Journal of the Society of Motion Picture Engineers ( Volume: 37, Issue: 10, October 1941)

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Electrical Equipment for the Stereophonic Sound-Film System

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Publisher: SMPTE — White Plains, NY, USA
Doc Type: Journal Article
Content Type: Original Research
Abbreviated Title: J SMPE
Volume: 37, No. 10, pp. 380–396
Abstract: An electrical system is described which permits the use of sound-film with its limited signal-to-noise ratio, as a recording medium for wide-range stereophonic reproduction of symphonic music. Noise reduction is accomplished both by pre-equalization, rising to 18 db above 8000 cycles, and by automatic signal compression and expansion of 30 db. To secure maximum suppression of noise and freedom from distortion, a pilot-operated, flat-top compandor system was selected. In each channel low-level signals are recorded on a separate track with constant gain 30 db above normal, which places them above the film noise. Higher-level signals cause automatic gain reductions and are recorded at substantially full modulation. These signals vary the intensity of a pilot tone, which in turn controls the compressor gain. There is a pilot frequency for each of the three channels, and the three are combined and recorded together on the fourth film track. During reproduction they are separated by filters, and operate expandors which restore the signals to their original forms but reduce the noise to inaudible levels. The compressor and expandor gains are made proportional to pilot level in db, and the expandor range over which this relation holds is 45 db. Therefore a 15-db variation in average pilot level during reproduction causes a corresponding average level change but no distortion. This is used to allow expansion of the original signal intensity range during recording or re-recording by simple gain controls in the pilot circuits. The paper describes the apparatus and circuits developed to accomplish these results, and discusses the frequency, load, distortion, noise, and dynamic characteristics of both constant and variable-gain elements. Also included are considerations of microphone and loud speaker arrangement and equalization to secure high fidelity of reproduction.
Publication Date: 1941-10-01
DOI: 10.5594/J09959
ISSN: Print: 0097-5834
Link: https://doi.org/10.5594/J09959
Author(s): W. B. SnowBell Telephone Laboratories, New York, N. Y.
A. R. SoffelBell Telephone Laboratories, New York, N. Y.
Copyright: © 1941 Society of Motion Picture and Television Engineers, Inc.

Bibliographic Reference(s)

1 Fletcher H. ,: “The Stereophonic Sound-Film System—General Theory” (this issue of the Journal, p. 331 ). EXTERNAL
2 Wente E. C. Biddulph R. Elmer L. A. Anderson A. B. : “Mechanical and Optical Equipment for the Stereophonic Sound-Film System” (this issue of the Journal, p. 353 ). EXTERNAL
3 Steinberg J. C. : “The Stereophonic Sound-Film System—Pre- and Post-Equalization of Compandor Systems” (this issue of the Journal, p. 366 ). EXTERNAL
4 Scriven E. O. : “Auditory Perspective—Amplifiers,” Electrical Engineering , LIII (April, 1934 ), p. 278 . EXTERNAL
5 Wente E. C. Thuras A. L. : “Auditory Perspective—Microphones and Loud Speakers,” Electrical Engineering , LIII (Jan., 1934 ), p. 17 . EXTERNAL
6 Bennett W. R. Doba S. : “Vario-Losser Circuits,” Electrical Engineering , LX , Trans. No. 17 (Jan., 1941 ), p. 17 . EXTERNAL

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W. B. Snow and A. R. Soffel; Electrical Equipment for the Stereophonic Sound-Film System, Journal of the Society of Motion Picture Engineers ( Volume: 37, Issue: 10, October 1941); SMPTE, 1941. Available at https://doi.org/10.5594/J09959

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W. B. Snow and A. R. Soffel; Electrical Equipment for the Stereophonic Sound-Film System, Journal of the Society of Motion Picture Engineers ( Volume: 37, Issue: 10, October 1941); SMPTE, 1941. Available at https://doi.org/10.5594/J09959

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<span class="citation">W. B. Snow and A. R. Soffel; <cite>Electrical Equipment for the Stereophonic Sound-Film System</cite>, Journal of the Society of Motion Picture Engineers ( Volume: 37, Issue: 10, October 1941); SMPTE, 1941. Available at <a href="https://doi.org/10.5594/J09959" target="_blank" rel="noopener">https://doi.org/10.5594/J09959</a></span>

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W. B. Snow and A. R. Soffel; <cite id="bib-10-5594-j09959">Electrical Equipment for the Stereophonic Sound-Film System</cite>, Journal of the Society of Motion Picture Engineers ( Volume: 37, Issue: 10, October 1941); SMPTE, 1941
<span class="doi">10.5594/J09959</span>
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History of Sound Motion Pictures [Active]