CTCSS an Explication

Continuous Tone-Coded Squelch System

In telecommunications, Continuous Tone-Coded Squelch System or CTCSS is one type of circuit that is used to reduce the annoyance of listening to other users on a shared two-way radio communications channel or frequency. It is sometimes referred to as tone squelch. It does this by adding a low-frequency audio tone to the voice.

In Amatur radio, it is most commonly used to keep out interference at the transmitter site. When there are other high-powered transmitters nearby they may cause interference and is heard on the receive channel as noise. The receiver on the transmitter site might hear signals from faraway transmitters on the same frequency and repeat them as if they were local.

When the repeater has employed the CTCSS system it will only respond to signals it hears that have the correct CTCSS code embedded in the transmitted signal. When the repeater hears the code it allows the received signal to be repeated. This keeps out most if not all unwanted signals.

The CTCSS tone is considered by most to be a sub-audible tone. That means that most people can not hear it. It also makes it harder to hear because most communication speakers do not reproduce very low tones well.

There may be other names for this process. Motorola calls it PL for Private Line. No matter the name they all work the same way.

Note 1 - Not all repeater systems use CTCSS.
Note 2 - The CTCSS tone used on a repeater system will usually have the specific tone listed in the information about the repeater (146.840 (-) 100Hz W7VOI Boise)

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