SURROUND SOUND: PAST, PRESENT, AND FUTURE

by Joseph Hull, Dolby Laboratories Inc.

(This document is complemented by a GIF file available in the Dolby 
Software Library.  All Figure references refer to this file.  Please 
see "SURRPPF.GIF" in the Dolby Software Library.)


Ten years ago, movie sound, television audio, and music playback 
formats were distinctly different products of industries often 
working in isolation.  Today, however, the increasing popularity of 
surround sound in the home is bringing these industries and their 
sound formats closer together.  This article traces surround sound 
back to the first stereophonic movie formats of the 1950Õs, and 
shows how over the years there has been technological cross-
fertilization -- not always planned -- among the entertainment 
media.

In the not-too-distant future, new digital technology and closer 
industry co-operation may lead to an even more consistent approach 
to home sound reproduction, easing the burden on both consumer and 
producer, and providing unparalleled fidelity not only to the 
tonality of live sound, but also to its spatiality.


Movie Stereo: The Origin of Surround Sound

The first commercially successful multi-channel sound formats were 
developed in the early 1950Õs for the movies.  At the time, films 
were heavily promoted as providing Òstereo-phonic sound,Ó a new 
concept to the public.  However, unlike the two-channel format later 
adopted for home stereo, movie stereo used, and continues to use, a 
minimum of four channels.

With most movie stereo formats, the multiple channels were recorded 
on stripes of magnetic material applied to each release print. To 
play the prints, projectors were fitted with special playback heads 
much like those on a tape recorder, and theaters were equipped with 
additional amplifiers and speaker systems.  

From the outset, movie stereo featured at least one channel played 
over speakers at the rear of the theater.  At first this was known 
as the ÒeffectsÓ channel, and its use was reserved as the name 
implies for the occasional dramatic effect (e.g. ethereal voices in 
religious epics).  In fact, some formats used trigger tones to 
switch this channel off when it wasnÕt needed, because the 
corresponding track on the film was particularly narrow, and thus 
very hissy.

Although film stereo lost favor in the 1960Õs and early 1970Õs due 
to high costs and a slump in the movie business, sound mixers 
continued to experiment with the effects channel.  With formats such 
as six-track 70 mm magnetic providing consistent signal-to-noise 
ratios on all channels, mixers began to use the effects channel to 
envelop the audience in continuous low-level ambient sounds, to 
conveying greater sonic realism as well as the occasional dramatic 
effect.  (Figure 1.)

This expanded and more naturalistic practice came to be known as 
surround sound; the effects channel thus became known as the 
surround channel.  The speakers at the rear -- and now at the sides 
of the theater as well to create a more diffuse soundfield -- came 
to be known colloquially within the industry as Òthe surrounds.Ó


Home Stereo and Quadraphonic Sound

In 1958 the stereo LP was introduced, providing two channels (left 
and right) on the familiar phonograph record.  Two-channel stereo 
music reproduction was such a dramatic advance over mono, and so 
comparatively easy to implement, that it soon became the norm.  A 
few years later, stereo FM broadcasting, also a two-channel format, 
was introduced, further entrenching two-channel sound as Òstereo.Ó  
As a result, all but dedicated film buffs were unaware that movie 
stereo, as rare as it was by now, continued to use four or more 
channels.

As home stereo grew in popularity, manufacturers of home stereo 
equipment began to look for ways to expand their market.  This was 
one motivation behind the ill-fated quadraphonic, or Òfour-channelÓ 
home stereo format promoted in the early 1970Õs.  It required two 
additional speakers in the rear corners of the listening room, to 
reproduce two extra channels from specially encoded program sources.  
(Figure 2.)

Because the existing home stereo media had only two channels and 
could not easily accommodate more, several schemes were developed to 
encode the two channels with additional sound information.  Most of 
these schemes were based on matrixing techniques, which enabled 
ÒhidingÓ extra channels within the two regular channels by recording 
them with different relative phase.

