Do stations have different processing for their main analog signal and their HD signal? Would seem important for AM stations on HD channels which sound over-processed in HD to my untrained ear. I would think the goal on HD would be to be as clean as possible. One could argue the same (as Kelly does) for strong FM signals. Nerd question, but I am curious from the engineers that frequent this board.
Back when FM/IBOC/HD started, stations would need to process the analog and HD-1 separately. They would also have to delay the analog audio by several seconds to match up with the digital audio. That way when a listeners radio would switch between digital to analog, and back, there would be minimal interruption. The problem with that chain was it required several different devices to keep everything synced-up from a time perspective, and that when a tuner switched back and forth between analog or HD, there would be a stark difference in sound between the two.
Processing manufacturers solved the aforementioned, sort of, by selling all-in-one audio processors. Modern processors have a delayed analog stereo generator output, and a second AES digital output for feeding the HD-1 side. The benefits are that for the most part, everything stays in sync analog plus HD, and sounds the same because it's the same processing settings. The bad part is; everything sounds the same. So if a PD, engineer, GM, owner, etc., wants the station to be loud and heavily processed on analog, then the HD-1 sounds equally processed.
Back in the day the original assumption was that stations would back down aggressive analog processing to preserve the more accurate sound and dynamic range of the digital side. Unfortunately, most stations chose the other way around.