DTMF decoder component

DTMF is a in-band signalling method which uses voice-frequency to instruct a telephone switching system of the telephone number to be dialed, or to issue commands. It was originally developed and used in the 1950s in AT&T to direct calls between switching facilities. Modern applications of DTMF also include caller ID transfer, menu selections and others.

DTMF is sometimes called multifrequency signaling or multifrequency pulsing because it uses a pair of frequencies to form a signal which will represent the key pressed or command issued.

	1209 Hz	1336 Hz	1477 Hz	1633 Hz
697 Hz	1	2	3	A
770 Hz	4	5	6	B
852 Hz	7	8	9	C
941 Hz	*	0	#	D

More about DTMF:
http://en.wikipedia.org/wiki/DTMF

Software decoding of DTMF signals

Software recognition of keys encoded as DTMF is a digital signal processing (DSP) technique, which identifies original frequencies of a signal. Usually it is a variation of general Fast Fourier transform (FFT) algorithm.

VC components use algorithm published by Dr. Gerald Goertzel in 1958 for DTMF decoding. A practical application of this algorithm is to recognize the tones produced by the keys pressed on a telephone keypad.

The vcDTMFdecoder demo shows how to use the TunavclDTMFDecoder class to decode DTMF signals. When activated, the components analyzes incoming audio stream and tries to recognize presence of DTMF frequensies.

When DTMF key is detected, the onDTMFCodeDetected event is called, so immediate action can be taken.

More about Goertzel algorithm:
http://en.wikipedia.org/wiki/Goertzel_algorithm

Download

Precompiled binary of this sample is included in the Demos package.