There are primarily 2 types of the telephone sets

    1. Pulse Dialing (also known as Rotary Dial, Trigger Dial)

    2. Multi-Frequency Dialing (also known as Touch Tone Dial)

1. Working Mechanism of Pulse Dialing

In this type of dialing, a train of the pulse is used to represent a digit in the subscriber number.



Successive digits in a number are represented by a series of pulse trains. Two successive trains are distinguished from one another by a pause in between them, known as interdigital pause, and is a minimum of 400ms.

 The pulse is generated by alternately breaking and making the loop circuit between the subscriber and exchange. 

The lost motion period(230ms) The moment of the trigger from left to right causes a delay of about (230ms). This lost motion period together with the time taken to rotate the finger plate initially ensures a minimum pause of 400ms occurs between digits.                                                      

 so, No of Break Pulses = No of Digit Dialed      

 

2. Multi-Frequency Dialing/ Touch Tone Dialing



Toggling a key generates a "tone" which is a combination of two frequencies, one of the Lower Frequency Band and the other from the Upper-Frequency Band.

Example: Pressing the 9 key transmits 1336 Hz and 770Hz.

The touch-tone signaling frequencies should be in the voice band since telephone lines are designed for voice signals and this creates a problem of "talk-off".

This means that the speech signals can be mistaken for a touch-tone signal and undesired control actions such as terminating a call may occur.

To eliminate "talk-off" two frequencies are used, one from the higher frequency band and the other from the lower frequency band. This scheme is also called "Dual Tone Multi-Frequency dialing (DTMF)".