[The 'conjecture' is that there exists a DTMF decoding solution simply by
sampling the output of a comparator.  A further implied condition, known
as the PICLIST criterion, is that such a solution is implementable on
a PIC microcontroller and that source code is forthcoming...]

> From: Scott Dattalo <sdattalo@UNIX.SRI.COM>
>
> [cut]
> Actually Chuck, I think this is problem that can be solved without
> resorting
> to analog filters. It is true that the phase between the sine waves will
> vary and this will likewise cause the zero crossing points to wiggle
> around.
> However IMO I think you can look at say 10 or 20 milliseconds worth of
> zero crossing information and still extract the high and low tones. Stay
> tuned.

Something tells me you're going to have a hard time doing this, Scott.
For a start, there are an awful lot of unknowns (including the frequencies).
If you don't know a-priori what the relative signal strengths, phases, noise
and slight errors in frequencies are going to be, there's just too much that
can go wrong with that nice bit stream.

Another problem: your 'sampling rate' is effectively fixed at an average
of 2 times per cycle - exactly the Nyquist limit.  Digital filtering can
oversample thus allowing some tradeoffs to be made.

If you are after a _practical_ solution i.e. doesn't use more registers
than are available in the typical, say, 16C84; and doesn't take more than
50ms of sampling; and is reasonably immune to all the variables listed above;
then I think real analogue sampling is required.  Digital filtering in
some form or another, I would be willing to bet, would be the most robust
method.

One of the most important properties of a practical decoder would be the
ability to determine when there is _not_ a DTMF signal.  Digital filtering
can easily determine this situation, whereas your zero-xing solution (if
one exists) would have more difficulty e.g. it is fundamentally insensitive
to the amplitude of the incoming signal.

Regards and good luck,
SJH
Canberra, Australia