller than the front loudspeakers. During encoding, prior to being mixed into LT and RT, the surround signal is also bandpass filtered at 100 Hz and 7 kHz before undergoing an approximately eomplementary spectrum- and level-dependent upward treble shelving. Specific rearward directed sound may also be encoded for the surround loudspeakers in addition to ambience effects, but to localize these sounds unambiguously rearward requires directional enhancement. The unmodified characteristics of the decoding matrix provide for complete separation of center encoded sounds such as dialogue from the surrounds, but other front sound effects require directional enhancement to prevent confusing surround loudspeaker output. For such sounds, the surround time delay alone is insufficient for listeners close to surround loudspeakers. 3 MATRIXING

AND PANNING

3.1 The Decoding _

_

_

"_

I Ji i~ ·'

® (&)

'

_ '(_ Ts['\ ,o, ,'/"°i'/ C)

speaker signals, ment,as

2O20 _. 15 J

10

$

0

R'

J5

I tO

I 15

LEF3'

[]

defined, prior to directional

enhance-

= LT

(3)

RT

(4)

=

S' = RT) · C' = 0.707(LT 0.707(LT -+ RT)

RIGI-_

_S_^KE.

_(_®,,_":_'_¢A-'_J _ Fig. 2. Apparent audio image position for various listener positions. Dashed line--two channels; solid line--two channels with derived center channel [17, Fig. 5]. J. Audio Eng. Soc., Vol. 35, No. 7/8, 1987 July/August

so far decodes four loud-

(2) (1)

20

_*_,a ,or_o_AGE POS_ION (FEET}

_,KE._

CONSIDERATIONS

Matrix

The system as described (_) 2o (_),,

SOUND FOR HOME VIDEO

Combined with complementary encoding and panning around a full 360 ° listening circle, this also essentially defines Scheiber's first "diamond array" two-channel quadraphonic matrix [25]. The most obvious departures from the earlier system are the movement of the side left and right loudspeakers to the front left and right positions, the rear surround loudspeaker time delay, and the spreading of the surround information about the rear half of the theater. This also, of course, departs from the "standard" quadraphonic layout of left-front, right-front, left-back, and right-back loudspeakers. The time delay between front and surround loudspeakers prevents any possibility of forming stable side phantom images. However, even with "ideally" panned signals between a left-front and a left-back loudspeaker, 539


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for example, controllable phantom side images cannot be formed (see [26] among others), even for an ideally positioned listener. If sufficiently elaborate means could be employed to form side images for a small seating area, this would still not represent the best use of the two channels of information available. Motional effects between the front loudspeakers and the surrounds, such as "fly-by's," can be effective throughout the theater though. These require "interior" pans through the theater, as do environmental sounds, which come approximately equally from all the loudspeakers. It is these interior pans which necessitate quadrature phase shifts in the encoder. A further examination of the interaction of the encoding and decoding matrices will be aided by the introduction of a "flattened Scheiber sphere" geometric representation of two-channel phase-amplitude encoding. 3.2 The Spherical Encoding AS pointed out by Scheiber the relative

amplitude

Model [27] and Gerzon

ILTI/IRTI and phase

[28],

Amplitude-Only Encoding

Relative amplitude-only reversals, is represented radius in the X- Y plane, 0 counterclockwise from point A is given by

0 = 2 tan-]

tan qJ0 LTT+ (8)

[ (0_] -- tan- l {_tan qJ0tan k/J

where t_0 is the angle away from the centerline of the left or right loudspeaker. This gives the direction of the total velocity vector from the left and right loudspeaker wave fronts (not including the center loudspeaker) and is in agreement with the low-frequency (<700 Hz) localization theories for the central listener of Leakey

encoding, including polarity by points on a circle of unity as shown in Fig. 3. The angle the positive X axis determining

[29], Makita [30], and Gerzon [31]. t_ will

/L--\

vary roughly proportionally with 0, except for very wide loudspeaker placements. The exact proportionality at qJ0 -- 45 ° corresponds to the directional encodings obtained with the crossed coincident bidirectional microphone technique. The addition of the center loudspeaker narrows the soundstage somewhat from that determined by the val-

_RTT) -- 90°

ues given by Eq.and(8). full stageto width can be restored the However, localizationsthe returned those (5)

\

Lw LT -+ RT) RT '

= 2tan -1

t_ = tan -]

(bLT -

(bRT = (l)of Li and Ri comprise the total information available to encode directionality in two channels. Every combination of these can be uniquely represented by a point on a sphere of unity radius, and every point on the sphere represents one such coding. This was termed the energy sphere by Gerzon [28] and has also been called the Scheiber sphere from its usage by Scheiber [27]. The axis orientation and use of (1)will be as in Gerzon [28], but the use of 0 will not. 3.2.1

These are represented by opposite points on the X axis. Since no relative phase shifts are introduced in the encoder between its L, C, and R inputs, then signals which are panned (pairwise mixed) between L and C, R and C, and L and R are represented by points on the "in-phase" half of the circle between these encoder points. This semicircle of points, shown heavier in Fig. 3, is the pairwise pan locus (as defined by Gerzon [28]) for front signals. It does not differ from an "optimal" or "ideal" locus, as is the case in other quadraphonic phase-amplitude matrices in at least some quadrants. A point on the front locus can be assigned an image localization angle ql counterclockwise from front center for a centrally positioned listener most conveniently according to the stereophonic law of tangents,

Point A's X and Y coordinates

of Eq. (8) for specific sources with the use of. appropriate directional Theofleft localizations will be more enhancement accurate than circuitry. with the use and right

are x ¢

X A =

cos

YA ---- sin0

(6)

0

.

