Optical Sound-Track Printing

Abstract

It is well known that the nearly universal practice of printing sound tracks by contact on the periphery of a rotating sprocket introduces significant amounts of flutter, and also amplitude modulation of the higher frequencies of the record, principally at the sprocket-hole frequency. — An optical one-to-one ratio printer in which the negative and the printing stock are driven separately by good constant-speed mechanisms has given substantially improved quality in printing 16-mm sound tracks. In contact prints of 7000-cycle records it is easy to demonstrate rapid fluctuations of output level of the order of 6 to 8 decibels. In optical prints from the same negatives such fluctuations do not exceed 1/2 decibel. High-frequency response is improved, to the extent of 5 decibels at 7000 cycles. The cross-modulation cancellation density for variable-area track prints is increased by about 0.1 in the region of practical interest. Listening tests show an immediately noticeable improvement in quality when optical prints are compared with contact prints made from the same negatives. — The optical system for sound-track printing must be able to resolve patterns several times finer than the highest frequencies on the sound negative, and, for printing on color films, must be especially free from the higher-order chromatic aberrations, secondary spectrum and chromatic variation of spherical aberration. Ordinary photographic lenses are unsuitable. One system which meets the requirements using commercially available microscope lenses is described.