CTCSS and DCS tones explained
A repeater has to decide, several times a minute, whether the signal on its input is a licensed operator or noise from a passing van. The oldest and still the most common answer is a tone your radio sends underneath your voice, too low to hear.
CTCSS
CTCSS — Continuous Tone-Coded Squelch System, and "PL" on Motorola equipment — is a single tone between 67.0 and 254.1 Hz, transmitted continuously alongside your audio. The repeater opens only when it hears the tone it expects. Everything else is ignored.
The tone is not a password and offers no security whatsoever: the standard set is publicly listed, and every radio can send any of them. It exists to stop a repeater keying up on interference, not to keep anyone out.
DCS
DCS — Digital-Coded Squelch — does the same job with a slow digital code instead of a tone, written as a three-digit octal number such as 023 or 754. It is more resistant to false triggering than CTCSS and less common on amateur repeaters, though some regions use it widely.
Why transmit and receive tones can differ
Some repeaters send a tone on their own output as well as requiring one on the input, so a receiver can be set to stay silent unless the repeater itself is talking. Where the two tones are not the same, a directory has to record both, and a radio programmed with only one may hear nothing at all.
This is common in North America and rare in Europe. It is also a frequent cause of a repeater that appears to be dead: the radio is muting a signal that is perfectly present.
What to set on your radio
| Radio setting | What it does |
|---|---|
| Tone / T | Sends the tone, ignores tones on receive |
| TSQL / T-SQL | Sends the tone and stays muted unless it hears one |
| DTCS / DCS | The digital equivalent of the above |
| Off | Sends nothing — most repeaters will not open |
Plain "Tone" is the right setting most of the time. Use TSQL only when you know the repeater sends a tone back, or you will sit in silence.