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

WAV is not a format, it's a container. In fat, a .wav file can contain 
MP3 data in it.

I'd use ADPCM format (of which there are a number of variations, pick 
one that's well supported), in a WAV container. Offers decent 
compression compared to raw PCM with minimal quality loss, and it's easy 
to implement.

> * 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.

Yep, USB is going to be really annoying. USB hosts are hard to 
implement, and not a good idea for a portable recorder - you'll end up 
needing a powerful microcontroller just for the USB. Using memory cards 
(CF or SD) is probably a much better idea.

> * 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 don't see why two mics would be a problem. Most everything these days 
comes in dual versions anyway (stereo sound). All you need to do is 
encode samples from each channel and interleave them.

> * I don't know what kind of pre-processing you'll need to do to the  
> Mic inputs...
As far as I know, electret microphone capsules are not hard to drive. An 
op-amp or two should suffice to condition the signal for the ADC.

-- 
Hector Martin (hector@marcansoft.com)
Public Key: http://www.marcansoft.com/marcan.asc

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