The DTMF encoding

The DTMF encoding system has become very common in telephones where it has replaced the old pulse dialer. But even on the air the DTMF encoding is widely used, for example, for the tone squelch.

A DTMF telephone keyboard

DTMF means Dual Tone Modulation Frequency. This systems is intended to transmit keys pressed on a keyboard through an audio channel such as a telephone line or a radio connection. Every time a key is pressed two audio frequencies are transmitted: one corresponds to the column in which the key is in, and the other one corresponds to the row. This encoding handles a maximum of 4 rows by 4 columns, that means 16 keys (from 0 to 9, *, #, and from A to D). The frequency corresponding to the row is emitted with less amplitude than the higher one, and precisely of 2 dB (that means that the amplitude is 1.259 times smaller).

Colimn 1: 1209 Hz (min 1189 Hz, max 1229 Hz) Colimn 2: 1336 Hz (min 1314 Hz, max 1358 Hz) Colimn 3: 1477 Hz (min 1453 Hz, max 1501 Hz) Colimn 4: 1633 Hz (min 1607 Hz, max 1659 Hz) Rows are -2 dB than columns
1 2 3 A Row 1: 697 Hz (min 685 Hz, max 709 Hz)
4 5 6 B Row 2: 770 Hz (min 756 Hz, max 784 Hz)
7 8 9 C Row 3: 852 Hz (min 837 Hz, max 867 Hz)
* 0 # D Row 4: 941 Hz (min 925 Hz, max 957 Hz)

This table shows graphically the layout of the keys on the DTMF keyboard with the corresponding frequencies and the minimum and maximum values for each tone. Wave files containing the corresponding tones for each key can also be directly downloaded.

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Bibliography and further reading

[1] A. Sbrana. Decoder DTMF. Progetto-Elektor, N.10, Ottobre 1988, pp. 64-67.