On Mon, 8 Jun 2009 19:47:29 -0700 (PDT), tonyjeffs wrote:

If you play them back at 45 and use software to speed the recordings
up to normal, you will lose the top frequencies.

Explain?

Original recorded 78 sound = 5kHz
Played at 78, frequency reproduced = 5kHz

Played at 45, frequency reproduced = 2.88kHz
Digitally speed up by 1.73, frequency = 4.98kHz

Close enough for Jazz...

--
Cheers
Dave.