(7)

Also shown are the four primary encoding/decoding points for the matrix under discussion, although these may be different for other matrices. LT only and RT only are encodings intended for the left and right loudspeaker positions, respectively, and are represented by opposite circle points on the Y axis. Equal in-phase Lw and RT encoding is intended for the center position, and equal-magnitude opposite relative polarity LT and RT encoding is intended for the surround loudspeakers. 540

A

e

L

Fig. 3. Amplitude-only encoding circle. 0 = 2 tan-_(LT/RT) - 90 °. d. Audio End, Soc., Vol. 35, No. 7/8, 1987 July/August


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SURROUND SOUND FOR HOME VIDEO

loudspeakers only, due to the closer spacing of the image-producing loudspeakers, The magnitude of the response of each of the decoded outputs to an encoded direction represented by point A is

IAI = cos (?)

(9)

where A0 is the angular difference between the encoding and decoding directions. Each decoded output (before directionalenhancement)containsnot onlysignalsfrom its intended direction but also signals from each adjacent loudspeaker's direction attenuated only 3 dB. Complete isolation is obtained only from signals encoded at the point that is diametrically opposed to the decoding point. A signal encoded at left center, 0 = 45 째, for example, appears at equal levels in L' and C' and at equal levels, 7.7 dB lower, in R' and S'. These relationships can, of course, be deduced from Eqs. (1)-(4) and their complementary encoding equations, but the graphic representation gives a more intuitive understanding. Mono reproduction is equivalent to decoding making the relative reproduced level

(0)

}AMi =

COS _

.

at C',

(10)

This can be stated equivalently as the relative power level being proportional to the distance from S' of the X coordinate of point A; _ iAMI2

X^ 2+ 1

(11)

Played back in mono, then, center-encoded signals are boosted 3 dB relative to left- and right-encoded ones, as with conventional stereo recording. Signals mixed only to surround, being pure difference information, do not appear in mono reproduction, a characteristic in common with the center-back direction of most

this, the representational model will be expanded to include the relative LT, RT phase shift qb. 3.2.2 Phase-Ampfitude Encoding 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 aangle sphere, A' is found from 0', the surface of qb aspoint shown. Tofirst locate point B on O' = 2 tan-l[ILz[h _IRTI/

symmetrical panning through the "interior" of the theater. To examine how encoder phase shifts allow J. Audio Eng. Soc., Vol. 35, No. 7/8, 1987 July/August

90 째 (5a)

i[[LTI -- [R_ = 2 tan- _l_-_x [ + [RT[J ' Then the plane of the circle is rotated by the angle qb to move A' to B. LT leading RT is represented by points in the upper hemisphere (intersecting the positive Z axis) and LT lagging RT by points in the lower hemisphere. Such a representation returns many useful insights as discussed in Scheiber [27] and Gerzon [28]. For instance, Eq. (9) 'remains valid if A0 is reinterpreted as a spherical angle. An encoding where (ILTI/IRT[) = 1 and qb = -90 째, for example, determines a point at the bottom of the sphere forming equal 90 째 angles with all four decoding Points, resulting in equal output from L', C', R', and S'. Eq. (11) also holds if X8 is substituted for XA. 3.3 Flattening the Sphere While useful, the sphere is sometimes difficult to represent and visualize in two dimensions.Two-dimensional projections have been introduced [32], [33], which clarify some encoding characteristics at the expense of others. The two-dimensional representation used here is simply a vertical projection as if looking downward on the sphere. This is obtained by setting the Z coordinate of point B, Za = 0, resulting in the projection of B to B', as shown in Fig. 4. As justification, 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 practice, critical sounds are mixed at least slightly forward of the surround direction, which can still result in good rearward localization with the aid of directional enhancement. If no relative phase shifts were introduced in the encoder between the front directions and the surrounds, then the pan loci for front-to-surround pans, including center-to-surround, would be confined to points on the circle of Fig. 3. Panning from center to surround would be confined to traversing the circle through either the left or the right points. There are no points on the circle which result in approximately equal output from all four decoded outputs and which, therefore, would allow

l

Z

B ,,--

Fig. 4. Forming the phase-amplitude sphere and projecting it to the X-Y plane (after [28, Fig. 2]). 541


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the X-Y plane and remain unmoved, and that only the lower hemisphere of phase encoding is used in the system under discussion, so no information is lost in this instance. The specification of point B' is shown also in the projected view of Fig. 5. Throughout the phase rotation and vertical projection, the Y coordinate remains, YB' = sin 0' .

-

(12)

3.4 The Encoding Locus Fig. 6 is a simplified block diagram of the Dolby Stereo/Dolby Surround matrix encoder. Not shown are

90°

and

ILTI 2 - IRTI 2 Yv = ILTI2 + [Rx12 (13) IL, ]2 _ [R'I2 [m,[2 + Ig, I2'

The X coordinate

(16)

(7a)

= cos [.2 tan _i[IRT['_] [[_)

-

IR'I 2 ---[c'l2 + Is'l2 .

