At 09:12 12/19/98 -0800, Scott Dattalo wrote:
>You may wish to look at Analog Devices' DSP apnotes. They discuss this
>more thoroughly. Their solution is to compute 16 DFT's: 8 at the DTMF
>tones and 8 at the 2nd harmonic of DTMF tones. The second set presumably
>allows one to ascertain whether a 'pure' DTMF tone was present or whether
>the DTMF energy is a harmonic of some lower frequency present signal. On
>comp.dsp I have read that people have found this technique to be flawed.
>But I haven't tried it my self (at least not as described here).

if the goal is to find out whether a certain filtered band is a base
frequency of a tone or a harmonic of some other, the second harmonic is not
very suitable, because it would be part of both, whether the filtered
frequency is a base frequency or a power of two harmonic of some other
frequency -- unless you can make sure the DTMF tones are pure sine. i'd
guess that is rarely the case.

ge