Multi-Tech Systems ISI5634PCI/4/8 manual About DTMF

ISI5634PCI/4/8 User Guide

About DTMF

DTMF (dual tone multi frequency) is the signal to the phone company that you generate when you press an ordinary telephone’s touch keys. In the United States and perhaps elsewhere, it’s known as “Touchtone” phone (formerly a registered trademark of AT&T). DTMF has generally replaced loop disconnect (“pulse” or “rotary”) dialing. With DTMF, each key you press on your phone generates two tones of specific frequencies. So that a voice can’t imitate the tones, one tone is generated from a high-frequency group of tones and the other from a low frequency group. Here are the signals you send when you press your touchtone phone keys:

Digit	Low	High
	Freq	Freq
1	697 Hz	1209 Hz
2	697 Hz	1336 Hz
3	697 Hz	1477 Hz
4	770 Hz	1209 Hz
5	770 Hz	1336 Hz
6	770 Hz	1477 Hz
7	852 Hz	1209 Hz
8	852 Hz	1336 Hz
9	852 Hz	1477 Hz
0	941 Hz	1336 Hz
*	941 Hz	1209 Hz
#	941 Hz	1477 Hz

When any key is pressed, both the high and low tones of of the row are generated, hence the name “dual tone”. For example, pressing the ‘5’ button generates the tones 770Hz and 1336Hz. The frequencies were chosen to avoid harmonics (no frequency is a multiple of another, the difference between any two frequencies does not equal any of the frequencies, and the sum of any two frequencies does not equal any of the frequencies).

94