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DTMF - Dual Tone Multi-Frequency
2 March 2006

DTMF was invited by the world famous telephone research institution in the USA called Bell Laboratories (part of American Telephone & Telegraph) in the late 1950's. Bell Labs also invented the transistor. When AT&T was split into smaller companies, Bell Labs was closed down.

DMTF is used for transmitting digits, usually telephone numbers, by audible tones.

DTMF is a two simultaneous tones signal. In a 2 tone DTMF system, each digit, letter or symbol is represented by two audible tones. For example a 5 is represented by tones 770 Hz and 1,336 KHz.

 

1209 Hz

1336 Hz

1477 Hz

1633 Hz

697 Hz

(1)

(2)

(3)

A (12)

770 Hz

(4)

(5)

(6)

B (13)

852 Hz

(7)

(8)

(9)

C (14)

941 Hz

(10)

(0)

(11)

D (15)

The figures inside brackets are the decimal codes. Flash or called Timed Break Recall (TBR), which always lasts 90 milliseconds, is decimal code 16.