Continuous Tone Coded Squelch System (CTCSS) is a feature in most conventional analog radios, using sub-audible tones to allow multiple groups of users to communicate on the same frequency (channel) without receiving radio traffic from each other; and to improve radio operation in high RF noise, interference and congested areas. The feature transmits a tone between 67 Hz to 254 Hz (below human hearing range) along with the voice traffic. The receiving radio may be programmed to only respond to signals transmitting the desired tone frequency.
Digital Coded Squelch (DCS) is a digital form of CTCSS. A third type of squelch system, called Selective Calling (SelCall) is used in some business radios, but is not used in Ham radios. Note that none of these squelch systems are used in digital radios.
CTCSS is also known by a number of other terms which are registered trademarks of individual manufacturers. Among them are Private Line (PL), owned by Motorola; Channel Guard (CG), owned by GE/Ericsson/Harris; and Quiet Tone (QT), owned by Kenwood.
Some Family Radio Service (FRS) radio manufacturers use other terms for CTCSS, including Privacy Code and Sub-Channel.
Below are tables of the tone frequencies used for CTCSS and the codes used for DCS.