pixelization?

You should check into having the signal amplified by your cable company. It sounds like you have a weak signal.

This is a natural consequence of how video compression works.

The images are split up into 8x8 sample blocks which are converted into the frequency domain where you have an average level for the block plus the changes across the whole block, that occur twice, etc.

Periodically a full still frame is sent.

Subsequent frames are encoded with how far different groups of blocks have moved plus the changes.

When there's fast motion or camera pans there are more changes which must be represented and the system starts discarding the high frequencies and small color/intensity changes that make up details.

Cramming more channels into the same system like cable and satellite companies do reduces the bandwidth available to each one so more information needs to be thrown out.

Thanks for all insightful responses. My only question
is why does this only happen on HD channels? The SD
channels of the same programs are fine. The improved
picture in HD far outweighs this issue. Watching my beloved NY Yankees win last night in HD was great! Pixels and all. Yankee bashers need not reply!

My guess is that HD, needing a larger bandwith, would display the compression more so, than SD, and, SD, being a lower resolution, would mask the compression better. I did a few installations in my friends houses, who use cable, and I was able to reduce the artifacts with better cables. I haven't played with HDMI on thier (cable) sets, but when I replaced the poor component cables that come with cable boxes, with top of the line Monster Component Cable the difference in picture was staggering. (a neighbor of a freind of mine went out and bought the same TV when they saw her picture and was very dissapointed when it didn't lok as good; the difference was, indeed, the cable.
When I had cable, I fought with my cable company to lay a new cable direct from junction to my home, instead of splitting it among the other residents in my development, and the picture was much, much better (almost as good as my sat picture-almost.

My only question is why does this only happen on HD channels? The SD channels of the same programs are fine.

With no compression or equal compression, hd would require vastly greater bandwidth than sd, because of its far higher pixel count. Therefore the cable company compresses the hd channels heavily, resulting in the fast-motion artifacts you are seeing, while compressing sd (on sd digital channels) to a much lesser degree. And sd analog channels, by their nature, have no compression at all.

Also, for those who may be concerned about signal strength on their incoming cable, if you have your internet service provided via the same cable you can usually determine your incoming signal strength (as it exists at the cable modem) by interrogating the modem from a web browser program on your computer. My Scientific Atlanta cable modem is interrogated by entering http://192.168.100.1, which will cause it to return various status information including incoming signal strength. If that number doesn't work for you, Google the modem's make and model number and you'll be able to find how to do this for your particular model.

But I don't think the op's issue has anything to do with signal strength. The sensitivity to motion in the picture, and the fact that it only occurs on (multiple) hd channels, strongly suggest that it is due to compression applied by the cable company.

Regards,
-- Al