User`s manual

Dolby
®
CP650 Digital Cinema Processor User’s Manual
B-1
Appendix B
The Evolution of Dolby Film Sound
Going 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
three decades closely mirrors the history of Dolby
®
film sound technologies.
Optical Soundtracks
The photographic (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. 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. The optical soundhead within the projector is also
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 sophisticated new 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