I've never done much with audio, but here are my initial thoughts:

* Does it have to be mp3?  I'd use the much simpler .wav (google .wav  
spec to see how to implement it)

* Does it have to be a USB stick?  I'd favor using a CompactFlash  
card.  Again, the spec is much simpler to implement.  see "PIC a  
CompactFlash Card" by Mark Samuels.

* It'll be tricky to get two microphones recording at the same time  
through the same PIC to the same CF card.  It'd be easier and cheaper  
to make two boards (the biggest sink is going to be your time and the  
PCBs anyway, and if you use a PCB service you'll have several of  
those, anyway).

* I know there are cheap high-sampling rate audio ADCs... check them  
out on digikey.

* I don't know what kind of pre-processing you'll need to do to the  
Mic inputs...

* By the time you get through all of this, are you really going to be  
beating (by whatever metric you're trying to beat them) a cheap-o  
digital audio recorder?

Regards,
Matt


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