Coded Squelch

Coded squelch systems are used in place of a carrier-squelch system traditionally used in 2-way radios. While carrier squelches will open up (allow audio to pass through) when a sufficiently strong signal is received, coded squelch systems will only open up when the correct code is received. This further prevents interference from stray electrical sources as well as from other users.

Security (or lack thereof)

It is important to note that coded squelch systems do not obscure the signal in any way. It is used only to block out signals on the receivers - any other radio with code squelch disabled, or a general communications receiver such as a scanner can still receive all transmissins. Some higher end scanners are also able to decode the code that is being used. Furthermore, they are not individual channels (frequencies) and if other radios are being used on the same channel at the same time, they will interfere (step on) each other, even though they may have different code squelch settings.

CTCSS

A radio employing the Continuous Tone Coded Squelch System transmits a low-frequency audio signal along with the transmission at all times. There are generally 38 standard assignments in a de-facto standard, although Motorola uses slightly different naming conventions.

Operation

Radios using CTCSS place a filter between the tuner and squelch stages that allows only the selected frequency to pass through - if the correct frequency is passed through and detected, then the signal is sent to the squelch stage.

Reference Table