Michael Watterson wrote:
> Decoding in SW isn't simple. there is a a couple of DSP shortcuts
> instead of a full FFT.

Yes, but of course nobody would attack this by doing a "full" FFT.  That
would be silly.

You can do the detecting by multiplying by sine and cosine of each tone
frequency plus some filtering and level detection.  That's 32 multiplies
each sample, which is well within the capability of even low end DSPs or
possibly even something like a PIC 18.  Certainly a cell phone processor is
up to the task.  The question is whether it has access to the audio stream
in the right format with real time capability or enough buffering.

A while back wanted to prove the sine/cosine algorithm for detecting narrow
band tones.  I set it up to detect one of the DTMF tones and then fed it
three adjacent DTMF tones in sequence with the one it was supposed to detec=
t
in the middle.  Each tone and space was 50ms, which is the minimum a DTMF
decoder must be able to react to.

A plot of the result is at http://www.embedinc.com/temp/dtmf.gif, and the
source code that produced that is at http://www.embedinc.com/temp/dtmf.pas.


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