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From Wikipedia, the free encyclopedia
DTMF - Dual Tone Multi-Frequency. Used for telephone signaling over the line in the voice frequency band to the call switching center. The keypad is layed out in a 4 x 4 matrix with each row representing a 'low' frequency, and each column representing a 'high' frequency. Pressing a single key such as '1' will send the two frequencies 697 and 1209 Hz, thus giving rise to the multi-frequencies.
1 | 2 | 3 | A | 697 Hz |
---|---|---|---|---|
4 | 5 | 6 | B | 770 Hz |
7 | 8 | 9 | C | 852 Hz |
* | 0 | # | C | 941 Hz |
1209 Hz | 1336 Hz | 1477 Hz | 1633 Hz |
The tone frequencies were intentially designed to avoid harmonics or natural occuring frequencies that could occur when the two tones are sent. The frequencies may not vary more that +/- 1.5% from their nominal frequency, or the switching center will ignore the signal. The 'high' frequencies are as loud or louder as the 'low' frequencies when sent across the line. The loudness is referred to as 'twist' and can be as large as 3dB.
The keys A, B, C, and D all use the high frequency 1633 Hz were originally used in the U.S. military's Autovon phone system. Their original names were Flash Override (A), Flash (B), Immediate (C), and Priority (D). Pressing one of these keys gave your call priority, over riding other conversations on the network. Flash Override is the highest priority. Many non-telephone applications use these keys, such as amateur radio repeater signaling and control.
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