Each time you hold down a key on your push botton telephone set a pair of audio frequency signals is transmitted over the telephone voice channel. Central-office switching facilities decode these tones and connect the desired circuits based on the sequence of tone pairs received. Each tone must last long enough and there must be adequate separation between them. A tone pair duration of about 150 ms and a separation of about 75 ms works.
Each of these tones is composed of two pure sine waves of different frequencies superimposed on each other. These two frequencies explicitly represent one of the digits on the telephone key-pad.
The telephone key-pad can be thought of as a 4 row by 3 column matrix. Associated with each row is a specific frequency belonging to the low group (697 to 941 HZ) and corresponding to each column is a unique frequency of the high group (1209 to 1633 HZ). All the keys in a given row or column have one tone in common (see Table 1.1). For example, pressing the digit "9" (row 3 and column 3) produces 852 Hz and 1477 Hz tones simultaneously, while pressing a "5" produces 770 and 1336 HZ tones.
The full DTMF-encoding standard defines four rows and four columns for a total of 16 two-tone combination. Standard telephones use only 12 of these combinations. Depending on the application, these extra codes may be useful. Most tone decoding devices allow a 2 per cent tolerance on DTMF frequencies. This creates a range of acceptable frequencies, which is demonstrated in Table (1.2).