IL'[2 +

The point (X, Y) = (0, 0) represents ILTI/IRTI -'-1, 4) = _+90°, and gives Im'l = Ic'l= Ig'l= Is'l.Points farther from the center are decoded less equally among the outputs with points on the perimeter representing the strongestdirectionalencoding.Eq. (11) stillholds with XB, substituted for XA.

Also,

YB' = sin 2ta

phase shifts. Relative values ofL', C', R', S', LT, and RT may be calculated from XB, and YB' with the added relationship.

described Sec. frequency 2 and frontresponse signal processing path phase-comthe surroundin input blocks pensating networks which match the phase shifts which the processingintroducesin the surroundpath. The desired 90 ° phase shift of surround relative to front can only be introduced by the use of all-pass networks in both the front and the surround paths with 90 ° relative phase shift across the audio frequency range [34], as shown. The unfortunate frequency-dependent phase shift introduced by the all-pass networks, and any consequent

is given by

degradation controversy

XB, = cos 0' cos qb .

(6a)

Also by analogy to Eqs. (12) and (13),

of sound quality, has been the subject of since such networks were introduced to

the technology of quadraphonics. At present, a probable consensus may be drawn from a recent study [35] on the audibility of midrange phase distortions, particularly

900] A'

[

= cos 2 tan

i_ I

= c°s[2tan-l(LL;-

(14) y_

RTIJJ Rx"_l+

and

lc'12- Is'12 x_,- ic,r + is'l2

Fig. 5. Specification of phase-amplitude encoding point B' in the X- Y plane.

(15) ILT + RTl2 -- ILT -- RTl2 = ILT + RTl2 + ILT-- RTl2 ' Thus phase-amplitude encoding and RT is represented by points on or within of the LT circle of Fig. 5, and lagging phase relationships of the same amount are representedidentically. Points on the perimeter of with the only loss amplitude of information being thatincluding leading the circle represent encoding only, polarity inversions, and points in the interior of the circle represent encodings with other than 0 ° or 180 ° 542

L'4

/

__

') .707

[--7-3---------,_O I_

.707_

=

LT

-'--"T c_ "a.o

s----_ ¢_

+ '?_L*O

Fig. 6. Simplified encoder block diagram. d.AudioEng.Soc.,Vol.35,No.7/8,1987July/August

'


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SURROUND SOUND FORHOME VIDEO

with reference to those resulting from loudspeaker crossovers (which are not dissimilar to those from the all-pass networks). Such distortions are clearly audible with appropriate test signals auditioned on headphones or anechoically, but with normal music over loudspeakers in a typical room, they are either inaudible or extremely subtle in their effect. It is also possible, if desired, to bypass the phase-shifting circuitry by feeding signals which are not to be panned between front and surround directly to LT and RT. Ignoring the phase shift common to all inputs, the encoder is characterized by the encoding equations LT = L + 0.707C

- j0.707S

(17)

RT ----R + 0.707C + j0.707S

(18)

where the j coefficient is used to denote an idealized frequency-independent 90 째 phase shift. The resulting front pan locus and the surround point are as shown in Fig. 3. However, following the guidelines of Gerzon [28] for determining pairwise pan loci in the spherical model, the pairwise pan locus from any front direction to the surround point will depart from the front semicircle at a right angle, following a vertical semicircular path on the lower half of the sphere of Fig. 4 on its way to the surround point. This projects onto the XYplane as the chord connecting the two perimeter points, A dual pan-pot arrangement such as shown in Fig. 7 is capable of reaching all encodable positions of the matrix. Fig. 8 shows several representative front-to-

FRONT-S

PAN

FRONT PAN

TO ENCODER

I._' s )

'"'"'"_V_2_/L? {FRON_ --xAAP _

Fig. 7. Dual pan-pot arrangement capable of reaching all encodable positions,

surround pans obtained by sweeping the first (front-S) pan pot in Fig. 7 through its range with fixed settings of the second (front) pan pot. Fully symmetrical panning capability through the interior of the theater is obtained at the inconsequential loss of L to S and R to S perimeter panning capability. An interesting similarity exists between Fig. 8 and Gerzon's Fig. 11 [28], which is used to help explain his "two-loudspeaker stereo localization theorem." According to low-frequency (<700 Hz) theories of sound localization such as those of Leakey [29], Makita [30], and Gerzon [31], the front-to-surround pan loci shown also represent loci of constant two-loudspeaker playback localization for a centrally positioned listener. For a sound panned from a front point to the surround point, the two-loudspeaker listener hears approximately constant localization with increasing "phasiness" and image vagueness. 3.5 Comparison with Other Matrices The graphic representation can also be used to aid comparison with other matrices. Fig. 9 shows the pairwise pan locus of the QS [36] matrix between adjacent loudspeaker positions. Its four primary encoding/decoding points correspond to the conventional quadraphonic loudspeaker layout and are located at 0 = _+45 째, _+135 째 in the X-Y plane. Again, the back points are given a 90 째 phase relationship to the front points, but with qbpositive instead of negative. This leads to vertical semicircular front-to-back pan loci through the upper hemisphere, projecting to the chords of Fig. 9. With the addition of a center-front loudspeaker at the usual decoding point, Fig. 9 represents a plausible theater system with interior pan capability, including left-back to right-back separation. However, left-front to right-front separation is also only 3 dB, severely limiting the front soundstage width in the absence of directional enhancement. Also, separation from center front to either left back or right back is only 8.3 dB, resulting back-loudspeaker the center inofsevere the circle, and a perhapsdialogue useful leakage. 3 dB of As noted in Willcocks [33], full front-stage width may be restored by introducing -7.7 dB of oppositepolarity crosstalk between the QS-encoded LT and RT,

