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PUSH-PULL RECORDING* J. K. HILLIARD**
Summary. — A discussion of some of the practical aspects of push-pull recording supplementing a discussion of the theoretical principles by 0. 0. Ceccarini on page 162 of this issue of the Journal.
Sound that is to accompany motion pictures must necessarily be reproduced with the greatest possible accuracy in order to create the greatest illusion of reality. In seeing motion pictures the audience is aroused to a sound expectancy that is not necessarily essential to the radio or the phonograph. It is necessary that the recording and reproducing systems have frequency and volume ranges commensurate with those of the original sounds, and a high degree of linearity. To approximate this condition, a frequency range of 50 to 8000 cps. is necessary, and a volume range of 50 to 60 decibels. Reproducing systems are now available, and are being installed in theaters, that will meet these requirements, and recording practice has been developed to such a degree that it also fulfills the requirements. However, it has been found necessary to limit the volume range of releases until such time as the majority of theaters are adequately equipped.
To date, standard methods of recording on film, both variabledensity and variable-width, have given a volume range of approximately 40 decibels, at the expense of considerable distortion. In the variable-density system the principal limitation has been the small linear range of density between the toe and the shoulder of the characteristic curve of positive stock, plus the distortion due to the noise-reduction system because of its slow operating time. Adoption of the push-pull feature would reduce these limitations by cancelling these internal distortions and, accordingly, development of such a project was undertaken.
The light-valve as shown in Fig. 1 has been modified to have two pairs of biplanar ribbons of the conventional type, each pair exposing
* Received October 11, 1937. The theoretical principles of push-pull recording are outlined by O. O. Ceccarini, p. 162 of this issue. ** Metro-Gold wyn-Mayer Corp., Culver City, Calif.
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