DTMF tones are most commonly used for sending commands over the air, including repeater control, autopatch access and IRLP access. Mobile rigs tend to have the keypad built into the microphone and handheld radios have a keypad on the front of the rig. Most modern FM radios include a DTMF encoder as part of the radio. The frequency accuracy is specified at 1.5%, fairly tight, to keep the tones separated. These frequencies were carefully chosen so that no frequency is a harmonic of another, which would have increased the possibility of a decoding error. The amateur radio world makes use of a fourth column of keys labeled A, B, C and D to provide some additional signaling options. Most telephones will just have the keys for 0 through 9, * and #. The frequencies listed in the left hand column are called the “low group”, while the frequencies shown on the top are the “high group”. Table 1: The matrix of DTMF keys and frequencies.įor example, pressing the number 6 produces these two frequencies: 770 Hz and 1477 Hz.
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