SQ is squelch.
Since the FM receiver will make an annoying "rustling" sound when there is no signal, people have designed a signal detection and control circuit. When there is no signal, the audio output is turned off, and when there is a signal, the audio output is turned on. This is the squelch function. The squelch control threshold can be adjusted by a potentiometer, and it is generally best to adjust it to just enough to shut off the noise.
How to adjust the SQ value?
We found that the potentiometer of DP-666 may have poor contact, so it is recommended to use the rotory to adjust the SQ value. Please make sure that in the menu:
Audio settings -> Use squelch -> On
Operation settings -> Rotory adjust squelch -> On
Then touch 'SQ' on the screen. When the SQ label is inverted, rotate the rotary to adjust the SQ value.