Pierce Nichols wrote:

>         You can deal with the noise problem by measuring the beat
> frequency over several cycles -- the DTMF spec also supplies a minimum
> pulse length, and it's long enough so that the code is extremely robust.

And in a personal message to William Chops Westfield, I wrote:

As far as the idea of the beat frequencies... some one clearly pointed
out that this will NOT work because the 780Hz beat frequency shows
up for two tone combinations. However, one could possibly do a beat
frequency detection and perhaps the lower half of the DTMF tones.
But even that sucks. Another problem with beat frequency detection
idea is that some of the beat frequencies are higher than some the
DTMF frequencies: e.g. the 697 1633 beat frequency is 934hz. Yet
another problem is that the number of beat frequencies is 4*4 =16
(excluding the redundancies of course) as opposed to the simpler 4 + 4
DTMF tones.

The whole idea of beat frequencies is useful for the error analysis
of the 1-bit A/D converter algorithm though. But it's unfortunately
beyond the scope of this thread, and not to mention beyond the
availability of my time, to describe here. However, one of these
days it will get added to my web page......

Scott