DTMF recognition

[rknobbe]

For the spy crowd there -- how hard would it be to use the microphone to record an audio sample then parse that audio sample for DTMF tones (then printing out the equivalent keypad numbers).  Seems you would need to do a fast fourier transform to get the tone frequency, don't know if the typical rockbox (like my e260) would have the horsepower.

Anybody know how hard this would be?

Perhaps the http://en.wikipedia.org/wiki/Goertzel_algorithm Goertzel algorithm would be better suited for DTMF?

[saratoga]

Anybody know how hard this would be?

You want to record PCM, and then FFT it to find out which two frequencies in contains?  Thats trivial for compared to actually decoding compressed audio.

[Multiplex]

You probably want to do it in real time, and DFTs like Goertzel are often used, there is plenty of stuff available, the 'interesting' bit is discerning tones from other noise.

There have also been discussions about Guitar Tuners, as well as more general purpose Frequency analysers.