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1950 SYNTHETIC COLOR-FORMING BINDERS 473
Magenta Sound Tracks
When a phototube with S-4 sensitivity is to be used for reproduction, a sound track is applied to Type 275 Color Film in a manner completely analogous to black-and-white practice. The sound negative is simply printed on the color positive using the blue-exposing filter to confine the image to the magenta-forming emulsion, and the film is developed as described without any additional treatments to the sound tr&ck.
The top density and contrast of the magenta image, while dictated by picture requirements, are quite appropriate for sound reproduction. In particular, the high resolving power of the magenta emulsion confers good high-frequency response to a magenta track.
Because the contrast of the magenta image as "seen" by a bluesensitive phototube is somewhat lower than black-and-white release positive, variable-density negatives intended for printing on Type 275 should have somewhat higher contrast than is used with black-andwhite positive. A Du Pont Type 228 negative developed to a IIB control gamma of about 1.2 will yield a magenta track with minimum intermodulation for positive track densities in the neighborhood of 0.6 density. Likewise, variable-area sound negatives should have higher track density than if intended for black-and-white use. In crossmodulation tests a magenta track, printed from a Du Pont Type 201 negative developed to a IIB gamma of 3.5 and having a track density of 2.5, had optimum cancellation for a 1.15 density.
Actual intermodulation data for magenta sound tracks are represented by the solid curve of Fig. 10, and cross-modulation data appear in Fig. 11. These distortion measurements were made with a 1P37 phototube in the sound reproducer, and the track densities read with a blue-sensitive phototube in the densitometer.
Sulfided Sound Tracks
A sulfided sound track is produced on Type 275 by an edge treatment following the bleach, but preceding the second fix. At this point in the processing the original silver image has been converted to silver ferrocyanide by the bleach. This compound reacts very rapidly with sodium sulfide to form silver sulfide, which has the desired opacity to near-infrared radiation.
The picture image also contains silver ferrocyanide, so it is obvious that the entire film should not be treated with sodium sulfide. Therefore, an applicator which treats only the sound track with the sulfiding solution must be used. Such applicators are not novel, and many