x

Y

x

y

L

Fig. 8. Representative pan loci for encoder of Fig. 6 with pan pots of Fig. 7 J. Audio Eng. Soc., Vol. 35, No. 7/8, 1987 July/August

Fig. 9. Pairwise pan locus of QS matrix between adjacent loudspeaker positions. 543


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giving the diagram of Fig. 10. (Compare Figs. 9 and 10 to the spherical representations in Figs. 13 and 14 of [33].) Left back and right back move to 0 = _160.5 째, reducing their separation to a negligible 0.5 dB. Centerfront to left-back and right-back separation improves to 15 dB. Complete center-to-back separation is achieved if left back and right back are combined to a single point at 0 = 180 째, as in Fig. 8. The optimal or "position"-encoded pan locus for the SQ matrix [37] is shown in Fig. 11. (Note that the side pans shown projected on the Y axis are not obtained by pairwise mixlng.) The front pan locus is the same as Fig. 8, as is the center-back point. The center-front and center-back positions are not normally used for loudspeaker-feed decoding. The left-back and rightback points represent [LTI/IRTI -- i and qb = 90 째 and -90 째, respectively, placing them at the top and bottom of the spherical representation. Interior sounds intended in the theater system to come approximately equally from all loudspeakers would, if played back through the decoding matrix implied by Fig. 11, appear most strongly in the right back and most weakly in the left back. Center-front dialogue and center-back sounds (encoded identically by the matrices of both Figs. 8 and 11) decode equally in all four loudspeaker points of Fig. 11, leading to diffuse localization. This could be improved with the addition of center-front and center-back loudspeakers, but the mere 3-dB separation of left-back and right-back from x

.F

LF

Fig. 10. Pairwise pan locus of QS matrix after -7.7-dB opposite-polarity blending of Lt and R-r. x

center-front dialogue is still a heavy handicap for subsequent directional enhancement to overcome. The difficulties of applying other matrices to film/video surround sound should not be surprising since this was not their original intended use. Both Scheiber [27] and Gerzon [38] have discussed the possibility of using the third axis of the Scheiber sphere (in this case the Z axis of Fig. 4) to encode a degree of height information. In the matrix of Fig. 8, as defined by encoding Eqs. (17) and (18), the lower hemispherepointsarealreadyusedforinteriorpanning, but the upper hemisphere remains undefined. It is plausible that the top of the sphere (ILTI/IgTI = 1, qb= 90o) could be designated as overhead or ceiling, but the difficulty of its being separated by only 3 dB from the perimeter directions, including center dialogue, would have to be overcome. 3.6 Representation of Average Directionality Gerzon [28] introduces the idea of a point interior to the sphere representing the average directionality of a multiplicity of simultaneous, independent signals. That point is given as the center of "mass" of all the individual signal points weighted by the relative energy levels of their respective signals. The orientation of that point with respect to the center of the sphere "represents essentially all the information" [28] that is available to control directional enhancement circuitry. These points inside the sphere can be projected onto the X-Y plane as the surface points were by setting Z -- 0. The X and Y coordinates are still given by Eqs. (12)-(15), and Eq. (16) is still valid if the IIsigns are reinterpreted as rms averages. Thus a point in the interior of the circle can have either of two related meanings. It can represent the directionality of an individual encoded signal, or the average directionality of a complex sound field made up of a plurality of independent sources. In this second context, the point will move about the interior of the circle dependent on the nature of the sound encoded in LT and RT and the averaging time considered. The point will generally spend more time in the front half of the circle than in the rear half, indicating more sum than difference information. It will approach the perimeter only in response strongly predominant directional sound.

to a

4 DIRECTIONALENHANCEMENT

(CF)

4.1 The Need and the Limitations

LF

y _

R8

.F

(em Fig. 11. Pan locus of"position" encoded SQ matrix, 544

The basic matrix of Fig. 8 as described provides complete center-to-surround isolation, symmetrical interior panning, a wide front stage, and appropriate ambience extraction from stereophonic music. With itself capable of stable image localizations over a wide listening area. By Eq. (9), center dialogue appears attenuated only 3 dB in the adjacent left and right loudjust two channels of information, however, it is not by speakers, allowing sideways image shifts for off-center listeners (although not as much as without the center loudspeaker). Far left and right sound effects appear J. Audio

