I work with the broadcasters that feed the cable companies. My business will design and deploy this technical infrastructure.

In many markets these days, you will find that the cable companies actually provide a better feed that what is available OTA. This is because broadcasters are limited to 19Mbps of MP2 to the transmitter. They will often stuff one HD and several SD's in that space. In some cases they will even do two HD's. An average HD might be around 12 Mbps of MP2. This isn't great,. but, the encoders and statistical multiplexers are getting better at making the most of it.

Starting years ago, the cable companies began to take feeds directly from the broadcaster to the head end. They might take uncompressed HD, but typically the broadcaster will provide a high bandwidth MP2 or MP4. Maybe 25Mbps.

The cable providers have a big advantage in that they encode to MP4, so if they are getting a good signal from the provider, they at least could get a very nice picture to the home, if they choose.

This varies market by market, but generally speaking Comcast is the best at this (it pains me to say), Time Warner maybe second best, and then it's all downhill. Direct and Dish will also install a mini head end in each market where they are beginning to get direct feeds too, so those are also getting better.

I live in Seattle and have Comcast. Surprisingly, all of the HD channels look excellent, and I'm super critical of these things.

Again, this varies wildly by market, and by the head end that serves your neighborhood.