Telephone keypad

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A standard telephone keypad, as used for text messaging.

A telephone keypad is a keypad that appears on a "Touch Tone" telephone. It was standardised when the dual-tone multi-frequency system in the new push-button telephone was introduced in the 1960s, which gradually replaced the rotary dial.[1] The invention of the keypad is attributed to John E. Karlin, an industrial psychologist at Bell Labs. The contemporary keypad is laid out in a 4×3 grid, although the original DTMF system in the new keypad had an additional column for four now-defunct menu selector keys. Most keypads have a star key or asterisk key on the bottom left and a hash key on the bottom right.

Key frequency[edit]

When used to dial a telephone number, pressing a single key will produce a dual-tone multi-frequency signaling pitch consisting of two simultaneous pure tone sinusoidal frequencies. The row in which the key appears determines the low frequency, and the column determines the high frequency. For example, pressing the '1' key will result in a sound composed of both a 697 and a 1209 hertz (Hz) tone.

DTMF keypad frequencies (with sound clips)
1209 Hz 1336 Hz 1477 Hz 1633 Hz
697 Hz 1 2 3 A
770 Hz 4 5 6 B
852 Hz 7 8 9 C
941 Hz * 0 # D

Layout and characters[edit]

The layout of the digits is different from that commonly appearing on calculators and numeric keypads. This was chosen to allow the letter associations from Rotary Dial phones to have an alphabetic ordering.

The "*" is called the "star key" or "asterisk key". "#" is called the "number sign", "pound key", "hash key", hex key, "octothorpe", "gate" or "square", depending on one's nationality or personal preference. These can be used for special functions. For example, in the UK, users can order a 7.30am alarm call from a British Telecom telephone exchange by dialling: *55*0730#.[2]

Most of the keys also bear letters according to the following system:

A standard telephone keypad.
0 = none (in some telephones, "OPERATOR" or "OPER")
1 = none (in some older telephones, QZ)
2 = ABC
3 = DEF
4 = GHI
5 = JKL
6 = MNO
7 = PQRS (in some older telephones, PRS)
8 = TUV
9 = WXYZ (in some older telephones, WXY)

These letters have had several auxiliary uses. Originally, they referred to exchanges. In the mid-20th century United States, before the advent of All-Number Calling, numbers were seven digits long including a two-digit prefix which was expressed as the letters rather than numbers e.g.; KL5-5445. The UK telephone numbering system used a similar two-letter code after the initial zero to form the first part of the subscriber trunk dialling code for that region – for example, Aylesbury was assigned 0AY6 which translated into 0296. (The majority of these original numbers have remained, particularly in the rural areas, and are currently still in service. The modern equivalent of 0AY6, namely 01296, still refers to Aylesbury.)

The letters have also been used, mainly in the United States, as a way of remembering telephone numbers easily. For example, an interior decorator might license the phone number 1-800-724-6837 but advertise it as the more memorable phoneword 1-800-PAINTER. Sometimes businesses advertise a number with a mnemonic word having more letters than there are digits in the phone number. Usually, this means that the caller just stops dialing at 7 digits after the area code or that the numbers are ignored by the switchboard.

In recent times, the letters on the keys are needed also for entering text on mobile phones, for text messaging, entering names in the phone book, etc.; multi-tap and predictive text systems are used.

Letter mapping[edit]

When designing or selecting a new phone, publishing or using phonewords, one should be aware that there have been multiple standards for the mapping of letters (characters) to numbers (keypad layouts, as with keyboard layout) on telephone keypads over the years.

The system used in Denmark was different from that used in the U.K., which was different from the U.S. etc.[3] The use of alphanumeric codes for exchanges was abandoned in Europe when international direct dialling was introduced in the 1960s, because, for example, dialling VIC 8900 on a Danish telephone would result in a different number than dialling it on a British telephone. At the same time letters were no longer put on the dials of new telephones.

Letters did not re-appear on phones in Europe until the introduction of mobile phones, and the layout followed the new international standard ITU E.161 / ISO 9995-8.

The keypad pictured above is mapped according to the current international standard. The ITU established an international standard (ITU E.161) in the mid-1990s, and that should be the layout used for any new devices.[4]

There is a standard that covers European languages and other languages used in Europe, published by independent ETSI organisation: ETSI ES 202 130; first published in 2003[5] and updated in 2007.[6] (Work describing some principles of the standard is available [7] .)

Since many newer smartphones (such as PalmPilot and BlackBerry) have full keyboards instead of the traditional telephone keypads, the user must execute additional steps to dial a number containing convenience letters. On certain BlackBerry devices, a user can press the Alt key, followed by the desired letter, and the device will generate the appropriate DTMF tone.[8]

See also[edit]

References[edit]