> > This may have some merit.
> > A few years ago we looked at software based DTMF decoders
> > that would meet CCITT spec's.
> >
> FFT's and A/D converters certainly require this power. One of the
> beauties of the zero-crossing you don't have to worry"too" much
> about the dynamic  range of the signal.
> Essentially what's needed is an AGC stage.
>
> I've got a 'sigma-delta' A/D converter on my web page (which
> coincidently, was introduced to me by Walter).
> The problem is that it's SLOW.

Sigma-delta is slow for a single conversion but after that it
is a continuos conversion on each update Sigma-delta has
a large dynamic range which is why we used it. It is quite
easy to get about 12 bits of conversion. The frequency
response of continuos SD is a function of sample frequency..
What you have is a fast converter whose results are time shifted.
Testing a single bit on one of the accumulation registers
gives zero crossing detection. Then use the zero crossing
algothrim to decode the DTMF tones.

As several PICster's have pointed out software DTMF decoding
that meets CCITT spec's is mostly the ability to deal with large
dynamic ranges in the incoming signal.

> Then thrown in the sea afterwards? :)

No far too useful to discard. Portable on any processor with the required
spec's It was actually developed and used in commercial modem
software by our sister company.. :))


Walter Banks