Re: [Asterisk] DTMF noise

[Thomas Tonino <ttonino@users.sf.net>]

Mark Spencer wrote:

> The DTMF detector in the linux kernel is fairly simplistic and doesn't do
> many relative energy tests.  The Zapata library has a much better tone
> detector, but it is FP, and so would have to be made fixed point.  If
> nothing else, it may provide some lessons for the ISDN folks.

I remember that a good DTMF decoder can be very simplistic: DTMF was designed 
for that.

The idea is:

- separate the high tones from the low tones.
- amplify clip the high band and the low band separately
- run the tone decoders on the clipped signals

The clipping stage would make sure that only relatively pure tones will trigger 
the detector.

See also http://groups.google.com/groups?selm=7462%40accuvax.nwu.edu


Thomas