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at an approximately equal level in the surround loudspeakers, considering the typical 3-dB surround level playback advantage. Surround sound effect localization difficulty caused by leakage to the left and right loudspeakers is exacerbated by the surround time delay, Ideally, sounds directed at each of the loudspeaker directions should appear only at their intended loudspeaker outputs. Signals panned between L and C, and between R and C, should appear only at those pairs of loudspeaker outputs, and at the appropriate relative levels. The ideal directional enhancement circuit would transform the basic decoded outputs L', C', R', and S' into enhanced outputs L", C", R", and S" with these characteristics for all encoded directions perfectly and simultaneously. This, of course, is impossible, A well-designed directional enhancement circuit can provide such a transformation for only one, or at most two, diametrically opposite (on the circle) directions at a time. In addition, the average sound field directionality point defined in the previous section, which is used to control the enhancement, can only indicate one direction at any given instant. These considerations establish firm limits on what enhancement circuitry can accomplish. Within these limits, however, careful design monitored by careful listening can result in circuitry enabling excellent directional acuity for predominant sounds over a wide listening area, with minimal audible side effects. An important part of achieving this is to design the enhancement circuitry to limit its action essentially to strongly predominant directional sounds such as sound effects and dialogue, and not overreact to complex sound fields which cannot be properly enhanced, It is also important that the overall subjective result obtained with such circuitry not differ substantially from that obtained with present theater system enhancement circuitry. Monitoring is normally done through this system when the movie sound track is originally mixed to confirm the result obtained after processing through the matrix and enhancement circuitry [1]. The front soundstage of the theater matrix is identical to conventional stereo, so the directional enhancement circuitry will act similarly on such material. This may or may not give desirable results if the material was not intended for such playback. For example, a close multimiked string orchestra may have an overexuberant violinist who leans into his microphone and becomes momentarily directionally predominant, causing the reproduced sound field to momentarily focus unnaturally on him. In general, the best results are obtained when the enhancement circuitry is used with material mixed for it, such as surround sound movie sound tracks. For such material, decidedly inferior results are obtained when the enhancement is not employed. The enhancement circuitry should find useful application, however, with upcoming stereo television productions in maintaining dialogue solidly on screen, panned effects accuratelypositioned, and even enhancing surround effects which may ultimately be specifically encoded, O.Audio Eng. Soc., Vol. 35, No. 7/8, 1987 July/August

SURROUND SOUND FOR HOME VIDEO

4.2 Circuit Considerations The means used to effect the directional enhancement have evolved in sophistication since the early days of quadraphonics. The earliest circuits concentrated on making a dominant sound at a loudspeaker position more "discrete" by attenuating the neighboring loudspeakers and slightly boosting the desired loudspeaker and its diagonal opposite to compensate the overall power level [25]. The loudspeaker level changes could be quite audible, particularly for an off-center listener, so subsequent developments have tended toward various cross-coupling and leakage cancellation schemes. These all effectively vary the decoding matrix dynamically to cancel unwanted components of a predominant directional signal from undesired loudspeaker outputs. The later versions of these perform at least approximate directional enhancement for all encoded directions, not just the loudspeaker locations (see [ 13], for example). Various circuit topologies may be employed to obtain the appropriate matrix modifications, but their results must, of necessity, be similar. For a decoded loudspeaker output not to contain a given encoded direction it must, by Eq. (9), decode at the diametrically opposite point on the circle of Fig. 8. When the left direction is enhanced, for example, the other three loudspeaker outputs must decode temporarily at the diametrically opposite right position (perhaps with differing signal polarities) if they are to produce output that does not contain L. This is not necessarily as audibly disconcerting as might be imagined since the shift of the reproduced sound field should normally be masked by the onset of the strongly predominant directional sound that triggered it. It is evident, then, that the audible differences among various directional enhancement circuits will not be due so much to the matrix modification topology chosen as to the directional sensing characteristics, time constants, matrix modifier control signal processing, and degree of enhancement. 4.3 Power Compensation A level boost is normally given to the enhanced direction's output or outputs to maintain constant total power for the predominant sound when its unwanted leakage is canceled. For a primary direction such as center, a boost of 3 dB is appropriate if the 3-dB down leakages at left and right are canceled. For the L to C and R to C pan positions, the appropriate power cornpensation is not so obvious. There are two possibilities based on discrete L-C-R panning or the nearly equivalent L-R panning. Fig. 12(a) shows the relative levels of LT and RT for the front pan locus when panned by a standard sinecosine pan pot applied between the L and R inputs of the encoder of Fig. 6. When decoded through either two (left and right) or four loudspeakers, the total power level shown in Fig. 12(b) remains uniform with the pan position (before the application of directional enhancement). Fig. 13(a) shows two sine-cosine pans between L and C, and between R and C, as would be 545


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done in the discrete four-channel format. Fig. 13(b) shows that, reproduced over three loudspeakers fed discretely, the total power remains constant. Fig. 14(a) shows the results in LT and RT if L, C, and R of Fig. 13(a) are fed to the inputs of the encoder of Fig. 6, as would be the case when encoding from a four-channel master. In Fig. 14(b) the total power, when reproduced over two or four loudspeakers, exhibits a 2.3-dB peak at left center and right center as compared to the L, C, and R positions. [The amplitude curve shown is given by (1 + Isin(20)l/V_)_.] The angular locations resuiting from this panning are still correct, The directional enhancement circuitry may equalize the panning level for enhanced sounds for either the curve in Fig. 12(b) or that in Fig. 14(b), but not both. Equalizing for Fig. 12(b) has the advantage that the power level for an enhanced direction remains the same after enhancement as before, possibly minimizing "pumping" effects. However, left to right sound effect pans which were smooth in the discrete format take on the uneven character of Fig. 14(b). In the home decoder discussed in the next section, the curve of Fig. 14(b) is compensated, giving left-toright sound effect pans as smooth as the original discrete pan. This means that sound effects panned according to Fig. 12(a) exhibit 2.3-dB level drops at left center and right center, if enhanced. There is also a 2.3-dB drop in the overall power level of a left-center or rightcenter sound when it is enhanced as compared to before enhancement. In practice, the directional enhancement

