KPIX picture quality has been discussed here recently and I've noticed it as well. The below images are screen caps from a TiVo transfer/decrypt program that shows the file size and average bitrate for each program recorded. Last Friday I decided to record both the OTA signal and the Comcast signal for 2 shows airing on KPIX. KPIX is the only major net I can receive reliably with my OTA antenna. KBCW is another local channel I can get and I will try to record both OTA and cable sometime this week.
If you note in the below images, the top 2 listings for each show are the 2 different feeds for the same program and the file size and average bitrate varies dramatically between the OTA feed and the Comcast feed.
I can't receive KNTV OTA but I'm curious if it has also been compressed(?) by Comcast as the 720p channels - KGO and FOX - seem unaffected as their file sizes/bitrates appear to be consistent for the last couple of months - I have a lot of unwatched ABC programs sitting on the DVR. But I can't be certain of that as I cannot receive either station OTA.
I'm not sure if this apparent compression is on purpose or not, not enough data for one thing, but what ever is happening the resulting quality is terrible with lots of movement blur and pixelating images. To be frank, a comparable download of one of these shows from an illegal file sharing site has better perceived quality than what Comcast is providing, they must be using an OTA feed as their source.
Also, from looking at those images above it looks, and I'm only guessing, that a change occurred around the first part of April as the file sizes and bitrates are considerably larger for the Blue Bloods previously.