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CTCSS (Continuous Tone Coded Squelch System) is a generic trade term used by the mobile radio industry, and is used to accept desired signals and reject undesired signals on a radio channel. It is also known as PL (Private Line) to Motorola users and Channel Guard to GE/Ericsson users. 

The system involves the use of a sub-audible tone injected into the voice modulation line of the transmitter and a matching tone decoder after the FM discriminator (detector) in the receiver. When activated, the presence of a CTCSS tone on a carrier, opens the audio squelch circuit of the receiver, allowing the signal modulation to pass through to the transmitter.

The sub-audible tone modulates the transmitter at about +/-1.5kHz deviation whereas the voice level is about +/-3.5kHz deviation. The tone is selected from a frequency range of 67.0 Hz to 254.1 Hz and is designed to fall below the normal audio pass band of 300Hz-3kHz in a radio, so that it is not heard.

If the repeater also generates a sub-audible tone when it transmits, then you can turn on your CTCSS and block all signals except the repeater. Useful to keep intermod from paging transmitters out of your receiver or distant repeaters during times of good propagation. This is most important on an HT since they usually have a very broad and hot front end. Many newer mobiles can also benefit from CTCSS due wideband coverage and broad front ends.

CTCSS will not block signals arriving at your receiver at the same time as a desired signal. It will only keep the squelch closed unless the correct sub-audible tone is heard. Sometimes, what sounds like audio hum, may be heard from a radio, where the transmitter deviation of the CTCSS is set too high.

Tone Squelch (CTCSS on transmit only) is useful to access a repeater with Tone squelch access, such as our club VHF repeater. The repeater controller hears the subaudible tone and opens up the receiver squelch to pass your signal and ignores all other signals.

A typical amateur grade mobile radio is supplied with tone squelch installed only and full CTCSS may be added as an option. Some newer models are coming fully CTCSS equipped. Most HTs have full CTCSS installed at the factory. Some of the newest generation radios, can also automatically search for CTCSS tones when you are listening to new repeaters. A useful feature when travelling.