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Dual-tone multifrequency (DTMF) is a method used to dial telephone numbers or to issue commands to switching systems. DTMF is widely used for telecommunication signaling between telephone handsets and switching centers over analog telephone lines in voice-frequency bands.
DTMF is used in push-button telephones for tone dialing. This version of DTMF is an AT&T registered trademark and is called Touch-Tone.
DTMF signaling was developed to signal the destination telephone number of calls without requiring a telephone operator. It was standardized by the International Telecommunication Union (ITU) Telecommunication Standardization Sector recommendation Q.23.
DTMF tones are also used by cable television broadcasters to indicate the start and stop times of commercial insertion points during station breaks for cable company benefit. The frequencies used prevent harmonics from being incorrectly detected by receivers as other DTMF frequencies.
DTMF keypads are laid out on a 4x4 matrix, in which each row represents low frequency and each column represents high frequency. With DTMF, each key pressed on a phone generates two tones of specific frequencies. One tone is generated from a high-frequency group of tones, while the other is from a low-frequency group. DTMF systems use eight different frequency signals transmitted in pairs to represent 16 different numbers, letters and symbols.