Editing Squelch (section)

==Squelch==
[[File:Squelch 4.png|thumb|upright=1.4|A squelch threshold]]
In some designs, the squelch threshold is preset. For example, television squelch settings are usually preset. Receivers in [[base station]]s, or [[repeater]]s at remote mountain top sites, are usually not adjustable remotely from the control point.

In [[two-way radio]]s (also known as [[radiotelephone]]s), 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]] [[Silence|mute]]s the audio or blanks the video on "empty" [[Channel (broadcasting)|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&nbsp;Hz (4kHz) and above, leaving only high frequency noise. The squelch control adjusts the [[Gain (electronics)|gain]] of an [[amplifier]] which varies the level of the noise coming out of the filter. This noise is [[rectifier|rectified]], producing a [[Direct current|DC voltage]] when [[Electronic noise|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'''.