What are DTMF tones?

What are DTMF tones?

Dual tone multi-frequency (DTMF) is a technique that has been used since 1963 to dial telephone numbers, and to send commands to switching systems. Prior to DTMF, telephone systems used a rotary dial, which was considerably less efficient, and as a result, soon after replacing the rotary dial, DTMF became the industry standard. Years ago, a lineman’s handset featured a rotary dial, but all modern handsets now use some configuration of the standard DTMF keypad.

Lineman and telephone testers routinely use DTMF for communicating between telephone handsets and switching centers over analog telephone lines, using only the voice frequency bands. DTMF is commonly used in telephones which have push button configuration, and by cable television broadcasters as a means of indicating the start points and end points for commercial insertion during station breaks. Because cable usage of DTMF stays within a specified set of frequencies, there is no concern about receivers incorrectly identifying them as other kinds of DTMF frequencies.

How DTMF works

DTMF keypads are configured in a 4 x 4 matrix, wherein all rows represent low frequencies, and all columns represent high frequencies. When any specific key is pressed on the keypad, two tones are generated, one being from the group of low frequencies and the other being from the group of high frequencies. Thus there can be eight different audio frequency signals transmitted in pairs to represent 16 different values, each of which might be a number, a symbol, or a letter. The values represented are the numbers 0 through 9, the letters A through D, and the two symbols ‘#’ and ‘*’.

Since the signals are audible tones which are all within the voice frequency range, they can all be sent over amplifiers and electrical repeaters, as well as over radio and microwave links. This strategy effectively eliminates the need to involve an operator when any long distance call is placed.

DTMF benefits

Compared to its rotary dial predecessor, DTMF is much simpler and much faster, and it provides other benefits as well. The reason two different tones are used to indicate a value is that it would be much more difficult to accidentally generate the sound and get an undesired result.

The DTMF approach also allows a caller to communicate with a computer system using the numbered keypad. For instance, all the menu options you routinely hear when calling a place of business are driven by recognizing the frequencies assigned to the specific number which the caller has entered.