HD / Multicast reception behavior

I have 3 Alpine CDE148BT's in my 3 primary vehicles. They all do this:

When listening to a primary (HD1) station the display shows "HD". I occasionally see the display revert to "antenna" which is regular FM broadcast processing. Sometimes I detect a degradation in sound quality, but no "drop out".I assume reception has degraded, and the digital reception can't be sustained, so it reverts to analog. Ok.

When listening to a Multicast channel "HD2", "HD3", etc I frequently get a drop out of sound and display shows "loss of signal". The drop out lasts about 5 seconds, and occurs fairly frequently going under overpasses, buildings, etc. Really annoying.

Question: Is this a trait of HD and Multicast broadcasts in general, or just a quirk of the way Alpine deals with a degradation of reception? I suspect that the multicast is digital-only, so when digital translation can't be maintained, it drops out. Like Sat radio does.

Is there anything I can do to improve reception? I remember doing installs in the 70's we'd open the tape slot and adjust the antenna trimmer. Anything like this on an Alpine? I remember that poor or no AM reception indicates antenna problems. All 3 of the units have similar good AM reception. I live in Los Angeles so there's lots of close-by AM stations, and this may not be a great indicator of antenna performance. (They're all foreign language, but there's lots of them :grin:)

Thanks for reading.

Digital broadcasts are very finicky. If you don't have line of sight between your antenna and the tower you get dropouts (trees, buildings, other vehicles all cause this.) Plus the broadcast distance max is around 40 miles. When you are listening to side bands like HD2, the effect is even more pronounced as these signals tend to be weaker. Is there anything you can do? I don't think so, unfortunately. HD radio is most likely on its way out...

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