Squelch mutes the speaker when nothing useful is being received, so you are not listening to hiss between transmissions. Tones extend the idea: a radio can send a tone to unlock a repeater, or stay muted until it hears the right tone.
How squelch decides
An FM receiver with no signal produces loud wideband noise. A real signal "quiets" the receiver: noise drops, and the squelch circuit watches for that drop. Below the threshold the audio is gated off; above it, the gate opens.
Audio plays only for signals that reach past the threshold line.
ResultWeak signal muted
Set the threshold just above the noise. Too high and weak signals stay muted. To hear a very weak signal, turn the squelch down until the noise is always on, then listen through it. Many radios have a monitor button that opens the squelch while held.
Tones that ride under the voice
A sub-audible tone is added below the voice band, so it is not heard but a receiver can detect it.
CTCSS: a tone below the voice band, sent with your voice, opens the repeater's squelch.
Method
What is sent
Typical use
CTCSS (often "PL")
one continuous low tone, roughly 67 to 254 Hz, one of dozens of standard values
repeater access and quiet channels
DCS (often "DPL")
a continuous low-speed digital code, a three-digit octal number
same job, a different signalling method
DTMF
two audible tones at once per key
control: remote commands, linking, and dialling on some systems
Tone burst
a short audible tone at the start of a transmission, commonly 1750 Hz
opening repeaters in Europe and some other regions; not used for US access
A tone is a key, not a lock. It tells a repeater "this signal is meant for you", or tells your receiver "only open for this group". It does not encrypt anything, and anyone can open their squelch and hear the conversation.
Using them
Tone: your radio transmits the tone; you still hear everything. This is what most repeaters need for access, and the required tone is listed in repeater directories.
Tone squelch: your radio transmits the tone and also stays muted until it receives the same tone. Useful for ignoring other users on a busy channel, but it can hide a station you actually want to hear.
If a repeater seems dead, a missing or wrong tone is the first thing to check. Use the monitor button to confirm the channel really is busy.
DTMF = two simultaneous audio tones per key. Tap a key.
Touch-tone keys are used for control rather than access: the low tone and high tone together identify the key, so a tone decoder can tell one key from another even in noise.
To see whether you can reach another station directly, swap your receive and transmit frequencies and listen on the repeater's input:
Example 2 m pair. Normal: you hear the OUTPUT and transmit on the INPUT.