The Aperture Effect

The earlier workers who recorded on, and reproduced from film will no doubt recall the poor high frequency response obtained. It has never been particularly difficult to obtain fair reproduction between the frequencies of 1000 and 2500 cycles from phonograph records, but many of the early film reproducers suffered severe losses in even this restricted range. It was soon found that the width of the beam of light which fell on the film played an important part in the reproduction of the higher frequencies. It was obvious that if the thickness of this beam of light should happen to be an integral number of wave-lengths of the recorded signal, no sound corresponding to that wave-length could be reproduced. It was also recognized that this effect was present in recording as well as in reproducing. Moreover, it is evident that this effect is oftentimes made more serious by the lens system. In order to explain the aperture effect, it is desirable to examine the complete process of recording and reproduction by means of a rectangular beam of light whose thickness in the direction of film travel is small. A little consideration shows that the same reasoning applies to both systems of recording under certain assumptions. For simplicity, the variable amplitude system will be examined. It will be evident that the ideal aperture is supposed to measure only the amplitude of the wave. In so far as area is involved, distortion results. This is mentioned because sometimes this system is called variable area recording. It is more correctly called variable amplitude recording.

Print ISSN

0097-5834

Published

1930-06

Content type

Original Research

DOI

10.5594/J13051