It's a carrier with nothing "riding" on it like a tone.
I copied the following:
A simple
carrier squelch or
noise squelch operates strictly on the
signal strength of the signal, 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. In some designs, the squelch threshold is preset. For example, television squelch settings are usually preset. Receivers in
base stations at remote mountain top sites are usually not adjustable remotely from the control point.
In devices such as
radiotelephones (also known as
two-way radios), the squelch threshold is set with an adjustable knob marked
squelch. This setting adjusts the threshold at which signals will open the audio channel. Backing off the control will turn on the audio, and the operator will hear
white noise if there is no signal present. The usual operation is to adjust the control until the channel just shuts off - then only a small threshold signal is needed to turn on the speaker. However, if a weak signal is annoying, the operator can adjust the squelch to open only when stronger signals are received.
A typical
FM two-way radio carrier squelch circuit takes out the voice components of the receive audio by passing the detected audio through a
high-pass filter. A typical filter might pass frequencies over 4,000 Hz (4 kHz). The squelch control adjusts the
gain of an
amplifier which varies the level of noise coming out of the filter. The audio output of the filter and amplifier is
rectified and produces a
DC voltage when
noise is present. The presence of noise creates a DC voltage which turns the receiver audio off. When a signal with little or no noise is received, the voltage goes away and the receiver audio is unmuted. Some applications have the receiver tied to other equipment that uses the audio muting control voltage as a "signal present" indication.