Samson Stage 412 Frequency-Agile Quad-Channel Handheld VHF Wireless System (173 to 198 MHz) - Manual (Page 14)

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All about squelch
Squelch (or muting) circuitry acts to silence the audio output of the receiver whenever the
desired RF signal is not present. If such circuitry is not present, the receiver may output a
different signal—or even extremely unpleasant white noise—whenever the RF signal is lost
due to dropout or excessive distance, or when the transmitter is turned off. This is due to a
phenomenon known as the capture effect (sometimes called capture ratio), which describes
the fact that an FM receiver is constantly seeking RF signal and will always utilize the strongest
signal of a particular frequency that it locates (with the strongest signal always suppressing
weaker signals of the same or nearly same frequency).
To combat this, there are several kinds of squelch circuits used by wireless systems. The most
basic kind, called carrier squelch, simply sets an RF signal threshold; when signals drop below
this threshold, the audio output is muted.
Another, more advanced design, called noise squelch, carries out an automatic comparison
of the incoming RF signal to a reference voltage (generally 200 kHz or higher) in order to
determine whether it is valid signal or simply radio noise (which has much greater high
frequency energy than valid signal), muting the audio output when noise is detected.
A third kind is called tone squelch (sometimes called “tone-key” or “tone-code” squelching).
The Stage 412 system features this method of squelch. Here, the transmitters add a supersonic
tone to all audio signals they transmit, with the receivers refusing to pass through any received
audio signal that does not contain this tone.