Hi,
As Chris has pointed out, it's likely your cable operator who is at fault. Some networks, ABC if memory serves, use 720p to distribute their signals in HD because it's better for fast-moving sports, but most broadcasters go with 1080i for HD because it requires less bandwidth and is easier to transmit.
Of course, MPEG compression has become much more sophisticated than in the old days. I recall the first demo of satellite DirectTV at the Consumer Electronics Show in Las Vegas, and during a live broadcast of a basketball game, every time a player raced down the court and jumped to get a basket, the image would freeze and pixelate. It simply couldn't handle a combo of fast horizontal and vertical action. Those drawbacks were overcome fairly quickly as more refined versions of MPEG were developed.
Regards,
Alan
