what is dtmf

Dual-tone multi-frequency signaling (DTMF) is a telecommunication signaling system that uses the voice-frequency band over telephone lines between telephone equipment and other communications devices and switching centers. It was first developed in the Bell System in the United States and became known under the trademark Touch-Tone for use in push-button telephones supplied to telephone customers, starting in 1963. DTMF works by assigning eight different audio frequencies to the rows and columns of the keypad, and each key press corresponds to two tones, one from the high group and one from the low group, allowing for 16 total keys. These tones are transmitted with the voice channel and are used to control automated equipment and signal user intent, such as the number they wish to dial. DTMF signals have the same frequency range as the human voice, so they can replace the intermediate operators who were necessary for transmitting long-distance calls. DTMF is standardized as ITU-T Recommendation Q.23 and is also known in the UK as MF4. DTMF can be transmitted over active telephone lines, the internet, and a radio.