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in Sec. 2.2), which do not receive directional enhancement. The delay adjustment range (in this case, 1636 ms) enables adaptation to differing seating and loudspeaker layouts and personal taste. 5.1 The Center Loudspeaker Revisited The processor will often be incorporated into an existing two-loudspeaker stereo system. For cost reasons, the center loudspeaker may not be incorporated initially. While its desirability has been thoroughly justified, acceptable results can be obtained without it for centrally positioned listeners with narrow left-right loudspeaker spacing. A switchable option must be provided to vary the characteristics of the directional enhancement circuitry so that center sounds are not enhanced to a nonexistent loudspeaker. The enhancement circuitry is still essential in separating off-center front sound effects from the surround loudspeakers and surround-directed sound effects from the front loudspeakers. It will also be found that for reasons of cost, acsthetics, or practicality, the center loudspeaker may not match the left and right loudspeakers. To the extent that the side and center loudspeakers do not match in amplitude and phase response, image stability will suffer for other than enhanced loudspeaker-directed sounds. The results should be acceptable, though, if the amplitude responses are in general agreement and the phase responses are matching at least up through the midfrequencies.

is applied sufficiently quickly that the preenhancement levelis not discerned.Theenhancement powereompensation curve chosen represents a compromise in favor of fidelity to sound effects as encoded from a discrete master.

OdB----.-_ _3d8_ ___ LT

5 THE HOME ENVIRONMENT

Fig. 12. Front sine-cosine panning from left to right. (a) LT and RT relative levels. (b) Total relative power over two or

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

four loudspeakers.

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

....

stage width. The benefitof a center loudspeaker with directional enhancement in keeping dialogue on screen and center screen is even more apparent for these viewers.

L C R L C R Fig. 13. Front sine-cosine panning between discrete front channels. (a) L, C, and R relative levels. (b) Total relative poweroverthreeloudspeakers.

L

_-

OdB TOTAL POWER

C

R

L

C

R

TOTAL POWER

Adapting the theater system to the home does suggest some special considerations. These have been taken soundaccount processor (Fig.of a15). unitvideo incorporates into in the[39] design newThe home surround directional enhancement circuitry [40] capable of accurately enhancing strongly predominant sounds from any encoded perimeter direction, and an adjustable wide-dynamic-rangedigital delay [41] for processing the surround loudspeaker signals. The surround loudspeaker time delay is needed to aid forward localization of less strongly predominant front'sounds (as discussed 546

ods _ f -3dB.... --_T--//__

/

L

C

TOTAL POWER

,

'x ,

L

C

a

Fig. 14. Front sine-cosine panning between L, C, and R encoded into LT and RT. (a) LT and RT relative levels. (b) Total relative power over two or four loudspeakers. J. Audio Eng. Soc., Vol. 35, No. 7/8, 1987 July/August


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SURROUND SOUND FOR HOME VIDEO

If the center loudspeaker is smaller than the side loudspeakers, it will probably have less bass response. Since bass frequencies are often mixed to the center position, activation of center enhancement (in response to dialogue, for example) could remove bass "leakage" from the side loudspeakers and direct the energy to the center loudspeaker with weak bass response. This could cause a drop in bass level and audible bass modulation, To avoid this problem, center-directed low bass frequencies should not receive directional enhancement, This does not introduce localization problems since the low bass frequencies do not contribute significantly to perceived directionality in the home environment in the presence of other, stronger directional cues.

:

5.2 Subwoofers This property of the low bass frequencies suggests the use of a common subwoofer, allowing smaller loudspeakers to be used for left and right also. The processor of Fig. 15 includes a subwoofer output with a fixed second-order Butterworth low-pass characteristic at 80 Hz. This balances well with many small, sealed loudspeaker systems with second-order rolloffs below about 65-100 Hz. The low-bass power-handling requirement of the main loudspeakers may be reduced in a more elaborate system by the use of additional high- and low-pass filtering to create higher order crossover networks, The level of the subwoofer required to balance the main front loudspeakers depends on whether one, two, or three front loudspeakers are operating, and should shift with changes in the operating mode (mono, stereo, or surround). When listening to movie sound tracks, though, some listeners will find it appropriate to use the subwoofer not as a "high-fidelity" bass extension, but rather as a visceral special effect. 5.3 The Surround

Loudspeakers

spectral

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occurring

hall ambience.

