On Friday 06 April 2001 23:57, Tim Goetze wrote:
> hi chris,
>
> >effort, just detecting whether a particular tone pair (DTMF) or
> > subaudible tone is part of the audio input.
>
> i'd think of two ways to do that: either do a fourier transform
> (fftw software package for example) and check for those
> frequencies' amplitudes that you're looking for, or apply a
> bandpass filter for every frequency and check the filtered signal
> amplitudes.
>
> better suggestions anyone?
We're talking about a 4x4 matrix of frequencies within a rather small
range, and we'll always see two of them ("column" + "row") at a
time... BP filters may well be sufficient, and moderate Q should be
sufficient, as we only need to detect which 2 of the 8 frequencies
are currently present.
There may be some "real" shortcuts as well, as we're dealing with
multiple periods of sine waves. (And some distortion + lots of noise,
of course...) No ideas right now, though, short of
1. Remove the DC (simple HP filter)
2. Boost and clip the signal to 1/0/-1 (three levels)
3. Analyze the frequency and pulse width phenomena of
the resulting rectangular wave. (Best match of 17
possible patterns, including silence.)
Might work; haven't tried it. :-)
//David
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