Why are turntable speeds 78 RPM, 45 RPM, etc?
The speeds were chosen because that is the speed that resulted when you used standard parts. Electric motors rotate at 1800 rpm, most shafts are 1/4″. Those combinations with the proper gears and idlers came out to 78 rpm. In reality it’s 78.26 rpm. Tape recorder speeds evolved the same way. The 78.26 was standardized after electric recording/playback occured. Prior to that, speeds were “in the neighborhood of” 78 rpm. Some lower and some higher. 80 rpm was used in many recordings.
