Telephone keypad
From Free net encyclopedia
A telephone keypad is a keypad that appears on a "touch tone" telephone. It was standardised when the Dual-tone multi-frequency system was introduced, and replaced the rotary dial.
The keypad is laid out in a 3×4 matrix, with each row representing a low frequency, and each column representing a high frequency. When used to dial a telephone number, pressing a single key such as '1' will send a sinusoidal tone of the two frequencies 697 and 1209 hertz (Hz).
| 1 | 2 | 3 | 697 Hz |
|---|---|---|---|
| 4 | 5 | 6 | 770 Hz |
| 7 | 8 | 9 | 852 Hz |
| * | 0 | # | 941 Hz |
| 1209 Hz | 1336 Hz | 1477 Hz |
The "*" is called the star or asterisk key. The "#" is called the number sign, pound key or hash key, 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#.
Most of the keys also bear letters according to the following system: Image:Telephone keys.JPG
0 = none 1 = none 2 = ABC 3 = DEF 4 = GHI 5 = JKL 6 = MNO 7 = P(Q)RS 8 = TUV 9 = WXY(Z)
These letters have had several auxiliary uses. Originally, the letters referred to area codes. In the US in the mid-20th century, numbers were seven digits long including a two-digit prefix, the latter expressed as letters rather than numbers. In the UK telephone numbering system, a similar two-letter code was added 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. The modern equivalent of 0AY6, namely 01296, still refers to Aylesbury.)
The letters have also been used, mainly in the U.S., 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 1-800-PAINTER.
In recent times, the letters on the keys have found a new use thanks to text messaging on mobile phones.