Pulse dialing

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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. Two exceptions to this are New Zealand, with ten clicks for 0, nine clicks for 1, and so on, and Sweden, with one click for 0, two clicks for 1, and so on.

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 "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) 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, e.g. in most ex-USSR countries.

[edit] Tapping

It is possible to trick a phone system into thinking that a rotary dial is being used. To do this, one finds the little button, switch, or hook that is pushed down when you hang up the phone. To "dial" the digit 1, tap it once. For the digit 2, tap it twice quickly (ten taps per second for UK phones and in North America). For the digit 3, use three taps, etc. The digit 0 is ten taps. (But see above if you are in Sweden, New Zealand or Oslo.)

In the UK it used to be possible to make calls free of charge from coin-box phones (payphones) by tapping. This was on phones with A and B button boxes. (These were phased out between 1958 and 1994 as subscriber trunk dialling was introduced.) A person caught tapping could be charged with 'abstracting electricity from the GPO'. (several cases of dishonestly using telephones without paying were prosecuted under this offence). The Post Office introduced a special Bellset for use in the A/B box to prevent tapping for use in areas where the problem was especially bad. These delayed by a second or so the closure of the line loop (and hence the return of dialling tone) when the handset was lifted.

In popular culture, tapping was used in the movie Red Dragon as a way for prisoner Hannibal Lecter to dial out and circumvent a phone with no dialing mechanism. This method was also used by the character 'Phantom Phreak' to call 'Acid Burn' when taken to prison in the movie Hackers