The push-button telephone is a telephone that uses buttons or keys for dialing a telephone number to place a call to another telephone subscriber.
Western Electric experimented as early as 1941 with methods of using mechanically activated reeds to produce two tones for each of the ten digits. But the technology proved unreliable and it was not until long after the invention of the transistor when the technology matured. On 18 November 1963 the Bell System in the United States officially introduced dual-tone multi-frequency (DTMF) technology under its registered Touch-Tone® mark. Over the next few decades Touch-Tone service replaced traditional pulse dialing technology and it eventually became a world-wide standard for telecommunication signaling.
Although DTMF was the major technology implemented in push-button telephones, some telephone manufacturers used push-button keypads to generate pulse dial signaling. Before the introduction of Touch-Tone telephone sets, the Bell System often used the term push-button telephone to refer to key system telephones, which were rotary dial desk sets that also had a set of push-buttons to select one of multiple telephone circuits, or to activate other key system features.
The concept of the use of push-buttons in telephony originated around 1887 with a device called the micro-telephone push-button, but it was not an automatic dialing system as understood later. This use even predated the invention of the rotary dial by Almon Brown Strowger in 1891. The Bell System in the United States relied on manual switched service until 1919, when it reversed its decisions and embraced dialed, automatic switching. The 1951 introduction of direct distance dialing required automatic transmission of dialled numbers between distant exchanges, leading to use of inband multi-frequency signaling within the Long Lines network while individual local subscribers continued to dial using standard pulses.
As direct distance dialling expanded to a growing number of communities, local numbers (often four, five or six digits) were extended to standardised seven-digit named exchanges. A toll call to another area code was eleven digits, including the leading 1. In the 1950s, AT&T conducted extensive studies of product engineering and efficiency and concluded that push-button dialing was preferable to rotary dialing. On November 18, 1963, the first electronic push-button system with Touch-Tone dialing was offered by Bell Telephone to customers in Carnegie and Greensburg, Pennsylvania.
This phone, the Western Electric 1500, had only ten buttons. In 1968 it was replaced by the twelve-button model 2500, adding the asterisk or star (*) and pound or hash (#) keys. The use of tones instead of dial pulses relied heavily on technology already developed for the long line network, although the 1963 Touch-Tone deployment adopted a different frequency set for its dual-tone multi-frequency signaling.
Although push-button touch-tone telephones made their debut to the general public in 1963, the rotary dial telephone still was common for many years. In the 1970s the majority of telephone subscribers still had rotary phones, which in the Bell System of that era were leased from telephone companies instead of being owned outright. Adoption of the push-button phone was steady, but it took a long time for them to appear in some areas. At first it was primarily businesses that adopted push-button phones. During the mid seventies, in the United Kingdom, Post Office Telecommunications, THQ (Telephone Headquarters), as it was then called, were investigating the replacement of the rotary dial with a push button telephone using different tones for each of the numbers 1 -0 on the existing rotary dial. A major problem with the dial telephone was premature dialling by the customer, which resulted in mis- directed or invalid calls. The correct procedure was on lifting the handset from the telephone cradle, the customer would wait until a dial tone was sent from the local telephone exchange, to indicate that the exchange equipment was ready to receive the dialled impulses transmitted by the customer. The customer then turned the dial clockwise with their finger placed in the appropriate hole of the dial, representing the number 1 to 0, until a finger stop was reached on the outside of the dial, preventing further movement. When the customer removed their finger, the dial would return to its original position, awaiting the next number, until all numbers had been forwarded for completion of the call. Often, customers would pick up the telephone and immediately commence dialling before the exchange equipment was ready, so that when dial tone was eventually forwarded by the local telephone exchange, a number of customer dialled digits would have been lost or clipped, and the call would either be part completed with the exchange waiting for further pulses from the customers dial in order to route the call onwards (but none would be forthcoming) or prematurely route the call from the fewer pulses received to an unknown or invalid route. In both cases, the call would have to be abandoned.
Within Post Office Telecommunications, THQ, Holborn, London, the final design of the touch-tone telephone, which finally replaced the rotary telephone, was credited to Roger TS Parr, who introduced new electronic technology employing, among other sophisticated devices, FIFO chips (First in, First out). This overcame all problems which might be encountered by customers trying to beat the telephone exchange dial tone by prematurely transmitting the different tones for each of the numbers 1 to 0 on the telephone keypad. A prototype of the new design was taken to Hurstpierpoint telephone exchange in Sussex, where it underwent stringent trials, the outcome of which was successful. The design was then handed to Plessey Telecommunications (PTL) who, in turn, then designed the interface equipment, which was to be inserted in the telephone exchange to receive the tone pulses for onward routing of the call. It was during the testing phase with the new interface that a major problem was encountered. As part of routine maintenance and testing of telephone exchange equipment by the telephone engineers, it was customary for the engineers to use a piece of equipment called a test lamp. This device, the body of which comprised a built-in lamp and a test probe, was plugged into a convenient power socket located near the piece of equipment to be tested and by means of a switch located on the test lamp, the engineer was able to determine different conditions on the equipment under test, e.g. a voltage, earth or disconnection present. One of these tests was to extend negative 50 volts from the probe of the test lamp to establish if an earth condition existed, which would complete or close an electrical circuit and light the test lamp. When this test lamp condition was used between the line connecting the new touch-tone telephone design and the interface within the exchange, due to the interface comprising diodes and transistors, the extension of negative 50 volts from the test lamp, blew these sensitive devices, rendering them useless. In collaboration with Plessey, (W. Finney), Parr suggested the use of optical relays,  a first of its kind to be employed in exchange interfaces and these devices provided the necessary barrier to allow the testing with a test lamp to be used with no fear of destroying the electronic devices, which were used to receive the tones from the touch-tone telephones for interpretation and decoding to route the call to its destination. By 1979, the touch-tone phone was gaining popularity, but it wasn't until the 1980s that the majority of customers owned push-button telephones in their homes; by the 1990s, it was the overwhelming majority.
