Russell McMahon wrote:
>> Basically multiply the incoming signal by the sine and cosine of
>> each of the DTMF frequencies. Add the products for each frequency,
>> then low pass filter those sums individually....
>
> Fourier would be proud.
> (SFT strikes again :-) )

I'm not sure what your point is.  I'm assuming SFT is supposed to mean Slow
Fourier Transform, as apposed to FFT (Fast Fourier Transform)?  Do you see a
way to apply the FFT algorithm to checking for the existance of a few known
frequencies that aren't in a nice evenly spaced progression?

Remember that the FFT algorithm is a very clever optimization for performing
time domain to frequency domain conversion under just the right
circumstances.  It's not clear to me the circumstances apply here to the
point where doing a FFT would be less computation than what I described.

What I described requires 16 multiplies per sample, plus some low pass
filtering and of course the integer indexing and bookkeeping operations.  If
this is done on a machine with a hardware multiplier, these really aren't a
big deal.


********************************************************************
Embed Inc, Littleton Massachusetts, http://www.embedinc.com/products
(978) 742-9014.  Gold level PIC consultants since 2000.
-- 
http://www.piclist.com PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist