When you order Hi-Def TV, you expect it to be on the shining Excalibur level, but it looks like Comcast is degrading the quality of some HD channels in order to make room for more channels. To test this theory out, Avsforum member bfdtv recorded the same shows from the same channel at the same time on both Comcast Hi-Def and Fios TV. The left is the FiOs. The right is the Comcast. As you can see, the Comcast signal looks like crap. The forum thread has more screenshots, a signal analysis, and some source videos. So far the Comcast channels receiving extra compression are: Discovery Channel, SciFi, USA, Food, NatGeo, UHD, A&E, HGTV, Starz, Cinemax, HBO, TLC, Animal Planet, Discovery HD Theater, and History HD.
Comcast HD Quality Reduction: Details, Screenshots [AVSforum] (Thanks to thedave!)