> I also thought of implementing a DTMF decoder with the '71 (with the built
> in A/D).  I soon realized that the limited ram on these chips severely
> restricts the resolution of any FFT program.
>
> You may want to use a PIC controlled bandpass filter and sweep
> the pass band around.  Have the PIC output a frequency that is multiplied
> by the incoming signal using a 4 quadrant multiplier.  Low pass filter the
> output of the multiplier and peak detect the result.  This can serve
> as the amplitude of your signal.
>
Edward,

Thank you for the suggestion.  I did a little more reading about the
digital filter routines written for the 17c, and I began to realize that
I don't have a hope in digitally processing above 5 kHz.  As a result,
I'll have to do some off pic work.

I like your idea of controlling a bandpass filter with the pic.  This
sounds like a really good method to do what I want.

Yours,
--
Derrick Early
early@finite.nrl.navy.mil