On Wed, 10 Sep 1997, Martin R. Green wrote:

> Actually, I thought the frequencies were chosen so no two DTMF frequencies
> are harmonically related.  Are you sure the beat frequencies are always
> different?  If so, it should be relatively easy to LPF your input (not BPF

        The first statement (I believe) implies the second. At any rate,
the beat frequencies are always different (the math to demonstrate that is
relatively simple -- anyone here should be able to do it).

> around the DTMF frequencies) so you get ONLY the beat, and check the
> frequency of that instead, as you suggest.  Of course, by using only one
> frequency, you run the risk of noise artificially triggering your decoder.

        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.

        Pierce Nichols