Why 33 / 45 / 78 RPM ?
24 December 2008
from: Explanations of why certain speeds were chosen for certain types of phonograph recordings...
The original gramophones used a steel sewing needle for a stylus. The stylus determined the size of the grooves in the record and the recordable frequency range limited by this groove size determined a speed between 70 and 90 rpm. This was standardized in the 1930s at 78rpm, although actual play speeds varied depending on the frequency of your AC mains : 77.922 rpm in Europe (50 hertz) and 78.261 in the US (60 hertz).
33 1/3 rpm was chosen in the 1920s so that playing time of a side matched the playing time of a reel of film (11mins). The speed was also a result of J.P. Maxfield's calculation of the optimal compromise between signal-to-noise ratio and playing time. His calculation determined 3 minutes per radial inch. In the 1940s Columbia adopted this speed for its vinyl micro-groove records.
RCA Victor developed the 7inch 45rpm single in 1948. Optimal use of a constant speed disc occurs when the innermost groove is half the diameter of the outermost groove. Thus a fat 3.5 inch label.
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