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Date: Wed, 08 Jan 2003 06:43:10 -0600
From: "David D. Hagood" <wowbagger@sktc.net>
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To: Thomas Tonino <ttonino@users.sourceforge.net>
CC: Roy Sigurd Karlsbakk <roy@karlsbakk.net>,
       linux-kernel <linux-kernel@vger.kernel.org>
Subject: Re: [Asterisk] DTMF noise
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Thomas Tonino wrote:
> Roy Sigurd Karlsbakk wrote:
> 
>> so - we DO NOT need a 'simplistic' DTMF decoder.
> 
> 
> You need a good one. But good can be simplistic, is what I'm saying.
> 
> DTMF was designed to be easy to decode reliably. Complex doesn't 
> automatically mean better.
> 

I haven't looked at the code, but I'd recommend using a bank of Goertzel 
filters -


http://www.google.com/search?hl=en&lr=&ie=ISO-8859-1&q=Goertzel+filter+DTMF&btnG=Google+Search

The basic idea is that you have 8 filters (for the 4 row and 4 column 
frequencies), as well as 8 filters looking at the first harmonic of the 
8 frequencies. You then compare the energies in each frequency - if you 
see significant energy in the harmonic filter bank, discard the signal. 
That prevents you from detecting speech as DTMF, since speech will 
usually have harmonics that a good DTMF signal won't.

Since the Goertzel filters are simple, they can be implemented in fixed 
point math rather than floating point. At work, we've done this on a 
Motorola 56301 DSP, which is a fixed-point DSP. I think there's an app 
note from Moto on this - I'll check when I get into work today.

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