I am sure most of you have experienced the superb HD picture offered by Blu-ray discs, and wonder why over-the air (OTA) HD broadcast do not exhibit the same quality. The answer to this question is compound and has been discussed in similar blurbs on this subject. However, one of the broadcasters' limitations to optimizing HDTV is inherent in the ATSC standard itself, and that is the obsolete MPEG-2 compression system.
When the digital (HD or SD) signal is originally digitized at the production head (camera, film scan chain etc.), normally the brightness component (Y) is digitally sampled at a rate twice that of the two color components (Pr, Pb). The ratio is referred to as 4:2:2. (The derivation of the "4" is somewhat arcane, dating back to the early days of digitizing color video signals.) Most broadcasters' video plants haul digital video around and through the production process in the 4:2:2 ratios. However, some high quality video production systems digitize at 4:4:4 ratios, meaning the color signal bandwidth is not compromised and has the same resolution as the brightness component. Although Blu-ray is normally encoded at
broadcasting levels, an increasing number of Blu-ray video disc encoding processes are incorporating 4:2:2 ratios, and the result is obvious to the viewer.
The ability of Blu-ray to offer 4:2:2 color quality levels is possible because of the use of the MPEG 4 compression process. Unfortunately, MPEG 2, because of encoding inefficiencies, processes video at ½ the 4:2:2 color component vertical resolution, called 4:2:0. Too bad. No way will 4:2:0 broadcast video look as good as 4:2:2 Blu-ray video - unless there is a way for ATSC to transmit MPEG 4 compressed video and still remain "legal."
There is a glimmer of hope or at least a possibility. The about-to-be minted ATSC Mobile/Handheld (M/H) candidate standard indeed specifies MPEG-4 (AVC) compression that could accommodate 4:2:2 video. Now, if the M/H channel were allowed to be extensible, say to ˜15Mb/s, then at least two full 4:2:2 HD programs plus one legacy (MPEG-2) program could be accommodated in the allocated 19 Mb/s digital channel. Wouldn't that be a hoot! However, legacy HDTV receivers would require a separate M/H box to select and decode the M/H sub-channel multiplex. (So, what's another box? You always wanted something to stick into that extra HDMI input. Right?) No doubt, as soon as the M/H Standard is finally blessed, all ATSC receivers will include built-in M/H capability.
Hopefully, ATSC will have the foresight to design bandwidth extensibility in the M/H standard. This will give broadcasters further market leverage by offering viewers true Blu-ray quality programming.
Posted by Ed Milbourn, March 26, 2009 8:49 AM
About Ed MilbournAfter graduating from Purdue University with degrees in Electrical Engineering and Industrial Education in 1961 and 1963 respectively, Ed Milbourn joined the RCA Home Entertainment Division in 1963. During his thirty-eight year career with RCA (later GE and Thomson multimedia), Mr. Milbourn held the positions of Field Service Engineer, Manager of Technical Training and Manager of Sales Training. In 1987, he joined Thomson's Product Management group as Manager of Advanced Television Systems Planning, with responsibilities including Digital Television and High Definition Television Product Management. Mr. Milbourn retired from Thomson multimedia in December 2001, and is now a Consumer Electronics Industry consultant.