Sub-audible tone (CTCSS / PL)
A very low tone mixed under FM voice so that a repeater or radio only opens for the right users. Also called a PL tone.
What does it sound like?
Usually filtered out, so you often can't hear it at all; sometimes a faint low hum.
Who uses it?
Repeaters, businesses and family radios.
Where and when can I hear it?
Under FM voice on VHF/UHF. The tone itself is between 67 and 254.1 Hz.
The science
The transmitter mixes one steady low tone, somewhere between 67.0 and 254.1 Hz, underneath the voice for the whole transmission. The receiver keeps its speaker muted unless it detects the tone it was set for, and a high-pass filter removes the tone so listeners normally do not hear its hum. Many radios end a transmission by flipping the tone's phase by 120 or 180 degrees, a trick called reverse burst that closes the receiver before the carrier drops and avoids a burst of noise.
Why use it?
It lets several groups share one frequency without each hearing the others' traffic, and it stops a repeater from being triggered by noise or distant stations. It is a convenience filter, not privacy: it only decides when a radio's speaker opens.
Lore
Motorola developed tone squelch in 1951 and trademarked it as Private Line, which is why hams still call it a PL tone. General Electric sold its version as Channel Guard, and RCA as Quiet Channel.
What do the decoded parts mean?
- Tone frequency: The tone in Hz (for example 100.0 or 127.3). Knowing it lets you program a radio to use that repeater.
Easily confused with
- Digital squelch code (DCS), from the same family