Q. Elucidate the nature of signals produced on subscriber's loop by a pulse dialer and a touch tone dialer. 

Ans:

DTMF is a simple two-of -eight encoding scheme where every digit is represented by linear addition of two frequencies. DTMF is severely for signaling between a subscriber's location and the nearest message switching center ortelephone office. DTMF is several times confused with another two-tone signaling system known as multi frequency signaling (Mf) that is a two-of-six code designed to be used only to convey information between two electronic switching machines.

Figure demonstrates four-row-by-four column keypad matrix used with a DTMF keypad. As the figure displays, keypad is consists 16 keys and eight frequencies. Most household telephones, though, aren't equipped with the special-purpose keys located in the fourth column (that is, A,B, C and D keys). Hence, most household telephones actually use two-of-seven tone encoding scheme. Four vertical frequencies (known as low group frequencies) are 697 Hz, 852 Hz and 941 Hz and four horizontal frequencies (termed as high group frequencies) are 1209 Hz, 1336 Hz, 1477 Hz and 1633 Hz. Frequency tolerance of the oscillators is + .5%. As displayed in Figure, digits 2 through 9 can also be used to represent 24 of the 26 letters (Q and Z are omitted). Letters were originally used to identify one local telephone exchange from another.