> ...the sound is after all coming *from* my PC. ;-)

Oh I mistakenly thought I read somewhere that you had your collection on 
real CDs.  Anyway, the difference is just one parameter in an API call.

> The logical thing to do with be to decode the Vorbis files back into WAV 
> files and analyse the resulting spectrum.

Would be more flexible to just read the mixed output from the soundcard, 
that way your program would calculate the BPM of whatever music your 
computer happened to be playing.  You wouldn't have to worry about decoding 
various formats either then.

> Although this is far, far less trivial than you'd imagine. (E.g., WinAmp 
> can't reliably decide what the beat rate is either, mainly because it 
> can't "hear" the beats reliably.)

Yeh I guess you'd need a good algorithm for detecting beats - presumably 
some kind of autocorrelation function on certain parts of the spectrum, easy 
to do both in one shot with FFTs), seems right up your street :-)

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