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. 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 adding a low frequency audio tone to the voice. 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. The CTCSS feature does not offer any security.

A receiver with just a carrier or noise squelch unmutes for any sufficiently strong signal; in CTCSS mode it unmutes only when the signal also carries the correct sub-audible audio tone. The tones are not actually below the range of human hearing, but are poorly reproduced by most communications-grade speakers and in any event are usually filtered out before being sent to the speaker or headphone. CTCSS can be regarded as a form of in-band signaling.

As a simple 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 have to listen to the pizza shop and the pizza shop has to hear about landscape activity. With CTCSS and a different tone for each group, radios only hear the activity from their own group. This is supposed to reduce missed messages and the distraction of unnecessary radio chatter for the other users.

Note that in the example above there are only two co-channel users. In dense two-way radio environments, many separate groups may co-exist 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 erroneously assume that the frequency is idle and then transmit on top of another user, thus accidentally interfering with the other group's transmissions. For example, in the above situation, a landscaper might be communicating with another landscaper. Meanwhile, a pizza delivery 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 clearly being understood. The more separate groups that share a single frequency and the more frequently they transmit, the more likely that this accidental interference will occur. Radios equipped with the "Busy Channel Lockout" feature will prevent transmitting in this case.

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