Without squelch, listening to radio signals can be noisy. The circuit that controls the squelch level eliminates background noise and interference so that desired signals pass through clearly. Radios with squelch operate quieter than those without since only the signals you want to hear pass through to the speaker.
The term squelch describes the process of muting undesired signals while allowing desired signals to pass through. It is used to silence a radio while no signal is being received to allow for quieter operation. Radios without squelch may pick up weaker signals -- such as a distant but inaudible broadcast -- or background noise, like the static you hear on a frequency with no transmitting signal.
Three common types of squelch exist: one controlled exclusively by a circuit, another that measures the amount of high-frequency energy of a signal, and a third that looks for a sub-audible tone to turn off squelch. In circuit-controlled squelch, the level of squelch is selected so that the receiver is unmuted when received signals reach a certain strength. In noise squelch, the circuit measures the amount of high-frequency energy in a signal. Poorer signals have a higher level of high-frequency energy and are thus muted. With tone squelch, the receiver will only unmute if it detects a sub-audible tone that matches the tone squelch setting and has a strong enough signal.
Squelch is not only used to keep radios quiet when a signal does not exist on a given frequency but also to limit interference. Tone squelch is used in ham radio repeaters, allowing ham radio operators to talk over greater distances by receiving and then “repeating” their signal. Most repeaters use what is called a "PL tone," a sub-audible tone that must be sent along with the voice signal when a ham radio operator tries to access a repeater. The PL tone is a form of tone squelch and it keeps the repeater from activating due to weak signals or interference.
Some fine-tuning may be required to achieve the desired level of squelch in most cases. Too much squelch will block out even stronger signals, while too little squelch causes the radio to unmute at undesirable times. Audio electronics maker Shure recommends that the squelch setting be placed at a point just above where all background noise is muted but not so low that the received signal is muted due to low levels of background noise.
- Photo Credit Comstock/Comstock/Getty Images