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.
As a simple example, suppose a two-way radio frequency is shared by a 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 delivery service. The delivery service 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. Note that in the example above there are only two co-channel users. In dense two-way radio environments a large number of groups may be present on a single radio channel. A disadvantage of using CTCSS in shared frequencies is that since users cannot hear transmissions from other groups, they may assume that the frequency is open when it is not and transmit simultaneously with another user, thus accidentally overriding or interfering with the other group's transmission. For example, in the above situation a landscaper might be communicating with another landscaper. Meanwhile, a delivery service driver—not hearing any transmissions—assumes that the frequency is clear and calls his dispatch office. Depending on several factors (locations, power, etc.), the two simultaneous transmissions could easily interfere with each other—resulting in one or both not being clearly understood. The more separate groups that share a single frequency and the more frequently that they transmit, the more likely that this accidental interference will occur. Radios with a "Busy Channel Lockout" feature will prevent transmitting in this case.