Pulse Dialing

Pulse dialing, dial pulse, 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 electromechanical or 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. Each digit in the number is represented by a different number of rapid clicks. In most countries one click is used for the digit 1, two clicks for 2, and so on, with ten clicks for the digit 0; this makes the code unary, excepting the digit 0. Exceptions to this are New Zealand (example dial), with ten clicks for 0, nine clicks for 1, and so on; Sweden (example dial), with one click for 0, two clicks for 1, and so on; and in Oslo, the capital city of Norway, where the 'New Zealand' system was used in inverse to the rest of the country.

Individual digits in a phone number need to be separated with a short pause so as not to bleed into each other (as well as, originally, to give the rotary relays at the exchange time to rotate) and in keypad based pulse dialing digits need to be "buffered" when dialed rapidly. In rotary systems this interdigit interval is provided by the slow rotation of the mechanical dial.

Pulse dialing replaced the two telegraph keys used by telephones connected to the original Strowger exchange to control its vertical and horizontal magnets. With the new system, rather than tap each telegraph key the correct number of times, the user first dialed the vertical number, and then the horizontal number. A slow-release relay detected the interval between the two pulse trains, later standardized to approximately half a second. Director telephone systems and panel switches had equipment to record and decode the digits, which earlier systems had used directly to drive the switches. Dial pulses were further standardized, with a percent break usually specified at approximately 66% and 10 pulses per second. Some telephone systems standardized at 20 PPS. With the spread of Stored Program Control exchanges in the 1970s, pulse counting became a software job.

Most fixed-line phones now use dual tone multi frequency (DTMF, also called touch tone or tone dialing), supported by push-button telephones, rather than pulse dialing, but most telephone equipment retains support for pulse dialing for backward compatibility. Some models of keypad phones have a tone/pulse switch which can be toggled to switch between the two, making these phones usable in areas where DTMF dialing is not accepted.