Tone Squelch is a selective calling method, most commonly implemented as Continuous Tone-Coded Squelch System (CTCSS), used in two-way radio communications to reduce interference from other users on a shared frequency.
It works by transmitting a low-frequency sub-audible tone (typically between 67 Hz and 254 Hz) alongside voice audio; a receiver only unmutes its speaker if it detects the specific matching tone.
While tone squelch prevents hearing unwanted co-channel traffic, it is not a security feature as it does not encrypt communications or prevent interception.
Tone squelch functions by superimposing a continuous, low-frequency audio tone (typically 67.0 Hz to 254.1 Hz) onto the standard voice transmission. This tone is sub-audible, meaning it sits below the typical voice frequency range 300 Hz–3000 Hz and is filtered out by the receiver speaker, rendering it inaudible to the human ear.
Inside the receiver, a specialized decoder circuit monitors incoming signals. The squelch remains closed (muting the speaker) unless two conditions are met simultaneously:
If a signal lacks the correct tone, the receiver ignores the audio content, effectively filtering out co-channel interference from other users or background noise. To prevent a loud burst of noise "squelch tail" when a transmission ends, modern systems often employ a Reverse Burst (or phase reversal) technique, where the tone's phase is shifted (usually 180° or 120°) for a few milliseconds before transmission ceases, allowing the decoder to mute the audio smoothly.