dtmfdecode is part of the isdn4bsd package and is used to detect DTMF tones in the audio stream.
It reads audio G.711 A-law coded data from stdin and outputs the detected numbers values as ASCII characters to stdout.
The detector is implemented as 8 narrow band-pass filters realized with an integer double-cross recursive algorithm. Various ad-hoc methods are employed to provide hysteresis and anti-bounce for the detected signals.