What is CTCSS?

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. 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 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.

For example, suppose a two-way radio frequency is shared by a pizza delivery service and a landscape maintenance service. Conventional radios without CTCSS would hear all transmissions from both groups. The landscapers would have to listen to the pizza shop. The pizza shop would have to hear about landscape customer complaints. If both installed CTCSS, units from each group would only hear radios from their own group. This is supposed to reduce missed messages and the distraction of unnecessary radio chatter.