5.4 Other Useful Features In addition to video surround sound processing circuitry, other features enhance such a unit's usefulness. Switching for conventional mono and stereo playback is, of course, necessary. With the requisite loudspeakers and amplifiers already in place, it is logical to provide surround sound synthesis capability from mono and conventional stereo signals, which still make up the bulk of available source material. Remote volume control is a worthwhile convenience, as is remote surround balance control to adapt to differing recorded mixes and individual taste. Left-toright output balance adjustment is not needed or appropriate once the system has been set up properly for the given listening area. A test disk or tape is helpful in performing this setup. An input balance control should be adjusted to a given source to balance incoming LT and RT levels for optimum directional decoding. An input level control and level indicator is needed to approximately calibrate the signal level through the decoding circuitry. Finally, a visual sound-direction display (at the right end of the panel in Fig. 15) is an aid in system setup and in confirming proper system operation. While different approaches may be taken in the development of a home processing unit, the end result should be a system that combines all the important elements needed for an accurate reproduction of cinema surround sound. In so doing, a standard is created for the future in providing not just a vague all-around sound, but the specific, accurate effect intended by the moviemakers. 6 SUMMARY

Revisited

In the theater numerous small surround loudspeakers are used to diffuse the rear sounds and provide even coverage. In the home, two surround loudspeakers are more practical. In the processor of Fig. 15 an imagespreading technique is employed to diffuse the rear image and discourage localization at the closer surround loudspeaker. This does not reduce the ability of the directional enhancement circuitry to direct intended sounds solidly rearward, The surround loudspeakers need not have extended bass response, since they receive signals intended in the theater system for loudspeakers with limited bass response. Also, as noted in Sec. 2, the surround encoding and decoding treble response is limited to 7 kHz, and some mild noise reduction is employed. Both are done to overcome practical limitations of the optical sound track-based theater system. It may be argued that these limitations do not necessarily apply to the emerging home video formats, and some extension of surround treble response for these formats may be justified in the future. Some degree of treble rolloff is often found to be appropriate in any case to match the

balance of naturally

Sound tracks encoded

with the surround

sound of

the movie theater are now available for home audio/ video playback. The directional information is encoded by a phase-amplitude matrix method similar to fourloudspeaker quadraphony, but with its roots in wellgrounded three-loudspeaker stereo and ambience extraction techniques. A new two-dimensional representation of two-channel phase-amplitude encoding aids in understanding and analyzing this and comparable matrices. The surround information can be accurately recovered for home reproduction with a processor incorporating a wide-dynamic-range delay for the surround loudspeakers and directional enhancement circuitry to stabilize the localizations of strongly predominant sounds over a wide listening area.

_

Fig. 15. Home video surround sound processor. 547


JULSTROM

7 ACKNOWLEDGMENT Of many people associated with the home surround sound project, the author wishes in particular to acknowledge the contributions of William Bevan, Mark Gilbert, Paul Jenrick, and Robert Schulein, and their dedication to the concept of home video surround sound, An additional note of thanks goes to Dolby Laboratories for their helpful input. 8 REFERENCES

PAPERS

Noise Reduction System," SMPTE J., vol. 84, p. 720 (1975 Sept.). [17] R. Uhlig, "Two- and Three-Channel Stereophonic Photographic Soundtracks for Theaters and Television," $MPTE J., vol. 83, p. 729 (1974 Sept.). [18] H. F. Olson, "Stereophonic Sound Reproduction in the Home," J. Audio Eng. Soc., vol. 6, p. 80 (1958 Apr.). [19] D. Robinson, "CP100 Cinema Processor--A New Audio Control Center for the Movie Theaters,"

[1] L. Blake, "Mixing Dolby Stereo Film Sound," Record. Eng./Produc., vol. 12, p. 68 (1981 Feb.). [2] J. Steinberg and W. Snow, "Auditory Perspective--Physical Factors," Elec. Eng., vol. 53, p. 64 (1934Jan.). [3] H. Fletcher, "The Stereophonic Sound Film System--General Theory," J. Acoust. Soc. Am., vol. 13, p. 89, (1941 Oct.). [4] H. Reeves, "The Development of Stereo Magnetic Recording for Film," SMPTE J., vol. 91, p. 947 (1982Oct.). [5] L. Blake, "The Evolution and Utilization of 70mm Six-Track Film Sound," Record. Eng./Produc., vol. 14, p. 64 (1983 Apr.). [6] J. G. Frayne, "Motion Picture Sound Recording--A Capsule History," J. Audio Eng. Soc., vol. 24, pp. 512-516 (1976 July/Aug.). [7] P. W. Klipsch, "Stereophonic Sound with Two Tracks, Three Channels by Means of a Phantom Circuit (2PH3)," J. Audio Eng. Soc., vol. 6, p. 118 (1958 Apr.). [8] A. Blumlein, "Improvements in and Relating tO Sound-Transmission, Sound-Recording, and SoundReproducing Systems," U.K. Patent 394,325, 1933 June 14. [9] H. Clark, G. Dutton, and P. Vanderlyn, "The 'Stereosonic' Recording and Reproducing System," Proc. IEEE, vol. 104, pt. B, p. 17 (1975 Sept.).

