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Pulse dialing is a signaling technology in telecommunications in which a direct current local loop circuit is interrupted according to a defined coding system for each signal transmitted, usually a digit. Each of the ten digits are encoded in sequences of up to ten pulses each. For this reason, the method is also called decadic dialling, primarily in the United Kingdom. Historically the most common device to produce such pulse trains is the rotary dial of the telephone, lending the technology another name, rotary dialing. The term loop disconnect dialing arises from its nature of interrupting the local loop circuit. The pulse repetition rate has historically been standardized based on the response time needed for electromechanical switching systems, and most telephone systems used the nominal pulse repetition of 10 pulses per second.
Early automatic exchanges
Automatic telephone exchange systems were developed in the late 19th and early 20th century. For identification, telephone subscribers were assigned a telephone number unique to each circuit. Various methods evolved to signal the desired destination telephone number for a telephone call directly dialed by the subscriber.
The first commercial automatic telephone exchange, designed by Almon Brown Strowger, opened in La Porte, Indiana on 3 November 1892, and used two telegraph-type keys on the telephone, which had to be tapped the correct number of times to control its vertical and horizontal magnets. But the use of separate keys with separate conductors to the exchange was not practical. The most common signaling system became a system of using direct-current pulse trains originating from the telephone sets of subscribers transmitted on the single pair of wires of the telephone circuit.
Strowger also filed the first patent for a rotary dial in 1891. The first dials worked by direct, forward action, with the pulses sent as the user rotated the dial to the finger stop starting at a different position for each digit transmitted. The error-free operation depended on smooth rotary motion of the finger wheel by the user. This mechanism was soon refined to include a recoil spring and a centrifugal governor to control the recoil speed. The user selected a digit to be dialed, rotated the dial to the finger stop, and released it so it would produce the correct number of pulses on the return to the home position.
Pulse rate and coding
The generated pulses of the local loop current operated electrical relays in these electromechanical switching systems at the central offices. The mechanical nature of these relays generally limited the speed of operation, the pulsing rate, to ten pulses per second. The British (BPO, later Post Office Telecommunications) standard for their Strowger exchanges was 10 impulses per second (allowable range 7 to 12) and a 66% break ratio (allowable range 63% to 72%)
In some telephones the pulses may be heard in the receiver as clicking sounds. Each digit is represented by a different number of pulses. In most countries one pulse is used for the digit 1, two pulses for 2, and so on, with ten pulses for the digit 0; this makes the code unary, excepting the digit 0. Exceptions to this are: (1) Sweden (example dial), with one click for 0, two clicks for 1, and so on; and (2) New Zealand (example dial), with ten clicks for 0, nine clicks for 1, etc. Oslo the capital city of Norway, used the 'New Zealand' system, but not the rest of the country
Some later switching system used digit registers which doubled the allowable pulse rate to 20 pulses per second, and the inter-digital pause could be reduced as the switch selection did not have to be completed during the pause. These included some Crossbar systems, the later (7A2) version of the Rotary system, and from the 1970s earlier Stored Program Control exchanges which were space-division rather than time-division.
In 1963 the Bell System introduced dual-tone multi-frequency (DTMF) technology under its Touch-Tone® trademark using push-button telephones. In the decades following, pulse dialing was gradually phased out as the primary signaling method to the central office, but many systems still support rotary telephones for backward compatibility. Some models of keypad telephones have a switch for the selection of tone or pulse dialing output.
As pulse dialing is achieved by interruption of the local loop, it was in principle possible to dial a telephone number by rapidly depressing the switch hook the corresponding number of times for each digit at approximately ten taps per second. However, many telephone makers implemented a slow switch hook release to prevent rapid switching.
In the United Kingdom, it used to be possible to make calls free of charge from coin-box phones (payphones) by tapping the switch hook. A person caught tapping could be charged with 'abstracting electricity' from the General Post Office and several cases were prosecuted under this offense
In popular culture, tapping was used in the film 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 film Hackers.
- Federal Standard 1037C
- Dual-tone multi-frequency (DTMF) aka 'touch-tone'
- Push-button telephone
- Strowger switch
- Telephony by J. Atkinson, Volume 1 p142 (1948, Pitman, London)