Vitaphone Soundtrack Discs
In 1924–1925, when Western Electric established the format of the system which would eventually be named Vitaphone, they settled on a 16 inch (40 cm) diameter disc rotating at 33 1/3 rpm as a good practical compromise of disc size and speed. The slow speed permitted the 11 minute playing time needed to match the maximum running time of a standard reel of film projected at 24 fps, yet the increased diameter preserved the average effective groove velocity, and therefore the sound quality, of a smaller, shorter-playing record rotating at the then-standard speed of about 78 rpm.
Like most ordinary records, Vitaphone discs were made of a shellac compound rendered lightly abrasive by its major constituent, finely pulverized rock. They were designed to be played with a very inexpensive, imprecisely mass-produced steel needle with a point that quickly wore to fit the contour of the groove, but then went on to wear out in the course of playing one disc side, after which it was meant to be discarded and replaced. Unlike ordinary records, Vitaphone discs were recorded inside out, so that the groove started near the scribed synchronization arrow and proceeded outward. As one consequence, the needle would be fresh where the groove's undulations were most closely packed and needed the most accurate tracing, and suffering from wear only as the much more widely spaced and easily traced undulations toward the edge of the disc were encountered.
Initially, Vitaphone discs had a recording on one side only, each reel of film having its own disc. As the sound-on-disc method was slowly relegated to second-class status, economies were effected, first by making use of both sides of each disc for non-consecutive reels of film, then by reducing the discs to 12 inches (30 cm) in diameter. The use of RCA Victor's new "Victrolac", a lightweight, flexible and less abrasive vinyl-based compound, made it possible to downsize the discs while actually improving their sound quality.
There were exceptions to the 16-inch standard size of 1920s Vitaphone discs. In the case of very short films, such as trailers and some of the earliest musical shorts, the recording, still cut at 33 1/3 rpm and working outward from a minimum diameter of about 7.5 inches, was pressed on a twelve-inch or ten-inch disc when the smaller size sufficed.
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