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DTMF Frequencies

Dual-Tone, Multiple Frequency (DTMF) tones for touch-tone phones. When a keypad button is depressed, a dual-tone signal is transmitted according to the table below. For example, if the "5" is depressed, then both a 770 Hz tone and a 1336 Hz tone are sent simultaneously. What you actually hear are the two original tones at 770 and 1336 Hz, as well as the sum (2106 Hz) and the difference (556 Hz) frequencies. Fundamental tones are chosen such that these sum and difference frequencies never overlap.

The frequencies were initially designed with a ratio of 21/19, which is slightly less than a whole tone. The frequencies may not vary more than ±1.8% from their nominal frequency, or the switching center will ignore the signal. The high frequencies may be the same volume or louder as the low frequencies when sent across the line. The loudness difference between the high and low frequencies can be as large as 3 decibels (dB) and is referred to as "twist." The minimum duration of the tone should be at least 70 msec, although in some countries and applications DTMF receivers must be able to reliably detect DTMF tones as short as 45ms. On the receiving end, the tones are discriminated and decoded.

  1209 Hz 1336 Hz 1477 Hz 1633 Hz
697 Hz 1 2 3 A
770 Hz 4 5 6 B
852 Hz 7 8 9 C
941 Hz * 0 # D

RF Cafe: DTMF Keypad

Here is a standard DTMF touch tone keypad:

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About RF Cafe

Kirt Blattenberger - RF Cafe Webmaster

Copyright: 1996 - 2024

Webmaster:

    Kirt Blattenberger,

    BSEE - KB3UON

RF Cafe began life in 1996 as "RF Tools" in an AOL screen name web space totaling 2 MB. Its primary purpose was to provide me with ready access to commonly needed formulas and reference material while performing my work as an RF system and circuit design engineer. The World Wide Web (Internet) was largely an unknown entity at the time and bandwidth was a scarce commodity. Dial-up modems blazed along at 14.4 kbps while tying up your telephone line, and a nice lady's voice announced "You've Got Mail" when a new message arrived...

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