Syndicated radio programming was distributed on 78 rpm discs beginning in 1928. The desirability of a longer continuous playing time soon led to the adoption of the Vitaphone soundtrack disc format. 16-inch 33⅓ rpm discs playing about 15 minutes per side were used for most of these “electrical transcriptions” beginning about 1930.
Transcriptions were variously recorded inside out like soundtrack discs or with an outside start. Some were recorded with a vertically modulated “hill and dale” groove, as this was found to allow a wider dynamic range and an extension of the high-end frequency response, not necessarily a great advantage in practice because of the limitations of AM broadcasting.
Initially, transcription discs were pressed only in shellac, but by 1932 pressings in RCA Victor’s vinyl-based “Victrolac” were appearing. Other plastics were sometimes used. By the late 1930s, vinyl was standard for nearly all kinds of pressed discs except ordinary commercial 78s, which continued to be made of shellac.