CB Radio AM Auto-Squelch characteristics
Since the introduction by President Electronics of the patented Auto-Squelch feature in CB radios in 1998-1999, auto-squelch (a.k.a. ASC or ASQ) for AM and FM has become widespread in CB radios. The auto-squelch feature allows one to detect weaker signals than would be practical with a manual squelch when driving around or scanning channels because the background noise is automatically accounted for. Auto-squelch is not implemented for SSB, which remains on manual threshold setting of AGC-based squelch. Algorithms exist for voice-activated squelch for SSB, but they are generally not yet implemented in CB (or amateur) radios.
A key innovation was including RF signal strength along with the out-of-band baseband noise level commonly used in FM auto-squelch to determine the squelch threshold. This allows the squelch threshold to be set lower when a strong signal is present, which allows weaker signals to be heard. The auto-squelch algorithm is not perfect, however, and is often fooled into closing by overmodulated AM signals, which can cause the squelch to open and close rapidly, making the signal difficult to understand. The movement by prominent CB radio technicians to avoid overmodulation is to be lauded generally for not fruitlessly disrupting adjacent channel users, and especially to preserve the functioning of auto-squelch. The false closing of squelch even for on-channel strong overmodulated signals is due to the large amount of out-of-band distortion products generated by overmodulation, which can cause the auto-squelch algorithm to incorrectly determine that this is an undesired strong signal present and close the squelch.
In practice, one might use auto-squelch for general driving around and scanning channels, but switch to manual squelch when trying to listen to a known channel with a strong signal that is overmodulated. For reception of weak signals, switching to the manual squelch and leaving the squelch open is generally optimal. These characteristics are why it’s so important when buying a CB radio to get one that also has the “NRC” noise reduction algorithms, which make listening far less fatiguing on any voice mode AM / FM / SSB. It’s a shame that so many are blindly buying the “Cobra 29” style legacy radios when for the same price and often less money, they could get a CB radio with NRC and other features like scanning that make the CB radio experience much more enjoyable and productive.