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In telecommunications, squelch is a circuit function that acts to suppress the audio (or video) output of a receiver. It is activated in the absence of a sufficiently strong desired input signal, in order to exclude undesired lower- power input signals that may be present at or near the frequency of the desired signal. (Contrast with noise suppression .)Source: modified from Federal Standard 1037C and from the NTIA Manual of Regulations and Procedures for Federal Radio Frequency Management in support of MIL-STD-188.
1 Types
Two types of selective squelch are commonly used. A continuous tone-coded squelch system (CTCSS) uses any one of about 50 tones from 67 to 254 Hz. Digital-coded squelch (DCS) systems use a continuous streamThe primary meaning of stream is a body of water, confined within a bed and banks and having a detectable current. Stream is the umbrella term used in the scientific community for all flowing natural waters. For other meanings of the word stream see Strea of digitalA digital system is one that uses discrete values rather than a continuous spectrum of values: compare analog. The word comes from the same source as the word digit: the Latin word for finger (counting on the fingers) as these are used for discrete counti dataA datum is a statement accepted at face value (a "given"). Data is the plural of datum''. A large class of practically important statements are measurements or observations of a variable. Such statements may comprise numbers, words, or images. Etymology T to identify themselves, running in the same audio frequencyAn audio frequency (abbreviation: AF) is any frequency from about 20 hertz to about 20 kilohertz, which is the approximate range of sound frequencies that is audible to humans. The variable whose frequency is being measured can be any physical property ca bandA band is a small spectrum of radio communication frequencies, usually used for the same purpose. Examples: Aircraft band (108~136 MHz) FM broadcast band (88~108 MHz) upper VHF TV band (174~216 MHz in the Americas) L band (1452~1492 MHz) for digital radio as the tones but at about 131 baudIn telecommunications and electronics, baud is a measure of the "signaling rate" which is the number of changes to the transmission media per second in a modulated signal. It is named after Emile Baudot, the inventor of the Baudot code for telegraphy..
CTCSS is usually called PL tone (for "Private Line", a trademarkSee also: brand Bass Red Triangle, was the first trademark registered in Britain in 1876. A trademark ( Commonwealth English: trade mark is a distinctive name, phrase, symbol, design, picture, or style used by a business to identify itself and its product of MotorolaMotorola started as Galvin Manufacturing Corporation in 1928. The name of the company was changed to Motorola in 1947, but the word had been used as a trademark since the 1930s. The company is based in Schaumburg, Illinois, a Chicago suburb. Founder Paul), or simply squelch tone. It can be regarded as a form of in-band signaling. See also radiotelephone.
Squelch can also be used based strictly on the signal strength of the signal, such as when a television mutes the audio or blanks the video on "empty" channels, or when a walkie talkie mutes the audio when no one is calling.
