Comcast is now using "HDTV Lite" for all new channels, with each channel configured for a max of 13.3Mbps ABR. When this 13.3Mbps ABR is not sufficient, you will see pixelization, macroblocking, and skipped frames.
Comcast is also doing the same for a number of existing cable channels when new channels are added.
Previously, all channels were allocated just under 20Mbps ABR.Long answer
Until recently, in most markets, Comcast passed HD channels as is from the content provider without any extra compression. No provider offered higher quality, although some providers like Verizon FiOS did the same to provide equal quality.
Last fall, Comcast's management decided to abandon its "quality over quantity" approach. With its SDV trials in Denver and New Jersey, Comcast found that that SDV technology alone wasn't sufficient to meet their targets for new channels. Comcast's Stephen Burke and other executives concluded that added compression -- "HDTV Lite" -- was needed to meet their targets on many systems. They decided it was necessary to trade some quality to remain competitive in channel count with DirecTV and Verizon FiOS. According to this CED article, they are no longer catering to "golden eye" viewers with larger screens.
Previously, Comcast limited itself to two HD channels per 38.8Mbps QAM channel. That assured full quality on every HD channel. To create space for new HD channels in a number of markets, Comcast deployed Imagine Communications' ICE to squeeze three HD channels per QAM using added compression. This practice started in a handful of markets late last year, but is now being expanded to dozens of other markets in January (and February) to make room for new HD channels such as FOOD-HD, SCIFI-HD, USA-HD, and National Geographic HD.
Here is video footage showing what Comcast Chicago users are seeing since the new channels were added:http://www.youtube.com/watch?v=nT30UW80N0I
As you can see, the picture breaks up and skips frames during intense scenes. It is not this bad on all channels, but users with larger screens also report reduced quality on some older HD channels that began when -- or shortly before -- the new channels were added.
As far as I know, Comcast is not yet applying this added compression to any local HD channels.