Dual
Tone Multi-Frequency is a format for transmitting data by telephone
and radio. The tones you hear when you press the keys on your
phone are the DTMF tones. It was first designed by engineers
at Bell Labs for sending data across long distances over a variety
of systems.
The diagram below shows the format of DTMF (not that you need
to know this unless you enjoy the technobabble). Each digit
is represented by two tones, determined by the intersection
of the row and column where the digit sits. Two tones are
used as opposed to one to provide protection against false
digits appearing during transmission due to noise.
| 1 |
2 |
3 |
A |
697Hz |
| 4 |
5 |
6 |
B |
770Hz |
| 7 |
8 |
9 |
C |
852Hz |
| * |
0 |
# |
D |
941Hz |
| 1209Hz |
1336Hz |
1477Hz |
1633Hz |
|
An example of DTMF tones in use would be, when your call
is answered by a recorded message which instructs you to "press
a key" to be transferred to a particular department or
extension. It is also used to allow users to enter, for example,
their credit card number to obtain a balance, when calling
your credit card company.
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