squelch

Squelch is a circuit function that cuts audio output when the received signal falls below a preset threshold. In radio receivers, the periods of silence between transmissions are filled with thermal noise and background hiss. Without squelch, operators hear constant static. When a signal strong enough to exceed the threshold arrives, the audio unmutes and becomes audible.

The mechanism works by measuring the signal strength of the incoming carrier wave. Most receivers use a noise-level detector or signal-strength indicator connected to an audio gate. The threshold is set using a knob or potentiometer labeled squelch level or squelch control. In analog systems, the operator adjusts this knob until the hiss just barely stops; stronger signals still break through. In digital receivers, the threshold is often set as a decibel value relative to the noise floor.

Setting and applications

On VHF and UHF two-way radios, squelch is essential because the audio bandwidth is narrow and thermal noise is noticeable. Mobile radio systems, marine band radios, and amateur radio equipment all rely on squelch. Setting it too high causes weak but valid signals to be lost; setting it too low reintroduces unwanted noise. Most professional radios require squelch settings between 1 and 9, with higher numbers raising the threshold. Some systems use tone squelch or digital squelch, which require the signal to carry a specific subaudible tone or data header before unmuting.

The term squelch likely derives from the verb meaning to suppress or crush, reflecting its function. The word is also used as a noun to refer to the audible pop or thump heard when squelch gates are abruptly opened by an incoming signal. This click is sometimes called squelch break-through or the squelch tail, and it can be reduced in modern receivers through slow audio ramping circuits.