Operating instructions

G-1
APPENDIX G
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.