presented at the 54th Convention of the Audio Engineering Society, J. Audio Eng. Soc. (Abstracts), vol. 24, p. 492 (1976 July/Aug.), preprint 1112. [20] D. Robinson, "CP200--A Comprehensive Cinema Theater Audio Processor," $MPTE J., vol. 90, p. 778 (1981 Sept.). [21] E. R. Madsen, "Extraction of AmbienceInformation from Ordinary Recordings," J. Audio Eng. Soc., vol. 18, pp. 490-496 (1970 Oct.). [22] H. Lauridsen, "Experiments Concerning Different Kinds of Room-Acoustic Recordings" (in Danish), Ingenioren, no. 47, p. 906 (1954). [23] D. Hailer, "A New QuadraphonicSystem," Audio, vol. 54, p. 24 (1970 July). [24] W. Sommerwerck, "The Dolby Labs SurroundSound System for Motion Pictures," MCS Rev., vol. 4, p. 5 (1982 Summer). [25] P. Scheiber, "Four Channels and Compatibility," J. AudioEng. Soc., vol. 19, pp. 267-279 (1971 Apr.). [26] P. Ratliff, "Properties of Hearing Related to Quadraphonic Reproduction," BBC Research Dept., Rep. BBC RD 1974/38, 1974 Nov. [27] P. Scheiber, "Analyzing Phase-Amplitude Matrices," J. Audio Eng. Soc., vol. 19, pp. 835-839 (1971 Nov.). [28] M. A. Gerzon, "A Geometric Model for TwoChannel Four-Speaker Matrix Stereo Systems," J. Audio Eng. Soc., vol. 23, pp. 98-1'06 (1975 Mar.). [29] D. Leakey, "Some Measurements on the Effects of Interchannel Intensity and Time Differences in Two

[10] H. Haas, "The Influence of a Single Echo on the Audibility of Speech" (transl. from German), J. Audio Eng. Soc., vol. 20, pp. 146-159 (1972 Mar.). [11] H. Wallach, E. Newman, andM. Rosenzweig, "The Precedence Effect in Sound Localization," Am. J. Psychol., vol. 62, p. 315 (1949 July). [12] P. Fellgett, "Directional Information in Reproduced Sound," Wireless World, vol. 78, p. 413 (1972 Sept.). [13] M. Willcocks, "Directional Enhancement System for Quadraphonic Decoders," U.S. Patent 3,944,735, 1976 Mar. 16. ,i14] J. Frayne, "A Compatible Photographic Stereophonic Sound System," SMPTE J., vol. 64, p. 303 (1955June). [15] R. Uhlig, "Stereophonic Photographic Soundtracks," SMPTE J., vol. 82, p. 292 (1973 Apr.). [16] I. Allen, "The Production of Wide-Range, LowDistortion Optical Soundtracks Utilizing the Dolby

Channel Sound Systems," J. Acoust. Soc. Am., vol. 31, p. 977 (1959 July). [30] Y. Makita, "On the Directional Localization of Sound in the Stereophonic Field," EBU Rev., pt.. A, no. 73, p. 102 (1962 June). [31] M. Gerzon, "Surround-Sound Psychoacoustics," Wireless World, vol. 80, p. 483 (1974 Dec.). [32] M. Gerzon, "Pictures of 2-Channel Directional Reproduction Systems," presented at the 65th Convention of the Audio Engineering Society, J. Audio Eng. Soc. (Abstracts), vol. 28, pp. 362, 364 (1980 May), preprint 1569. [33] M. E. G. Willcocks, "Transformations of the Energy Sphere," J. Audio Eng. Soc. (Engineering Reports), vol. 31, pp. 29-36 (1983 Jan./Feb.). [34] W. Albersheimand F. Shirley, "Computation Methods for Broadband 90 째 Phase-Difference Networks," IEEE Trans. Circuit Theory, vol. CT-16, p. 189 (1969 May). [35] S. P. Lipshitz, M. Pocock, and J. Vanderkooy,

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"On the Audibility of Midrange Phase Distortion in Audio Systems," J. Audio Eng. Soc., vol. 30, pp. 580-595 (1982 Sept.). [36] R. Itoh, "Proposed Universal Encoding Standards for Compatible Four-Channel Matrixing," J. Audio Eng. Soc., vol. 20, pp. 167-173 (1972 Apr.). [37] B. B. Bauer, R. G. Allen, G. A. Budelman, and D. W. Gravereaux, "Quadraphonic Matrix Perspective--Advances in SQ Encoding and Decoding Technology," J. Audio Eng. Soc., vol. 21, p. 342-

350 (1973 June). [38] M. A. Gerzon, "Periphony: With-Height Sound Reproduction," J. Audio Eng. Soc., vol. 21, pp. 210 (1973 Jan./Feb.). [39] Shure Brothers Inc., model HTS 5000, Data Sheet 27A8083, 1985 [40] S. Julstrom, "Directional Enhancement Circuit," patent applied for. [41] M. Gilbert and S. Julstrom, "Delta Modulation Encoding/Decoding Circuitry," patent applied for.

THE AUTHOR

Stephen Julstrom studied electrical engineering at the University of Iowa where he was later employed in the recording studios of the School of Music. Since 1981 he has been employed in the electronics development department at Shure Brothers Incorporated,

J.AudioEng.Soc.,Vol.35,No.7/8,1987July/August

where he is now a senior staff engineer. In addition to surround sound system development, his duties have included work on automatic microphone systems, microphone preamplifiers, and teleconferencing systems. He holds one patent in automatic microphone control.

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AES Paper; Shure Acra-Vector Decoder