Channels broadcast in HD are all the same right? Not necessarily.
According to Glen Dickenson of Broadcasting & Cable, the signal distributed by cable companies can definitely vary.
“That's because many multichannel operators have recompressed broadcasters' HD video to reduce the bandwidth needed to pass along the signal.” he writes. “For example, a cable operator might receive an 18 megabit-per-second HD stream and recompress it to 15 Mbps before passing it down the pipe.”
15 Mbps sounds like a lot, but with multicasting more and more prevalent, that space gets eaten up quickly. Stations commonly have had a main, high def feed, and substations in standard definition, but with more of these substations switching to HD, there’s less bitrate to go around. When bit rate goes down, picture suffers, and when picture suffers, consumers get mad. The difficulty is finding the acceptable point of picture quality for the average consumer.