In telecommunications, the caller needs to dial the recipient's number. Earlier versions of phones had rotating dials that are now obsolete. Almost all landlines and cell phones now use push-button keypads.
Figure 1: Image of old and new landlines
What is DTMF?
DTMF is a signaling system for identifying the keys or rather saying the number dialed on a DTMF button or keyboard . Early telephone systems used pulse dialing or loop disconnect signaling. This has been replaced by multi-frequency (MF) dialing. DTMF is a multi-frequency tone dialing system used by push-button keypads on telephones and mobile devices to transmit the number or key dialed by the caller. DTMF allowed long-distance signaling of numbers dialed in the voice frequency range over telephone lines. This eliminated the need for a telecommunications operator between the caller and the receiver and developed automated dialing in telephone switching centers.
DTMF ( Dual Tone Multi-Frequency ) as the name suggests uses a combination of two sine wave tones to represent a key. These tones are called row and column frequencies because they correspond to the layout of a telephone keypad.
Figure 2: Image showing DTMF frequencies
The DTMF Keyboard (generator or encoder) generates a sinusoidal tone that is a mixture of row and column frequencies . Line frequencies are low group frequencies. The column frequencies belong to high group frequencies. This avoids misinterpretations of harmonics. Also the frequencies for DTMF are chosen so that none have a harmonic relationship with the others and that the mixture of frequencies does not produce a sum or product of frequencies that could imitate another valid tone. The frequencies of the high group (the speaker tones) are slightly higher than the low group to compensate for the high frequency drop of voice audio systems.
The row and column frequencies corresponding to a DTMF Keyboard have been indicated in the figure above.
DTMF tones are capable of representing one of 16 different states or symbols on the keyboard. This equates to 4 bits of data, also known as a nibble.
Most DTMF decoders can process at least 10 tones per second under worst-case conditions, so DTMF can easily transmit 40 (10 x 4) bits or 5 bytes of data per second, which is nowhere near the performance of a good communications modem , which can operate almost 600 times faster (28,800 bits per second). But DTMF signaling is much more robust in noisy line conditions.
It should be noted that the numbers and symbols on the keyboard do not always correspond to the binary values of DTMF decoders. Most notably, the '0' on the keyboard is represented in DTMF by a decimal value of 10 or a binary value of 1010.
The binary codes corresponding to symbols on a DTMF keyboard are listed below.
|
DECIMAL VALUE
|
BINARY VALUE
|
KEYBOARD SYMBOL
|
|
0
|
0000
|
D
|
|
1
|
0001
|
1
|
|
two
|
0010
|
two
|
|
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
|
W
|
Forms
DTMF tones are therefore mainly used in telephone switching centers to detect the dialed/called number. They are also used by certain radio and cable TV networks. These networks use DTMF tones to signal a local network station or cable operator when a local advertisement should be inserted or for station identification. In broadcasting, this is known as local insertion. DTMF tones have also been used by ground stations to turn remote transmitters on and off.