<html> Hi Sarah, <blockquote type=cite class=cite cite>Actually the lonlinearities would have to be in the playback process, not the recording process.   I'll point out that 20 and 20.5 kHz wav files are somewhat suspicious, considering that those frequencies are so close to the Nyquist rate (max sampling rate before aliasing).  It is possible that 500 Hz heterodyne products are being created as a result of sampling peculiarities.  </blockquote> I agree- it would be in the playback.  I used a high sampling rate to try to eliminate the possibility of interference with the usual 44.1 kHz CD sampling rate. Zooming in on the waverform view in Cool Edit (the program I used to create the file) shows 24 samples for each 5 cycles, so this verified the 96kHz sample rate. I don't really know how the computer DACs deal with this, though.  Maybe there are distortions taking place there, if they cannot deal directly with the 96kHz sample rate.  But the 500hz tone is easily identified, sounds like a sine wave, and the pitch is spot on, as verified by tunelab (thanks, Bob!) on my Pocket PC. Ric does come up with some fun questions, doesn't he? Don Mannino</html>