Surround Sound 159345.1 The Origin of Surround Sound
The first commercially successful multichannel sound
formats were developed in the early 1950s for the cin-
ema. At the time, stereophonic sound, as it was called,
was heavily promoted along with new wide-screen for-
mats by a film industry feeling threatened by the rapid
growth of television. Unlike the two-channel format
later adopted for home use because of limitations
imposed by the phonograph record, film stereo sound
started out with, and continues to use, a minimum of
four channels.
With such film formats as four-track CinemaScope
(35 mm) and six-track Todd-AO (70 mm), multiple
sound channels were recorded magnetically on stripes
of oxide material applied to each release print. To play
these prints, projectors were fitted with magnetic play-
back heads like those on a tape recorder (only much
larger), and cinemas were equipped with additional
amplifiers and loudspeaker systems.
From the outset, multichannel film sound featured
several channels across the front, plus at least one
channel played over loudspeakers towards the rear of
the cinema. At first the latter was known as the effects
channel, and was reserved for the occasional dramatic
effect—ethereal voices in religious epics, for example.
Some formats even switched this channel off by means
of trigger tones when it wasn’t needed because the
magnetic track on the film was particularly narrow, and
thus very hissy.
As time went on, sound mixers continued to experi-
ment with the effects channel. In particular, because
six-track 70 mm magnetic provided 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. This expanded,
more naturalistic application came to be known as
surround sound, and the effects channel as the surround
channel, Fig. 45-1.
45.2 Surround from Optical Soundtracks
Under the best conditions, the multichannel mag-
netic-stripe formats provided superb sound, way beyond
anything the home listener could experience, and it was
widely adopted in the 1950s. By the1970s, however, the
expense of magnetic release prints, their comparatively
short life compared to those with traditional optical
soundtracks, and the high cost of maintaining the play-
back equipment led to a massive reduction in the num-
ber of magnetic releases and cinemas capable of playing
them. Magnetic sound came to be reserved for only a
handful of first-run engagements of big releases each
year.
As a result, by the mid-1970s, most films were being
released with only low fidelity, mono-optical sound-
tracks, a technology that had hardly improved since the
late 1930s. Then, in 1975, came a new breakthrough:
the introduction by Dolby Laboratories of a highly prac-
tical 35 mm multichannel optical release print format
originally identified as Dolby Stereo.
In the space allotted to the conventional mono-
optical soundtrack are two soundtracks that not only
carry left and right information as in home stereo sound,
but are also matrix encoded with a third center-screen
channel and—most notably—a fourth surround channel
for ambient sound and special effects. The matrix
process in essence “folds” four channels down to two
tracks on the film, and “unfolds” them in the theater by
means of a sound processor-decoder (a common
industry term for the process is 4:2:4), Fig. 45-2.
This format not only enabled multichannel sound
from optical soundtracks, but higher-quality sound as
well, thanks to such techniques as noise reduction and
loudspeaker equalization. The result was multichannel
capability on easily manufactured, compatible 35 mm
optical prints that rivaled that of four-track 35 mm
magnetic, which soon became obsolete.
The multichannel optical format proved so practical
that within a decade of its introduction, virtually all
major releases could be heard in most local cinemas in
four-channel surround sound. It was dramatically
improved in 1987 by the application of spectral
recording (SR), a new recording process developed by
Dolby Laboratories that both lowered optical track
noise still further and increased headroom, making it
possible to record loud sounds with wider frequency
response and lower distortion.
Today, virtually all 35 mm movie prints, including
those with digital soundtracks, feature a matrix-encoded,
four-channel SR analog optical soundtrack. The SR track
makes it possible for the print to play in any theater in
the world, and also acts as a backup on digital prints in
case there are problems with the digital track(s).45.3 Digital and 5.1 SurroundIn the 1980s, as the success of the compact disc estab-
lished with consumers the idea that digital sound meant
better sound, the film industry began to investigate what
it wanted by way of digital sound in the cinema. While
not defining how it would be achieved, the industry
agreed on several objectives, including fully discrete
channels providing better channel separation than