About dtmf – Multi-Tech Systems ISI5634PCI/4/8 User Manual
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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 telephones touch keys.
In the United States and perhaps elsewhere, its 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 cant 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:
ti
g
i
D
w
o
L
q
e
r
F
h
g
i
H
q
e
r
F
1
z
H
7
9
6
z
H
9
0
2
1
2
z
H
7
9
6
z
H
6
3
3
1
3
z
H
7
9
6
z
H
7
7
4
1
4
z
H
0
7
7
z
H
9
0
2
1
5
z
H
0
7
7
z
H
6
3
3
1
6
z
H
0
7
7
z
H
7
7
4
1
7
z
H
2
5
8
z
H
9
0
2
1
8
z
H
2
5
8
z
H
6
3
3
1
9
z
H
2
5
8
z
H
7
7
4
1
0
z
H
1
4
9
z
H
6
3
3
1
*
z
H
1
4
9
z
H
9
0
2
1
#
z
H
1
4
9
z
H
7
7
4
1
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).