The Continuous Tone Coded Squelch System, commonly referred to as CTCSS, has been in used in the land mobile radio arena from the late 1960’s. It is known by a number of different trade names such as Private Line® (PL) by Motorola, Channel Guard® (CG) by General Electric and generically as tone squelch.

PL / CTCSS is an analog squelch scheme in which an audio tone is sent by a transmitter to control the squelch opening of a receiver.

It is a use of sub-audible tones that are transmitted along with the speech portion of the transmission which allows more than one agency (or fleet) to use the same radio frequency without causing undue interference to another agency on the that frequency. Receivers for agency XYZ are set to only open their audio squelch when the proper sub-audible frequency tone is part of the transmission.

Today the sharing of frequencies by agencies is less common than it once was, CTCSS is more commonly used by repeater systems to prevent noise or interference from causing the repeater squawk obnoxiously, and by receivers as an extra measure of squelch (for instance, to prevent engine noise from breaking squelch).

CTCSS is a system whereby a (not really) sub-audible tone is encoded into the transmit audio of a radio and a receiver on another radio on that frequency will only open up (decode) when it picks up that specific tone. While the tones are called “sub-audible”, most people can hear them as a hum in their radio’s speaker when monitoring a radio or repeater that is encoding such a tone.

Encoded tones are sent by the transmitter and decoded tones are used by the receiver. When used, the receiver must receive the programmed tone or it will mute the audio output (i.e. the operator will not hear anything from the radio unless the programmed tone is present).  There are 42 standard Electronic Industries Association CTCSS tones from 67 Hz to 254.1 Hz.

When you press the PTT button and have a tone set, then the radio will send that tone. That tone  is received by the repeater and that opens up, or decodes, the signal.

If you have a tone for encode and a tone for decode, or ENC-DEC set on your repeater, then to hear and to talk you have to match the same tones.

Encoding on the output also allows travelers and newcomers to find out what the access tone required to get into the repeater is, via the use of their radio’s “Tone Scan” feature. Setting the repeater up to encode on the output (IMHO) shows superior engineering and pride in the repeater by the owner or trustee, and something quite the opposite in those who won’t spend the extra few seconds or minutes to set up encode on the output, if such is enabled by a few key presses.

If you have encode set, any radio one the same freq and same code will decode the radio. And all can talk. If you don’t have the code set you can still hear, but you will not be able to talk to the other radios.

Encode is only sent when the radio is in Transmit.

Decode is a radio will not hear anything unless the expected tone is present/transmitted to it.

Carrier Squelch (CSQ) is used when PL / CTCSS coded receive is not used. Even if any PL / CTCSS tone is present in the received signal, the receiver just ignores it and the squelch opens strictly when any received signal is strong enough to overcome the squelch level setting.

If your radios busy light comes on and you don’t hear anything, it could mean your tone is not turned on, or it is set to the wrong tone. (Or your volume is turned down). I have made all three errors. 🙂

Taken from various websites/blogs:

A good YouTub video on the topic can be found below.