In telecommunications, squelch is a circuit function that acts to suppress the audio (or video) output of a receiver in the absence of a strong input signal.^[1] Essentially, squelch is a specialized type of noise gate designed to suppress weak signals. Squelch is used in two-way radios and VHF/UHF radio scanners to eliminate the sound of noise when the radio is not receiving a desired transmission.

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In some designs, the squelch threshold is preset. For example, television squelch settings are usually preset. Receivers in base stations, or repeaters at remote mountain top sites, are usually not adjustable remotely from the control point.

In two-way radios (also known as radiotelephones), the received signal level required to unsquelch (un-mute) the receiver may be fixed or adjustable with a knob or a sequence of button presses. Typically the operator will adjust the control until noise is heard, and then adjust in the opposite direction until the noise is squelched. At this point, a weak signal will unsquelch the receiver and be heard by the operator. Further adjustment will increase the level of signal required to unsquelch the receiver. Some applications have the receiver tied to other equipment that uses the audio muting control voltage, as a "signal present" indication; for example, in a repeater the act of the receiver unmuting will switch on the transmitter. Squelch can be opened (turned off), which allows all signals to be heard, including radio frequency noise on the receiving frequency. This can be useful when trying to hear distant or otherwise weak signals, for example in DXing.

Carrier squelch is the most simple variant of all. It functions strictly on the signal strength, such as when a television mutes the audio or blanks the video on "empty" channels, or when a walkie-talkie mutes the audio when no signal is present. Carrier squelch uses receiver Automatic_gain_control (AGC) to determine the squelch threshold. Single-sideband_modulation (SSB) typically uses carrier squelch.

Noise squelch is more reliable than carrier squelch. A noise squelch circuit is noise-operated and can be used in AM or FM receivers, and relies on the receiver quieting in the presence of an AM or FM carrier. To minimize the effects of voice audio on squelch operation, the audio from the receiver's detector is passed through a high-pass filter, typically passing 4,000 Hz (4kHz) and above, leaving only high frequency noise. The squelch control adjusts the gain of an amplifier which varies the level of the noise coming out of the filter. This noise is rectified, producing a DC voltage when noise is present. The presence of continuous noise on an idle channel creates a DC voltage which turns the receiver audio off. When a signal with little or no noise is received, the noise-derived voltage is reduced and the receiver audio is unmuted.

Noise squelch can be defeated by intermodulation present in the high-pass band. For this reason, many receivers with noise squelch will also use a carrier squelch set at a higher threshold than the noise squelch.

Tone squelch, or another form of selective calling, is sometimes used to solve interference problems. Where more than one user is on the same channel (co-channel users), selective calling addresses a subset of all receivers. Instead of turning on the receiver audio for any signal, the audio turns on only in the presence of the correct selective calling code. This is akin to the use of a lock on a door. A carrier squelch is unlocked and will let any signal in. Selective calling locks out all signals except ones with the correct key to the lock (the correct code).

In non-critical uses, selective calling can also be used to hide the presence of interfering signals such as receiver-produced intermodulation. Receivers with poor specifications—such as inexpensive police scanners or low-cost mobile radios—cannot reject the strong signals present in urban environments. The interference will still be present, and will still degrade system performance, but by using selective calling the user will not have to hear the noises produced by receiving the interference.

Four different techniques are commonly used. Selective calling can be regarded as a form of in-band signaling.

Squelch was invented first and is still in wide use in two-way radio. Squelch of any kind is used to indicate loss of signal, which is used to keep commercial and amateur radio repeaters from continually transmitting. Since a carrier squelch receiver cannot tell a valid carrier from a spurious signal (noise, etc.), CTCSS is often used as well, as it avoids false keyups. Use of CTCSS is especially helpful on congested frequencies or on frequency bands prone to skip and during band openings.

Professional wireless microphones use squelch to avoid reproducing noise when the receiver does not receive enough signal from the microphone. Most professional models have adjustable squelch, usually set with a screwdriver adjustment or front-panel control on the receiver.

• Dynamic noise limiter
• Noise gate