In telecommunications, Continuous Tone-Coded Squelch System or CTCSS is a circuit that is used to reduce the annoyance of listening to other users on a shared two-way radio communications channel. It is sometimes called tone squelch. Where more than one user group is on the same channel (called co-channel users), CTCSS filters out other users if they are using a different CTCSS tone or no CTCSS.
Receivers equipped with a CTCSS circuit usually have a switch that selects normal mode or CTCSS mode. When enabled, the CTCSS radio circuit, instead of opening the receive audio for any signal, causes the two-way radio receiver's audio to open only in the presence of the normal RF signal AND the correct sub-audible audio tone (sub-audible meaning that the receiver circuitry can detect it, but is not apparent to the users in the audio output). This is akin to the use of a lock on a door. A carrier squelch or noise squelch receiver not configured with CTCSS has no lock on its door and will let in any signal. A receiver with CTCSS circuitry (and with it enabled) locks out all signals except ones encoded with the correct tone. CTCSS can be regarded as a form of in-band signaling.
Read more about Continuous Tone-Coded Squelch System: Example, Theory of Operation, List of Tones, Vendor Names, Reverse CTCSS, Interference and CTCSS
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