In the early 1960s, AT&T introduced a method of providing
in-band signaling so telephone customers could place telephone calls.
The system was called dual-tone multi-frequency, or DTMF. AT&T,
through its Bell divisions, marketed the technology under the name
The DTMF system uses eight different frequency signals transmitted in
pairs to represent sixteen different numbers, symbols and letters. The
table below shows how the frequencies are organized. These frequencies
were chosen to prevent harmonics from being incorrectly detected by the
receiver as other DTMF frequencies.
The frequencies along the top are referred to as the High Group. The
frequencies along the left column are called the Low Group. By
transmitting one frequency from each group, a corresponding DTMF is
ANSI standards state that DTMF tones used in telephony must meet the
The DTMF duration must be at least 50ms.
The inter-digit interval (silence) between tones must be at least
The receiver should receive the DTMF digits with a signal strength
of at least -25dBm and no more than 0dBm.
The energy strength of the high-group frequency must be -8 dB to +4
dB relative to the energy strength of the low-group frequency as
measured at the receiver.
When the level between the two frequencies is not equal it is said to
have a twist. Most modern equipment is fairly tolerant of
unbalanced levels between tones.
The buttons below are hyperlinked to WAV files of their corresponding
Audio samples created by Carla Kay Barlow.
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