Dual-Tone Multi-Frequency

DTMF, or Dual-tone multi-frequency, is a type of signal used for telephone tone dialing. This type of dialing was trademarked as Touch-Tone, although it is no longer widespread today after it was canceled in 1984. Meanwhile, other telephone networks implement an internal multi-frequency system.

History of DTMF

Before DTMF became prevalent, telephone systems used the pulse system, also known as Dial Pulse in the US, and the loop disconnect in dialing telephone numbers. The latter works the same way like that of switching on and off a light switch as it connects and disconnects the line of the calling party. In this process, the dial pulses are counted to identify the caller number. However, this system proved to be inefficient for long distance calls as telegraphic distortion were recorded.

This is why the Dual-Tone Multi-Frequency was developed to instruct the telephone switching system of the number to be dialed. The DTMF uses eight different frequency signals that are transmitted in pairs.

More on Tone Frequencies

The eight DTMF tones basically represent the tones you hear whenever a standard telephone keypad is pressed. For each button, it produces a tone which is the sum of the row and column tones, hence, the term “dual tone”. For standard telephones, there are no ABCD keys.

The standard DTMF tones have 16 keys; however, most telephones nowadays only use 12 out of these 16 keys. The other unused tones are still being used by telephone networks.

The difference in loudness for each frequency can go up to 3 decibels (dB), and is also known as twist. Each tone can run to at least 70 milliseconds, although it can also run to as low as 45 milliseconds, depending on the country and the type of DTMF receiver application.

Widespread Use of DTMF Today

Aside from telephone signaling, DTMF is also used for cable television broadcasting. This helps in signaling the start and stop times for insertion of local commercials during program breaks. Commercially speaking, this benefits the cable companies.

The DTMF tone sequences used to be widespread in cable channel companies all over the United States and in other parts of the world. However, the use of this method began to experience a slow decline with the advent of more advanced out-of-band signaling methods at the start of the 1990s.