Squelch

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.

Further adjustment will increase the level of signal required to unsquelch the receiver.

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.

The presence of continuous noise on an idle channel creates a DC voltage which turns the receiver audio off.

Tone squelch, or another form of selective calling, is sometimes used to solve interference problems.

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.

General Electric's implementation of CTCSS is called Channel Guard (or CG).

[citation needed] For those PMR446 radios with 38 codes, the codes 0 to 38 are CTCSS Tones: Selcall (Selective Calling) transmits a burst of up to five in-band audio tones at the beginning of each transmission.

Several tones were used, the most common being 1,750 Hz, which is still used in European amateur radio repeater systems.

DCS is also referred to as Digital Private Line (or DPL), another trademark of Motorola, and likewise, General Electric's implementation of DCS is referred to as Digital Channel Guard (or DCG).

Squelch of any kind is used to indicate loss of signal, which is used to keep commercial and amateur radio repeaters from continually transmitting.