1. Field of the Invention
This invention relates to sound recording for motion pictures, and more particularly to apparatus and a method for reducing noise during the playback of optical sound tracks.
2. Description of the Prior Art
Variable area motion picture sound tracks are made by recording a negative sound track on special high-contrast recording film, which is then used to make a contact print of the motion picture print to be released. The amount of open area in the finished print sound track is kept as small as possible, but the open area must still be large enough to record the full range of the sound track. Thus, when the maximum sound volume being recorded is at a low level, the open sound track area is small, but the open area increases as the maximum sound level increases to accommodate the increased level.
A primary source of noise in optical sound tracks is dirt and other debris in this open area. As the open areas of the sound track increase, more noise is generated as a result of the dirt and debris. This problem is particularly severe when optical sound tract negatives are played. With negatives, the open area on the sound track is greatest when the sound signals are at a minimum; since low level signals are not nearly as effective as high level signals in masking noise, the greater area available for noise and debris can be compounded by the low signal level to product unacceptably noisy sound. In some cases it would be desirable to playback a negative directly, but because of the noise problem an expensive positive print must be made from the negative and the print played back. Even then, the imprint of dirt and debris on the negative may be transferred onto the positive.
Additional noise in playing back an optical sound track originates as quantum noise from the photocell which is used to detect light transmitted through the film. This noise increases in intensity with the amount of light striking the photocell. Since a greater amount of light strikes the photocell during the low signal portions of a sound track negative, this type of noise is then an additional limitation on the ability to directly play back a sound track negative.