John Erdmann wrote:
>With regard to the three record speeds, is it just a coincidence that
>33 + 45 = 78?
Pretty much. 33 is really 33 1/3 and 78 was really 78.26 at the end of that era.
33 1/3 originated as part of the Vitaphone sound for picture system. A 16" 33 1/3 disk fit into a film can and ran the proper time for a reel of 35mm. film. The Scully electrical lathe was part of the same system and they all had the 33 1/3 option. The revolution of the LP was the microgroove leadscrew and styli.
45 RPM was RCA's answer. They claimed, justifiably, that it was the slowest speed that permitted full response in the inner grooves. Of course this was at the expense of giving up the outer grooves of the 12" LP.
As an experiment I once cut a half-speed microgroove 78 rpm acetate on the back of something we were throwing away. The quality was amazing.
Bob Olhsson Audio Mastery, Nashville TN
Mastering, Audio for Picture, Mix Evaluation and Quality Control
Over 40 years making people sound better than they ever imagined!