Some exchanges no longer support pulse-dialing[unreliable source?] or charge their few remaining pulse-dial users the higher tone-dial monthly rate as rotary telephones become increasingly rare. Dial telephones are not compatible with some modern telephone features, including interactive voice response systems, though enthusiasts may adapt pulse-dialing telephones using a pulse-to-tone converter.
The international standard for telephone signaling utilizes dual-tone multi-frequency (DTMF) signaling, more commonly known as Touch-Tone dialing. It replaced the older and slower pulse dial system. The push-button format is also used for all cell phones, but with out-of-band signalling of the dialled number.
The Touch-Tone system uses audible tones for each of the digits zero through nine. Later this was expanded by two keys labeled with an asterisk (*) and the pound or hash sign (#) to represent the 11th and 12th DTMF signals. These signals accommodate various additional services and customer-controlled calling features.
The DTMF standard assigns specific frequencies to each column and row of push-buttons in the telephone keypad; the columns in the push-button pad have higher-frequency tones, and rows have lower-frequency tones in the audible range. When a button is pressed the dial generates a combination signal of the two frequencies for the selected row and column, a dual-tone signal, which is transmitted over the phone line to the telephone exchange.
When announced, the DTMF technology was not immediately available on all switching systems. The circuits of subscribers requesting the feature often had to be moved from older switches that supported only pulse dialing to a newer crossbar, or later an electronic switching system, requiring the assignment of a new telephone number which was billed at a higher monthly rate. Community dial office subscribers would often find the service initially unavailable as these villages were served by a single unattended exchange, often step by step, with service from a foreign exchange impractically expensive. Rural party line service was typically based on mechanical switching equipment which could not be upgraded.
While a tone-to-pulse converter could be deployed to any existing mechanical office line using 1970s technology, its speed would be limited to pulse dialling rates. The new central office switches were backwardly compatible with rotary dialling.
DTMF keypad layout
How DTMF dialing sounds.
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The now standard layout of the keys on the Touch-Tone telephone was the result of research of the human-engineering department at Bell Laboratories in the 1950s under the leadership of South African-born psychologist John Elias Karlin (1918–2013), who was previously a leading proponent in the introduction of all-number-dialing in the Bell System. This research resulted in the design of the DTMF keypad that arranged the push-buttons into 12 positions in a 3-by-4 position rectangular array, and placed the 1, 2, and 3 keys in the top row for most accurate dialing. The remaining digits occupied the lower rows in sequence from left to right, however, placing the 0 into the center of the fourth row, while omitting the lower left, and lower right positions. These two positions were later assigned to the asterisk and pound key when the keypad was expanded for twelve buttons in 1969.
The engineers had envisioned phones being used to access computers, and surveyed a number of companies to see what they would need for this role. This led to the addition of the number sign (#, ''pound'' or "diamond" in this context, "hash", "square" or "gate" in the UK, and "octothorpe'' by the original engineers) and asterisk or "star" (*) keys as well as a group of keys for menu selection: A, B, C and D. In the end, the lettered keys were dropped from most phones, and it was many years before these keys became widely used for vertical service codes such as *67 in the United States of America and Canada to suppress caller ID.
The DTMF keyboard layout broke with the tradition established in cash registers (and later adopted in calculators and computers) of having the 0 key at the bottom and the remaining keys increasing in rows above. This was due to research conducted by Bell Labs using test subjects unfamiliar with keypads. Comparing various layouts including two-row, two-column, and circular configurations, the study concluded that while there was little difference in speed or accuracy between any of the layouts, the now familiar arrangement with 1 at the top was the most favourably rated.
Historically, not all push-button telephones used DTMF dialing technology. Some manufacturers implemented pulse dialing with push-button keypads and even Western Electric produced several telephone models with a push-button keypad that could also emit traditional dial pulses. Sometimes the mode was user-selectable with a switch on the telephone. Pulse mode push-button keypads typically stored the dialed number sequence in a digit collector register to permit rapid dialing for the user.
As telephone companies continued to levy surcharges for Touch-Tone service long after any technical justification ceased to exist, a push-button telephone with pulse dialing capability represented a means for a user to obtain the convenience of push-button dialing without incurring the Touch-Tone surcharge.
Electronics within push-button telephones may provide several usability features, such as last-number re-dial and storage of commonly called numbers. Some telephone models support additional features, such as retrieval of information and data or code and PIN entry.
Most analog telephone adapters for Internet-based telecommunications (VoIP) recognise and translate DTMF tones but ignore dial pulses, an issue which also exists for some PBX systems. Like cellular handsets, telephones designed for voice-over-IP use out-of-band signalling to send the dialled number.
- History of the telephone
- Interactive voice response
- Mobile phone
- Timeline of the telephone
- call logging
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- solid state relay
- BT/THQ RTS Parr (retired) See also Call Logging
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- Why do telephone keypads count from the top down, while calculators count from the bottom up?
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- The 'Busted' Edition, CBC Marketplace, Canadian Broadcasting Corporation, 2012
- Mobile Phone As Home Computer - Philip Greenspun