This may have some merit. A few years ago we looked at software
based DTMF decoders that would meet CCITT spec's. What we found was
that using a FFT to get a power spectrum we needed a A/D converter
between 10 and 12 bits,  a processor with 10 MIPs and a hardware multiply.

DTMF generation in the same system was accomplished by synthesizing
the sigma-delta signal in software and sending it out a single bit with
a RC low pass filter. The sample rate on the sigma-delta transmitter
was 7200 Hz.

The whole application was written in C of course.

Walter Banks


> From: Peter L. Peres <plp@ACTCOM.CO.IL>
>
> Just a thought: How complicated would it be to implement a sigma-delta
> converter using the PIC itself, an analog switch and the preamp
comparator
> ? I think that the 2 component frequencies can be detected easier from
the
> differential of the signal envelope (which is the output of the
> sigma-delta) than from zero crossings. Perhaps without Fourier. If there
> would be enough time, then the PIC could manipulate the switch to cancel
> out one of the frequencies and detect the other by period measurement. I
> mean, a sigma-delta whose sample & zero frequency would be varied in
> suitable ways. Maybe using the differential AND the zero crossings would
> yield a better starting data set.