Pulse dial

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. This lends the method the often used name loop disconnect dialing. In the most common variant of pulse dialing, decadic dialing, each of the ten arabic numerals are encoded in a sequence of up to ten pulses. The most common version decodes the digits 1 through 9, as one to nine pulses, respectively, and the digit 0 as ten pulses. 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 pulse repetition rate was historically determined based on the response time needed for electromechanical switching systems to operate reliably. Most telephone systems used the nominal rate of ten pulses per second, but operator dialing within and between central offices often used pulse rates up to twenty per second.

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. An automatic switch-hook was designed by Hilborne Roosevelt.

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 operated the correct number of times to control the vertical and horizontal relay magnets in the exchange. 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.

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