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CTCSS - Continuous Tone-Coded Squelch System.

In telecommunications, CTCSS is a circuit that is used to reduce the annoyance of listening to other users on a shared two-way radio communications channel. 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.

Instead of turning on the receive audio for any signal, the two-way radio receiver's audio turns on only in the presence of a signal AND the correct tone. This is akin to the use of a lock on a door. A carrier squelch or noise squelch receiver is unlocked and will let any signal in. A receiver with CTCSS locks out all signals except ones encoded with the correct tone. CTCSS can be regarded as a form of in-band signaling.