A squelch circuit is provided with fast-acting means for unsequelching a receiver when a strong FM signal is received and for squelching a receiver when a strong FM signal is terminated. The squelch circuit is also provided with slow-acting means for unsquelching a receiver when a weak or slowly increasing...http://www.google.com/patents/US4132953?utm_source=gb-gplus-sharePatent US4132953 - Squelch circuit for a radio receiver