The failure of quadraphonic sound had several causes.  Different 
groups of record companies and stereo equipment manufacturers backed 
different, incompatible encoding/de-coding systems, creating 
consumer confusion.  Producers and recording engineers couldnÕt 
decide on how to best utilize the extra channels.  And to top it all 
off, most consumers perceived little advantage from the format.  As 
a result, they thoroughly rejected the whole idea by not buying the 
new equipment in large numbers.

Quadraphonic home stereo was never associated with the multi-channel 
stereo formats used in movie theaters, and the term Òsurround soundÓ 
still wasnÕt heard outside the film industry.  After all, the only 
home visual medium at the time was broadcast TV, which provided 
distinctly low-fidelity mono sound.  Indeed, for the next decade 
home stereo, movie stereo, and TV sound would remain separate 
entities supported by essentially singular, isolated industries.


Dolby Stereo Film Sound

In the mid-1970Õs, Dolby Laboratories introduced a practical new 
film sound technology for 35 mm prints called Dolby Stereo.  Instead 
of the magnetic striping of existing movie stereo formats, it was 
based upon the optical soundtrack that had been the classic method 
for putting (mono) sound on film since the 1920Õs.

To enable compatible playback in mono theaters, it was necessary to 
fit the new stereo optical soundtrack into the same space on the 
print occupied by the traditional mono track.  Experiments showed 
that two channels, treated with Dolby A-type noise reduction, could 
give excellent fidelity when substituted for the mono track.  But 
trying to squeeze in more than two channels raised noise to an 
unacceptable level, even with noise reduction.  

Two channels, however, are not enough for movie stereo.  For one 
thing, movie screens are so wide that a separate center channel and 
speaker is required to localize dialogue for viewers seated off-
center.  For another, "stereo" and "surround" had come to mean one 
and the same thing to most of the film industry.  Therefore, to 
ensure mono compatibility, high audio performance, and acceptance by 
the industry, some way had to be found to encode just two physical 
tracks on movie prints with four channels of information.

The method developed has its roots in the matrixing techniques first 
tried for quadraphonic home stereo.  However, the playback channels 
are configured in the movie stereo norm --  left, center, right, and 
surround -- and the Dolby Stereo process is itself far more 
sophisticated than the earlier home systems. (Figure 3.)

The new stereo optical technology proved so practical that today 
there are more than 16,000 Dolby Stereo theaters worldwide, and 
virtually every major title is released in the format, for a total 
of more than 3,000 films thus far.  WhatÕs more, the film industry 
experienced a kind of renaissance in the 1980Õs, helped by a new 
commitment to high-quality presentations exemplified by the adoption 
of Dolby Stereo.  That rebirth was also helped by whatÕs often 
called "the video revolution."


The "Video Revolution"

The first video cassette recorders were introduced in the early 
1970Õs.  They were promoted initially as Òtime shiftingÓ devices to 
record TV broadcasts for viewing at more convenient times.

Before long, however, the video cassette took on another and hugely 
popular function:  the playback at home of theatrical films.  This 
created a voracious appetite for program material which the film 
industry was more than happy to satisfy.  Thus a whole new business 
grew up around the production, sales, and rental of prerecorded 
video cassettes.  

While the increased penetration of broadcast television had 
contributed to the film industryÕs woes in the 1950Õs and 1960Õs, 
the video cassette did quite the opposite.  Movie attendance 
actually increased with the growth of the prerecorded video 
cassette.  Films that proved popular in theatrical exhibition proved 
just as popular for home viewing, and in some instances, films which 
did not do well at the boxoffice fared better in video release.

Of course, the video cassette was not the only element of the video 
revolution.  More homes than ever were connected to cable systems, 
providing viewers with still more program sources (and the film 
industry with still more program-producing opportunities).  The 
high-performance Laser Disc, finally coming into its own today, was 
introduced.  Television set manufacturers began to offer products 
with what might be called Òhigh-fidelityÓ picture quality, and 
consumers bought them.  Thus ÒtelevisionÓ became Òvideo,Ó and the 
home TV set became a Òvideo monitorÓ to display a wide variety of 
visual program sources, much as stereo systems play music from a 
variety of sources.


