Also known as: muting, carrier-operated relay, COR, tone squelch
Squelch is the circuit or algorithm that mutes a receiver’s output until an incoming signal meets a chosen presence criterion, so the listener hears active traffic instead of the open-channel hiss of the noise floor.1 It decides, moment by moment, whether the channel is busy or idle and gates the audio (or the downstream decoder) accordingly. Squelch is what lets a scanner sweep many channels and stop only on the ones actually carrying a transmission.
How it works
A squelch continuously measures some proxy for signal presence and compares it to a threshold. The classic implementations differ in what they measure:
- Carrier (level) squelch compares received signal strength — the RSSI or, in an SDR, the in-channel power estimate — against a set level. Simple and fast, but it opens on any carrier, including interference.
- Noise squelch looks instead at energy in a band above the audio passband (typically several kHz up). A strong FM signal quiets that band as the discriminator captures; the drop in high-frequency noise is a reliable “signal present” cue that is largely independent of absolute level, which is why it is the traditional choice for analog FM.
- Tone squelch (CTCSS) additionally requires a specific subaudible tone (67–250 Hz) to be present, so the receiver stays muted for other users sharing the same RF channel.
- Digital code squelch (DCS) does the same with a continuous low-rate digital codeword instead of a tone.
Two refinements matter in practice. Hysteresis uses a slightly higher level to open than to close, so a signal hovering near threshold does not rapidly toggle (“squelch chatter”). A hang time keeps the gate open for a short interval after the signal drops, so brief fades mid-transmission do not clip the audio.
A distinct control often confused with squelch is the attack/decay of the level estimator. The presence measurement is a smoothed average of the channel power; a fast estimator reacts quickly to a signal appearing but is jittery, while a slow one is steady but sluggish to open and close. Tuning that time constant against the hang time is what separates a squelch that snaps cleanly onto short transmissions from one that either clips their start or lingers on noise after they end.
In practice
Setting squelch is a trade-off. Too tight (threshold too high) and weak but wanted signals are rejected; too loose and the channel opens on noise. The useful reference is the signal-to-noise ratio: a good squelch opens a few dB above the noise floor so that anything it passes is actually intelligible. Digital voice systems fold this decision into the modem — a P25 or DMR receiver “opens” only when it detects a valid frame sync pattern and produces good symbols, which is a far more selective test than raw energy. That sync-based gate is why a digital scanner stays silent on a strong analog interferer sitting on the channel: energy is present, but the expected digital structure is not, so the squelch correctly stays closed. It is also why digital squelch tends to be more nearly all-or-nothing than analog carrier squelch — either the frame decodes or it does not, with little of the marginal, noisy-but-audible zone that analog FM has near threshold.
Relevance to SDR
Every scanner and SDR receiver applies squelch of some kind. Analog FM applications (marine VHF, amateur, business band) still use carrier or CTCSS/DCS tone squelch. In a software receiver the measurement is just a power or sync estimate computed from the I/Q stream, so the same logic scales to many channels at once. GopherTrunk’s channel-activity logic is a squelch in spirit: on a trunked system it does not gate on audio energy but on control-channel decodes and, on a granted voice channel, on valid digital sync — it passes traffic only when the decoder confirms a live transmission, and treats CTCSS/DCS as identifying sub-channels on the conventional analog channels it can also monitor.