I've noticed various levels of compression used for ESPN and CSN HD baseball programming.
With baseball, the effects are most visible when camera shots contain movement and show lot's of grass or dirt infield (base paths) in the background. The movement can be panning or zooming-in or out. When the data is over-compressed you'll slowly become aware that these backgrounds sometimes look unnatural. Sometimes, the signals are being compressed too much, and the actual MPEG pixel tiling becomes visible to anyone who knows what to look for. Worse, the compression levels (and background detail) constantly changes during camera or lens movements!
Sometimes, I've actually been getting sick to my stomach (for a second or two) as I watch this occur onscreen. As soon as the movement stops, a less-compressed, more natural-looking version of the background grass and dirt re-appears, and my queasiness disappears.
I have noticed this in two or three games. Most recently it was very evident during the Chicago White Sox game of 4/19, which I think was broadcast on CSN (if I recall correctly). It is a very odd sensation. After awhile I could not keep watching, and turned off the White Sox game.
I wonder where the compression is occurring and who is controlling it?