Stereo Sound Comes To Video

At first the video cassette, in the tradition of broadcast 
television, offered only relatively low-fidelity mono sound.  
However, at the time the video revolution got underway, consumers 
were not only accustomed to high quality stereo sound from their 
music systems, but were also becoming accustomed to high-quality 
stereo sound in the theater as Dolby Stereo spread throughout the 
film industry.  

In recognition of the gap between video and these other media, video 
cassette machines and prerecorded tapes with stereo sound were soon 
offered, first with linear soundtracks and then by means of the 
ÒHiFiÓ formats.  The Laser Disc was conceived from the outset to 
provide high-quality stereo sound.  And soon a stereo TV 
broadcasting system was adopted for over-the-air and cable use.  
Thus the familiar two-channel stereo format long established for 
home music reproduction was adopted for all video media. 


Surround Sound Comes Home

In 1958 when the stereo LP was introduced, high-fidelity sound 
reproduction was itself a rarity.  But 25 years later, high-
performance stereo music systems were the norm.  Audio as well as 
video had expanded to include new program sources (the audio 
cassette and the CD).  Car stereo was coming of age, and the 
Walkman* was providing a whole new way to listen to music.  A new 
generation had grown up with rock music as dependent upon audio 
technology as musicianship, and was being regularly exposed to 
multi-channel surround sound at the movies. 

It was against this background of increased consumer sensitivity and 
sophistication that Dolby Surround was introduced late in 1982 for 
the playback of Dolby Stereo theatrical films released in stereo 
video formats.  The original four-channel encoding of Dolby Stereo 
film soundtracks remains intact when they are transferred to stereo 
video cassettes and Laser Discs, or broadcast on stereo TV.  Dolby 
Surround made it possible to decode the surround channel at home.  
Soon Pro Logic circuitry made it possible to decode the center 
channel as well, and to take advantage of the sophisticated steering 
circuitry developed originally for theatrical playback.  (Figure 4.)

Unlike quadraphonic stereo, Dolby Surround has gained considerable 
marketplace acceptance, for several reasons.  First, the channel 
configuration and how to best utilize it was firmly established 
within one industry in advance of its introduction to another.  
Second, it was developed with a clear focus, specifically as a 
sound-with-picture format to enhance the viewing experience.  Third, 
software and hardware standards for both them film and consumer 
electronics industries are defined by one organization (Dolby 
Laboratories) having credibility with both.  Fourth and most 
important, more than five million consumers so far have perceived 
Dolby Surround as worthwhile.  Thus surround sound has crossed 
traditional boundaries, to the advantage of the consumer, and the 
film, prerecorded software, and consumer electronics industries.  
And today it is crossing yet another boundary, to be used 
increasingly for television broadcasting -- the remaining sound-
with-picture format.


Dolby Stereo SR.D: Will History Repeat Itself?

In June of this year, Dolby Laboratories demonstrated to the film 
industry a newly-developed optical sound format for 35 mm release 
prints.  In its familiar location adjacent to the picture is a 
matrix-encoded, four-channel analog soundtrack, which as before can 
be played in virtually any theater, stereo or mono.  Located between 
the sprocket holes on one edge of the film, however, is an entirely 
separate digitally-coded optical soundtrack providing six discrete 
channels.  

This new format is called Dolby Stereo SR.D (ÒSRÓ denotes that the 
analog track is processed with Dolby SR rather than A-type noise 
reduction).  Release prints in the new format and decoders enabling 
theaters to play back the digital track will be available next year.  

The new digital track provides five full-range channels with CD-like 
performance.  They are assigned to left, center, and right screen 
speakers as with current formats, and to separate left surround and 
right surround speaker arrays in a configuration the industry calls 
Òstereo surroundÓ (optional with todayÕs Dolby Stereo 70 mm magnetic 
format).  A sixth bass-only track is provided for subwoofers.  
(Figure 5.)

