Specifications

APPENDIX E
THE EVOLUTION OF DOLBY FILM SOUND
Thanks to such developments as multichannel sound, the motion picture viewing
experience today is more exciting and involving than ever before. And what the
audience hears today is very much the result of a continuing effort to improve film
sound originally undertaken by Dolby Laboratories more than twenty years ago.
Indeed, the evolution of motion picture sound over the past two decades is, in great
part, that of Dolby film sound technologies.
Optical Sound
The photographic or “optical” soundtrack was the first method of putting sound on
film, and today it remains the most popular.
An opaque area adjacent to the picture contains narrow, clear tracks that vary in
width with variations in the sound. As the film is played, a narrow beam of light
from an exciter lamp 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.
Several advantages of optical sound have contributed to its universal acceptance,
the foremost being economy. For one thing, the soundtrack is printed
photographically on the film at the same time as the picture. For another, the
soundtrack can last as long as the picture, which—with care—can be a long time
indeed. A further benefit is that the optical soundhead within the projector is itself
economical and easily maintained.
Motion pictures with sound were first shown to significant numbers of movie-goers
in the late 1920s. By the mid-1930s, the “talkies” were no longer a novelty, but a
necessity, and many thousands of theaters were equipped in that short time to show
films with optical soundtracks. This phenomenally rapid acceptance of a
sophisticated new technology was not without drawbacks, however. Equipment
was installed in theaters so rapidly that there was no time to take advantage of
improvements which were occurring on an almost daily basis.
A good example is loudspeaker design. The first cinema loudspeakers had very
poor high-frequency response. Speakers with superior high-frequency capability
became available within just a few years. But there was no time to retrofit the
original systems with new units, because engineers were too busy equipping other
theaters 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