User Manual
75
Chapter 4—AT Commands, S-Registers, Result Codes
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:
tigiD
woL
qerF
hgiH
qerF
1zH796zH9021
2zH796zH6331
3zH796zH7741
4zH077zH9021
5zH077zH6331
6zH077zH7741
7zH258zH9021
8zH258zH6331
9zH258zH7741
0zH149zH6331
*zH149zH9021
#zH149zH7741
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).
Dual Tone Multi-frequency (DTMF) was a technique proposed by the phone company to replace “pulse
dialing” to make dialing faster and more reliable. In anticipation to adding other “customer services”, 6
additional digits were included, totaling 16 digits. Most commercial phones only include 12 digits. Many
specialized phones and telephone equipment (such as ADSI phones, PBXes, etc.) utilize all 16 digits.
DTMF was first introduced in the 1960s to the general public as “touch tone” dialing. The conversion from
“pulse dialing” to “touch tone” dialing took over 20 years in the US. To this date (and in the foreseeable
future) all phone companies support pulse dialing, and most phones you buy still allow you to switch to
pulse. In the 80s, DTMF found a complete new use as a “key pad” for voice mail and interactive voice
response systems. Since these systems have become a way of life now, DTMF has become the most
common “man machine” interface.