Continuous Tone-Coded Squelch System

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 referred to as tone squelch or sub-channel since it has the effect of creating multiple virtual channels which are all using the same radio frequency. It does this by superimposing an extra audio tone over the voice transmission on a channel which can be heard by the radio circuitry but not by the human ear. Where more than one group of users is on the same radio frequency (called co-channel users), CTCSS circuitry mutes those users who 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 unmuting 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). A carrier squelch or noise squelch receiver not configured with CTCSS will receive 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 signalling.