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Does anyone know the correct percent increase for 33rpm to 45rpm and 78rpm?? I have the preset for the 33 to 45, but it seems to be incorrect. It is set for a 74.07% increase and that is too fast. I am guessing that it should be about 35%. As for the 33 to 78, there is no preset. Thanks!
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Re: Correct speed increase for 33 to 45 and 78 is?
If you have played a 45rpm disc at 33.3333 rpm, you need to make the new file size smaller than the original. 33.33333/45 = .740740... so the new file should be ~74% as long as the original file.
33.33333/78.26 = .42593 (the new file should be .42593 x as long as the original file).
Of course not all "78 rpm" discs were recorded at exactly 78.26 rpm... so you may need to listen and tweak. In fact, if you know the key of the original composition, you could "re-tune" it to the correct key by varying the multiplier.
I have done this by generating a sine wave tone at the exactly correct frequency (for the composition's key), superimposing that at a low level (~~ -10 dB) over the audio, and listening for beat notes, the way a piano tuner listens for beats between the tuning fork and the string. Then you can tell whether your audio is sharp or flat, and correct the speed accordingly.
To find the correct frequency, start with A = 440 Hz. Each half step up is multiplied by 1.0594631 (2 raised to the 1/12 power, since an octave is a frequency ratio of 2:1, and there are 12 steps of equal ratio in an even-tempered octave). In other words, C would be 440 x 1.059 x 1.059 x 1.059 = 523.25 Hz (that's the C one octave above middle C; middle C is 261.626 Hz).
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