The squelch setting are controlling when the scanner should decode digital signals as well as analog. If you have the squelch set too high any weak stations, being it digital or analog, will not be monitored.
Sometimes it helps to set the squelch to 0 when monitoring digital systems as the squelch could interfere and cut the signal, especially if you are trying to monitor simulcast systems. But when doing so it will stop on analog channels that are not programmed with any CTCSS/DSC tones and also it will think that it receives a carrier and tries to decode a signal until it gives up, which makes the scan rate slow down to 4ch/s.
/Ubbe
