33 3 ms

66 3 ms

Open circuit


Clear down

Fig. 3.6. Pulse train of rotary dial telephone.

It is necessary to indicate the end of a pulse train so that a decoding circuit can know where one digit ends and the next begin. This is done by mechanical design, which ensures a minimum make period between any two consecutive digits. This period is called the inter-digit pause and is typically 200 msec minimums. The completion of dialling is identified if any break, last for few hundred milliseconds.

This method of dialling is slow. For nationwide and international dialing, the routing signals to the switching center is fairly slow and inconvenient. The method, which is replacing the rotary dial telephone, is the push button telephone, which uses the multifrequency dialing.

Multifrequency dialing. The touch-tone dialing scheme is shown in Fig. 3.7 the rotary dial is replaced by keypad. This is called a dual tone multifrequency (DTMF) dial.

1209 1336 1477

1209 1336 1477

Fig. 3.7. Touch tone dialing scheme.

The principal method uses a pair of tones to signal each digit. These two tones are shown in Fig. 3.7. There is sixteen possible pair of tones. For the digits 0 to 9, ten combinations are used. Other six may be used for additional control signals. Allowing 40 msec interdigit pause, a maximum rate of about 12 digits/sec is feasible. When pressing a button (for example the digit 4), the corresponding tones are sent to the central exchange (for digit 4, 770 Hz and 1209 Hz). The central exchange decodes the different combinations of tones received into equivalent dialed digit.

The telephone set uses dialler IC and associated circuits. The dialer IC's of 16 pin HM 91C02A, 18 pin UM 91214B, 18 pin UM91214 E, 16 pin HT 9202G, 18 pin M 2560 G, 18 pin UM 1032 CP and 22 pin HM 91501, B are pupular. Fig 3.5 (b) shows the dialler IC HM 91C02A.

Sound amplifier. The two wires of transmitter and the receiver are connected to the sound and speech IC for the effective transmission and reception of speech signals. This IC performs various functions like Automatic Gain Control (AGC), dial tone, biasing etc. Popular speech or sound amplifier IC are 20 pins KMC 34214 P, 18 pin PBI 3726/11, 18 pin TEA 1060. The IC pin diagram of 16 pin TEA 1062 is shown in Fig. 3.5 (c).

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