Continuous Tone-Coded Squelch System

In telecommunications, Continuous Tone-Coded Squelch System or CTCSS is one type of in-band signaling that is used to reduce the annoyance of listening to other users on a shared two-way radio communication channel.

CTCSS tone codes are sometimes referred to as sub-channels, but this is a misnomer because no additional radio channels are created.

The ability of a receiver to mute the audio until it detects a carrier with the correct CTCSS tone is called decoding.

Pressing one half of the button, (often marked with a speaker icon or the letters "MON", short for "MONitor") disables the CTCSS decoder and reverts the receiver to hearing any signal on the channel.

The filter's output is amplified and rectified, creating a DC voltage whenever the desired tone is present.

The repeater receiver must first sense the carrier signal on the input, then decode the CTCSS tone.

When that occurs, the system transmitter turns on, encoding the CTCSS tone on its carrier signal (the output frequency).

During this fixed delay period (the amount of which is adjusted during installation, then locked down), the CTCSS decoder has enough time to recognize the right tone.

This way the problem with lost syllables at the beginning of a transmission can be overcome without having to use higher frequency tones.

As electronic components age, or through production variances, some radios in a system may be better than others at rejecting nearby tone frequencies.

A later Digital-Coded Squelch (DCS) system was developed by Motorola under the trademarked name Digital Private Line (DPL).

When the user releases the push-to-talk button the CTCSS tone does a phase shift for about 200 milliseconds.

The end-of-transmission phase reversal (called "Reverse Burst" by Motorola (and trademarked by them) and "Squelch Tail Elimination" or "STE" by GE [8]) caused the reed to abruptly stop vibrating which would cause the receive audio to instantly mute.

If a transmitter without the phase reversal feature is used, the squelch can remain unmuted for as long as the reed continues to vibrate—up to 1.5 seconds at the end of a transmission as it coasts to a stop (sometimes referred to as the "flywheel effect" or called "freewheeling").

Receivers with poor specifications—such as scanners or low-cost mobile radios—cannot reject the strong signals present in urban environments.

It will still degrade system performance but the user will not have to hear the noises produced by receiving the interference.

These do not afford privacy or security, but serve only to reduce annoying interference by other users or other noise sources; a receiver with the tone squelch turned off will hear everything on the channel.