Telephone Keypad - Key Frequency

Key Frequency

When used to dial a telephone number, pressing a single key will produce a 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.

Note that the layout of the digits is different from that commonly appearing on calculators and numeric keypads.

The "*" is called the "star key" or "asterisk key". "#" is called the "number sign", "pound key", "hash key", "hashtag" or "octothorpe", 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:

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

8 = TUV

9 = WXYZ

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 you just stop 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.