Dolby 53 evolutionofsound schematic

G

oing to the movies
today is more

exciting and
involving than ever before,
thanks in large part to a
continuing effort to improve
film sound undertaken by
Dolby Laboratories in the
early 1970s. Indeed, the
history of cinema sound over
the past two decades closely
mirrors the history of Dolby
film sound technologies.

Optical
soundtracks

The photographic, or
“optical,” soundtrack was the
first method of putting sound
on film. Today it remains the
standard, in both analog and
digital forms.

The classic analog optical
soundtrack consists of an
opaque area adjacent to the
picture containing narrow,
clear tracks that vary in
width according to variations
in the sound (Figure 1). As
the film is played, a beam of
light from an exciter lamp or
LED in the projector’s
soundhead shines through
the moving tracks.
Variations in the width of the
clear tracks cause a varying
amount of light to fall on a
solar cell, which converts
the light to a similarly
varying electrical signal.
That signal is amplified and

ultimately converted to
sound by loudspeakers in
the auditorium.

Economy, simplicity, and
durability are among the
advantages that have
contributed to optical sound’s
universal acceptance. The
soundtrack is printed
photographically
on the film at the
same time as
the picture and
can last just as
long, which—
with care—can
be a long time
indeed. And the
optical sound­head within the
projector is itself
economical and
easily maintained.

Success gets
in the way
of progress

Motion pictures with sound
were first shown to significant
numbers of moviegoers in
the late 1920s. Within a few
years, many thousands of
theatres were equipped to
show “talking pictures” with
optical soundtracks.

This phenomenally rapid
acceptance of a new,
sophisticated technology was
not without drawbacks,
however. Equipment was
installed in cinemas so

rapidly that there was no time
to take advantage of the
improvements that occurred
almost daily.

A good example is
loudspeaker design. The first
cinema loudspeakers had
very poor high-frequency
response. Speakers with

superior
response
became
available within
just a few years,
but there was no
time to retrofit
the original
systems with
new units.

Engineers were
too busy
equipping other

cinemas with their first sound
installations.

This caused a dilemma for
soundtrack recordists. Should
the tracks be recorded to
take advantage of the
improved speakers, or should
they be prepared to sound
best on the many older
installations already in place?
Given that it was impractical
to release two versions of a
given title, the only
alternative was to tailor
soundtracks to the older
speakers. The result was to
ignore the improved high­frequency response of the
newer, better units.

The Evolution of

Dolby Film Sound

Figure 1: 35 mm
optical print

2

To forestall compatibility
problems, in the late 1930s a
de facto standardization set
in, the cinema playback
response that today is called
the “Academy” characteristic.
Cinema owners knew what
to expect from
the films, and
therefore what
equipment to
install. Directors
and sound
recordists knew
what to expect
from cinema
sound systems,
and thus what
kind of
soundtracks to
prepare. The
result was a
system of sound recording
and playback that made it
possible for just about any
film to sound acceptable in
any cinema in the world. The
problem was that the system
lacked the flexibility to
incorporate improvements
beyond the limitations that
existed in the 1930s.

Magnetic striping
and multichannel
sound

In the early 1950s, as the
film industry sought to woo
viewers away from their
fascinating new television
sets, a new method of putting
sound on film was
introduced. After the picture
was printed, narrow stripes of
iron oxide material (similar to
the coating on magnetic
recording tape) were applied

to the release print
(Figure 2). The sound was
then recorded on the
magnetic stripes in real time.
In the cinema, magnetic
prints would be played back
on projectors equipped with

magnetic heads
similar to those
on a tape
recorder,
mounted in a
special
soundhead
assembly called
a “penthouse.”

Magnetic
sound was a
significant step
forward, and at

its best provided

much-improved
fidelity over the conventional
optical soundtrack. It also
enabled the first multichannel
sound reproduction, dubbed
“stereophonic sound,” ever
heard by the public. The
voice of an actor appearing
to the left, center, or right of
the picture could be heard
coming from speakers
located at the left, center, or
right of the new wide screens
also being introduced at this
time. Music took on a new
dimension of realism, and
special sound effects could
emanate from the rear or
sides of the cinema. The two
main magnetic systems
adopted were the four-track
35 mm CinemaScope
system, introduced with
The Robe, and the six-track
70 mm Todd-AO, first used
for Oklahoma!

Magnetic falls
into disuse

Magnetic sound was widely
adopted in the 1950s. By the
1970s, however, when the
film industry experienced an
overall decline, the expense
of magnetic release prints,
their comparatively short life
compared to optical prints,
and the high cost of
maintaining the playback
equipment led to a massive
reduction in the number 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.

By the mid-1970s, then,
movie-goers were again
hearing low-fidelity, mono
optical releases most of the
time, with only an occasional
multitrack stereo magnetic
release. Ironically, just as the
industry was reverting to
mono optical, more and more
moviegoers were enjoying
better sound at home over
superior hi-fi stereo systems.

Dolby gets
involved

By the late 1980s, the
situation that prevailed in the
mid-1970s had completely
changed. Thanks to new
technology and a turnaround
in the financial decline of the
industry, almost all major
titles by that time were being
released with wide-range
multichannel stereo

Figure 2: 70 mm
magnetic print

very little maintenance was
required. The result was
multichannel capability
equaling that of four-track
magnetic 35 mm (which
soon became obsolete), with
consistently higher fidelity,
greater reliability, and far
lower cost.

The next step:
Dolby SR

In 1986, Dolby Laboratories
introduced a new
professional recording
process called Dolby SR
(spectral recording). Like
Dolby noise reduction, it was
a mirror-image, encode­decode system used both
when a soundtrack is
recorded and when it is
played back. It provided
more than twice the noise
reduction of Dolby A-type,
and, moreover, permitted
loud sounds with wider
frequency response and
lower distortion.

The 35 mm optical
soundtracks treated with

3

soundtracks, as is the
case today.

The breakthrough was the
development by Dolby
Laboratories of a highly
practical 35 mm stereo
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
encoded with a third center­screen channel and—most
notably—a fourth surround
channel for ambient sound
and special effects
(Figure 3).

This format not only
enabled stereo sound from
optical soundtracks, but
higher-quality sound as well.
Various techniques were
applied to the soundtrack
during both recording and
playback to improve fidelity.
Foremost among these was
Dolby A-type noise reduction
to lower the hissing and

popping associated with
optical soundtracks, and
loudspeaker equalization to
adjust the cinema sound
system to a standard
response curve.

As a result, stereo optical
prints could be reproduced
in cinemas installing Dolby
cinema processors with far
wider frequency response
and much lower distortion
than conventional
soundtracks. In fact, the
Dolby optical format led to a
new worldwide playback
standard (ISO 2969) for
wide-range stereo prints.

An important advantage of
the Dolby optical format was
that the soundtracks were
printed simultaneously with
the picture, just like mono
prints. Thus four-channel
stereo optical release prints
cost no more to make than
mono prints, and far less
than magnetic prints. In
addition, conversion to
stereo optical proved
relatively simple, and once
the equipment was installed,

Figure 3: Dolby analog 35 mm playback