Decadic pulse dialing

Western Electric dialing model 500 by pulses

pulse decadic dialing is a telecommunications signaling technology in which a local loop circuit is interrupted according to a defined coding, usually a digit. Each of the ten digits is encoded in sequences of up to ten pulses, hence its name. Historically, the most common device for producing these pulses is the telephone dial disk. Another term, loop disconnect dialing, arises from the local circuit interruption method used.

Pulse generation speed was historically determined based on the response time necessary for electromechanical switching systems to operate reliably. Most telephone systems used a speed of 10 pulses/second, but operator dialing within and between telephone exchanges sometimes used speeds of 20 pulses/second.

First automatic exchanges

Automatic telephone exchange systems were developed at the end of the XIX century and beginning of the XX. In order to be identified, subscribers to the service were assigned a unique telephone number for each circuit. The first automatic telephone exchange designed by the American businessman Almon Brown Strowger and his nephew Walter S. Strowger was opened in La Porte (Indiana), United States on November 3, 1892, with equipment manufactured by the Strowger Automatic Company. Telephone Exchange and used telephone devices that had three telegraphic keys, which had to be pressed an adequate number of times to control, indicating to the central team the number of the receiver and a fourth key was used, if the user made a mistake when marking. However, the use of that system was impractical. The most common signaling system was the interruption of the local loop by means of a train of direct current pulses generated in the telephones of the subscribers.....

Dial Disc

Decay marking phone for New Zealand. Watch the layout of the digits, reverse the phone of Western Electric.

Carcass telephone for the Swedish market, by the company Ericsson. Note that the "0" digit position is different from the Western Electric phone.

In view of the inconvenience of the push button system, on January 11, 1898, three of Almon Strowger's company partners, Alexander Keith, John, and Charles Erickson, were granted patent 597,062 for "a device for call for telephone exchanges" in which a rotating disk was included for dialing the destination telephone, but this disk did not have holes but rather edges similar to those of a cogwheel. The pulses were sent when the user turned the disk until a different stop position for each digit transmitted. Error-free operation of the drive required smooth rotational motion by the user, but this system was considered unreliable. This mechanism was soon refined to include a recoil spring and a centrifugal governor to control recoil speed, an innovation that appeared as early as 1905. The user selected a number by inserting a finger into the corresponding rotating disc hole as far as it would go.. When released from this position, the dial contacts would open and close several times, thus breaking the loop current. The telephone exchange decoded the pattern of each digit transmitted in this mode of transmission by step relays or by accumulation in the so-called digit registers. Initially, dial disks were made of metal, until on June 3, 1941, the United States Patent Office granted American Frank A. Cosgrove, an employee of AT&T, patent 2,244,609 for inventing a dial disk. plastic, which became standard thereafter.

Pulse rate and encoding

When the electromechanical switching system was still in use, the current pulses generated by the rotating disk activated relays in the switches or switches of the telephone exchange. The mechanical nature of these relays, and the capacitance of the loop, affected the shape of the pulse, typically limiting the speed of operation to ten pulses per second.

Bell Systems US specifications required service personnel to adjust dials on customer equipment with an accuracy of 9.5 to 10.5 pulses per second, but the tolerance of dials switching equipment was generally between 8 and 11 percentage points.

On some phones, the pulses can be heard on the receiver as clicks. Each digit is represented by a different number of pulses. In most countries, one pulse identifies the digit 1, two pulses 2, and so on, with ten pulses for the digit 0; which makes the code unary, except for the digit 0. Exceptions to this occurred in Sweden, with a single pulse for 0, two pulses for 1, and so on; and in New Zealand, with ten pulses for 0, nine pulses for 1, etc. In Oslo, the capital of Norway, the New Zealand dialing system was used, but the rest of the country did not. Systems that use this encoding of 10 digits in a sequence of up to 10 pulses are sometimes known as decadic dialing systems.

Later, some switching systems used digit registers that doubled the allowable pulse rate to 20 pulses per second, and which reduced the pause between digits since switch selection did not have to be completed during the pause. Some of these systems included crossbar exchanges, the 7A2 version of the Rotary switching system, and the early stored-program control telephone exchanges of the 1970s.

Replacement Technologies

In 1963, the Bell System introduced tone dialing technology also known as Dual-Tone Multifrequency (DTMF) under the trademark Touch-Tone using keypad phones. In subsequent decades, pulse dialing has been phased out as the primary signaling method to the exchange, but many systems continue to support rotary phones for compatibility. Some models of keypad telephones have a switch to select tone or pulse dialing.

In Voice over Internet Protocol (VoIP) systems, digit dialing can be completely replaced when initiating a telephone call by specifying the recipient's uniform resource identifier.