DTMF Tones
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.
