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224 E. W. KELLOGG [J. S. M. P. E.
sound), and no rise and fall of the ground-noise imperfectly following the increase and decrease in modulation which is often noticeable in all systems depending on shifting bias. In the push-pull system the moment the output voltage drops the track is black again.
These advantages of the new sound recording system were pointed out by Dimmick and Belar, but are restated here for the reason that the availability of this system is a factor to be weighed in comparing variable-density with variablewidth. Although variabledensity recordings may benefit in some measure from application of the push-pull principle,17 the full advantages are realizable only if the print transmission can be brought practically to zero whenever the galvanometer current is zero (or the sound-pressure zero). For this, definite and reproducible linear relationship between galvanometer current and print transmission is essential, and it is in such linear relationship that the variable-width system has its great advantage.
REFERENCES
1 HARDY, A. C.: "The Rendering of Tone Values in the Photographic Recording of Sound," Trans. Soc. Mot. Pict. Eng., XI (1927), No. 31, p. 475.
2 ZWORYKIN, V. K., HANNA, C. R., AND LYNN, L. B. : "The Kerr Cell Method of Recording Sound," Trans. Soc. Mot. Pict. Eng., XII (1928), No. 35, p. 748.
3 HARDY, A. C.: "Optics of Sound Recording Systems," Trans. Soc. Mot. Pict. Eng., XH (1928), No. 35, p. 760.
4 KELLOGG, E. W.: U. S. Patent No. 1,740,406.
5 DIMMICK, G. L.: "Galvanometers for VariableArea Recording," /. Soc. Mot. Pict. Eng.t XV (Oct., 1930), No. 4, p. 428.
6 SACHTLEBEN, L. T.: "Characteristics of the Photophone Light-Modulating System," /. Soc. Mot. Pict. Eng., XXV (Aug., 1935), No. 2, p. 175.
7 JONES, L. A.: "Photographic Reproduction of Tone,". Opt. J Soc. Amer., V (May, 1921), No. 3, p. 232.
8TurrLE, C., AND MCFARLANE, J. W. : "The Measurement of Density in Variable-Density Sound-Films," /. Soc. Mot. Pict. Eng.,. XV (Sept., 1930), No. 3, p. 345.
• MACKENZIE, D.: "Straight-Line and Toe Records with the LightValve," J. Soc. Mot. Pict. Eng., XVII (Aug., 1931), No. 2, p. 172.
10MEES, C. E. K.: "Some Photographic Aspects of Sound Recording," /. Soc. Mot. Pict. Eng., XXIV (April, 1935), No. 4, p. 285.
11 SANDVIK, O., AND HALL, V. C.: "Wave-Form Analysis of VariableDensity Sound Recording," /. Soc. Mot. Pict. Eng., XIX (Oct., 1932), No. 4, p. 346.
12 COOK, E. D.: "The Aperture Effect," /. Soc. Mot. Pict. Eng., XIV (June, 1930), No. 6, p. 650.
18 FOSTER, D. : "The Effect of Exposure and Development on the Quality of Variable-Width Sound Recording," /. Soc. Mot. Pict. Eng., XVII (Nov., 1931), No. 5, p. 749.