There was a big article back in September on the subject of sync detectors an how they can take the "crunchiness" out of long-range signals. <link> There are some pretty impressive demo sound clips included.
TLDR: "Selective fading" effectively scrolls a notch filter over a signal. When the notch happens to hit the carrier frequency, the carrier level drops and overall it's the same as an over-modulated signal – hence the crunchy sound. A sync detector substitutes in a full-powered carrier, so the crunches never happen. Also if your radio's sync has a USB/LSB option, you can use it to put a little more distance between what you're tuning in and some adjacent station's signal splatter.
Since reading this, I've been using the sync detector on my PL-660 more and more. It's especially effective for medium wave AM stations out towards west, where the longer propagation paths frequently exhibit selective fading. I've been using sync for years to eliminate spatter from adjacent stations, but for cleaning up WOAI (1200 KHz, San Antonio), that article was a real eye-opener.
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