This new technology is not only likely to impact the film industry.  
If the history of Dolby Surround is any indication, it has 
significant potential for home entertainment media as well.


Digital Comes of Age

The key factor in the wider potential for DolbyÕs new digital 
development can be gleaned by examining a sample of SR¥D film.  The 
digital soundtrack is about the same width as the analog track, but 
because it is located between the sprocket holes, it occupies only 
half the total area of the analog track. Yet it provides six 
discrete channels!  What ever happened to digitalÕs infamous 
appetite for storage/ transmission space?

What happened is a powerful new digital coding technique -- a high 
quality, discrete multi-channel delivery system so efficient it 
could be applied to Laser Discs and CDÕs, let alone future formats 
such as DAB and HDTV.

Efficiency is not the only advantage of the new coding system.  Were 
it to be adopted for a variety of media, producers would have to 
create only one soundtrack mix, rather than the several required 
today for different playback formats and circumstances.  This is 
because the digital data can be decoded in a configuration best 
suited to the listenerÕs preference, budget, and listening space.  
Regardless of the medium, upon playback the original multi-channel 
digital data could be reconstructed as a mono channel, as two 
conventional stereo channels, as three channels in a left-center-
right configuration, with a single surround channel as with Dolby 
Surround today, or in a full-bore Òstereo surroundÓ configuration.  
The low-bass channel could be incorporated into the main channels, 
or separately decoded if the playback system includes a subwoofer.  
The new technology could even let the listener choose a dynamic 
range tailored specifically to the listening environment.  The same 
recording could be played with a reduced dynamic range in a car, for 
example, and with the full dynamic range of a CD in a quiet living 
room.

So far, digital audioÕs most direct consumer benefit has been the 
Compact Disc.  Yet for all its ergonomic virtues, in terms of 
fundamental sound quality the improvement afforded by the CD over 
the best analog formats is only incremental -- and some finicky 
audiophiles would dispute even that.  With the new Dolby technique, 
on the other hand, digital audio might bring about a true revolution 
in the way sound is reproduced in the home.  Only time will tell.


Surround Sound and Music Reproduction

Dolby Surround was developed specifically as a sound-with-picture 
format.  That hasn't discouraged record companies like RCA 
Victor/BMG Classics from releasing music-only CDs utilizing Dolby 
Surround.  The recordings have been either re-mastered or originally 
recorded in Dolby Surround with the assistance of Dolby consultants.

While Concord Jazz uses surround to enhance the sense of "being 
there" at jazz concerts recorded live, RCA Victor's releases of film 
scores strive to capture the sound of multi-channel sound in the 
theatre.  With its most recent releases, however, Victor is 
branching out with re-masterings of Tomita's synthesized renditions 
of classical works, and the firs musical originally recorded in 
Dolby Surround, the London production of Stephen Sondheim's "Into 
the Woods."


Dolby and Digital

Because of its reputation for analog noise reduction systems, those 
outside the broadcast and satellite communications fields are often 
surprised to hear that Dolby laboratories has developed several 
digital audio coding systems, the first of which has been in use 
since 1983.  These systems reduce the space required to store and 
transmit digital audio, an objective not unlike reducing the noise 
added by analog formats.  For example, to get the same performance 
as an audio cassette processed with the new Dolby S-type noise 
reduction system, an unprocessed cassette would have to utilize tape 
three feet wide!  Conventional audio coding, such as that used for 
the CD, is the digital equivalent of that three-foot-wide cassette 
tape.  Such brute-force approaches are likely to give way as digital 
audio comes of age.

(C) 1991 Dolby Laboratories A91/0015
(December 1991)

(A hardcopy version of this document is available from Dolby 
laboratories.  Please request document S91/9295 by leaving your name 
and address via e-mail to DLLC.)