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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. For this reason, the method is sometimes called decadic dialling. Historically the most common device to produce such pulse trains is the rotary dial of the telephone, lending the technology another name, rotary dialing. Another term, loop disconnect dialing, arises from the method of interrupting the local loop circuit.
The pulse repetition rate has historically been standardized based on the response time needed for electromechanical switching systems to operate reliably. Most telephone systems used the nominal pulse repetition of 10 pulses per second, but operator dialing within and between central offices often used pulse rates up to 20 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 generated in the telephone sets of subscribers by interrupting the single-pair wire loop of the telephone circuit.
Strowger also filed the first patent for a rotary dial in 1891. The first dials worked by direct, forward action. The pulses were sent as the user rotated the dial to the finger stop starting at a different position for each digit transmitted. Operating the dial error-free required smooth rotary motion of the finger wheel by the user, but was found as too unreliable. 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 by inserting a finger into the corresponding hole and rotated the dial to the finger stop. When released from this position, the dial pulsing contacts were opened and closed repeatedly, thus interrupting the loop current in a pattern on the return to the home position. The exchange switch decoded the pattern for each digit thus transmitted by stepping relays or by accumulation in digit registers.
Pulse rate and coding
When electromechanical switching system were still in use, the current pulses generated by the rotary dial on the local loop operated electrical relays in the switches at the central office. The mechanical nature of these relays and the loop capacitance, effecting pulse shape, generally limited the speed of operation, the pulsing rate, to ten pulses per second.
The specifications of the Bell System in the US required service personnel to adjust dials in customer stations to a precision of 9.5 to 10.5 pulses per second (pps), but the tolerance of the switching equipment was generally between 8 and 11 pps. The British (BPO, later Post Office Telecommunications) standard for 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: Sweden (example dial), with one click for 0, two clicks for 1, and so on; and 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 the rest of the country did not. Systems that used this encoding of the 10 digits in a sequence of up to 10 pulses, are sometimes known as decadic dialing systems.
Some later switching systems 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 version (7A2) of the Rotary system, and the earlier 1970s stored program control exchanges.
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 from coin-box phones (payphones) by tapping the switch hook without depositing coins. 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 on 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.
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.
- AT&T Specification No. 4566, February 1926, p.113
- J. Atkinson, Telephony Volume 1, p.142 (1948, Pitman, London)