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DTMF (Dual Tone Multiple Frequency)

Table of Contents:

Written By: 

Nikhil Agnihotri

Most DTMF decoders can process at least 10 tones per second under the worst of conditions, so DTMF can easily convey 40 (10 x 4) bits or 5 bytes of data per second which is nowhere near to the performance of a good communication modem, which can operate nearly 600 times faster (28,800 bits per second). But DTMF signalling is lot more robust under noisy line conditions.

It should be noted that the numbers and symbols on the keypad do not always match the binary values of DTMF decoders. Most notably, the ‘0’ on the keypad is represented in DTMF by a decimal value of 10 or binary value of 1010.

The binary codes corresponding to symbols in a DTMF keypad are listed below.
 
DECIMAL VALUE
BINARY VALUE
KEYBOARD SYMBOL
0
0000
D
1
0001
1
2
0010
2
3
0011
3
4
0100
4
5
0101
5
6
0110
6
7
0111
7
8
1000
8
9
1001
9
10
1010
0
11
1011
*
12
1100
#
13
1101
A
14
1110
B
15
1111
C

Applications
DTMF tones are thus mainly used at the telephone switching centres for detection of dialled/called number. They are also used by certain radio and cable TV networks. These networks use DTMF tones to signal a network station or local cable operator when a local advertisement is to be inserted or for station identification. In broadcasting, this is known as local insertion. DTMF tones were also used by terrestrial stations for turning on and shutting off remote transmitters.

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