Pulse dialing
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Pulse dialing or loop disconnect dialing, also called Rotary or Decadic dialling in the United Kingdom (because up to 10 pulses are sent), is pulsing in which a direct-current pulse train is produced by interrupting a steady signal according to a fixed or formatted code for each digit and at a standard pulse repetition rate.
Dial pulsing originated with a rotary dial integrated into telephone instruments, for the purpose of signaling. Subsequent applications use electronic circuits to generate dial pulses.
The pulses are generated through the making and breaking of the telephone connection (akin to flicking a light switch on and off); the audible clicks are a side-effect of this. As a result, all that is really needed to dial a number with pulse dialing is a switch (i.e., the telephone receiver button). Each digit in the number is represented by a different number of rapid clicks. (In the U.S. one click for 1, two clicks for 2, etc. except 0 which has ten clicks -- note that in some countries different numbers of clicks are assigned to different digits, which may not match the example given here).
Individual digits in a phone number need to be separated with a short pause so as not to bleed into each other, and in keypad based pulse dialing, digits need to be 'cached' when dialed rapidly -- in rotary systems the user must wait for the rotor to turn back to the correct position before the next digit can be dialed.
Most fixed-line phones now use dual tone multi frequency (also called touch tone or tone dialing) rather than pulse dialing, but most telephone equipment retains support for pulse dialing for backward compatibility. ISDN and GSM mobile phones perform call setup using digital signalling systems.