The DTMF decoders are implemented on digital signal processors. Goertzel algorithm is used for DTMF decoding. Goertzel algorithm is a digital signal processing technique for identifying frequency contents of a signal. Goertzel algorithm is used to recognize the tones produced by buttons pushed on the telephone keypad. The decoder for DTMF signaling is implemented by several band pass filters centered at the sinusoidal frequencies corresponding to the numbers in the DTMF keypad. Figure 1 indicates the decoded DTMF signal frequencies for key presses 0, 3, 2, 1, 8, 2, 1, 3, 2, 7, 9.

Figure 1: Implementation of Goertzel algorithm to recover DTMF signals for key presses 0, 3, 2, 1, 8, 2, 1, 3, 2, 7, 9.

At the end of the experiment, students are encouraged to perform dial-tone multi-frequency decoding for the keys pressed at the DTMF transmitter.