Squelch
|
|
In telecommunications, squelch is a circuit function that acts to suppress the audio (or video) output of a receiver in the absence of a sufficiently strong desired input signal. This excludes 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.
Types
A simple squelch operates 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. Often the squelch threshold is preset, but it is common in some devices, such as radiotelephones, to find an adjustable knob marked 'squelch'. This adjusts the threshold at which signals will open the audio channel. Backing off the control will open the channel regardless, and the operator will hear noise if there is no signal. The usual operation is to adjust the control until the channel just shuts off - then only a small signal is needed to open the channel. However, if there is interference from a distant station, but the wanted signal is much closer, the squelch can be adjusted to only open when the stronger of the two signals is received.
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 254Hz. Digital-coded squelch (DCS) systems use a continuous stream of digital data to identify themselves, running in the same audio frequency band as the tones but at about 131 baud.
CTCSS is usually called PL tone (for "Private Line", a trademark of Motorola), or simply squelch tone. It can be regarded as a form of in-band signaling.
Uses
Basic squelch is always used in amateur radio to keep repeater stations from being keyed-up constantly. Squelch tones are very often used as well since they keep other nearby repeaters on the same input frequency from keying the squelch-equipped one unnecessarily.
Many Family Radio Service (FRS) and PMR446 radios also use 38 different squelch tones, also erroneously called "sub-channels". While these do not add to the available number of conversations which can take place at once in a given area, they do reduce annoying interference experienced by users.