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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. - Ed
Posted by Ed Milbourn, March 26, 2009 8:49 AM
Reader Commentary When a mobile version of 8-VSB was first proposed those proposing were adamant that all legacy receivers would at least be able to receive and view the signal if not take advantage of the added reception sensitivity. Later that changed to when some suggested that broadcasters would use an advanced codec with any mobile bandwidth that legacy receivers could not decode. Proponents then suggested that an additional "box" would be needed to decode the advanced codec. Now we will need an additional box because the M/H modulation is not itself receivable by legacy receivers. M/H is new in my book if a legacy receiver can't use it. Whatever and however you twist it any bandwidth used for any purpose by broadcasters that cannot be received and viewed using a legacy receiver is NOT legal in my opinion. Congress and the FCC would not allow broadcasters to use DVB-T as an ADDITIONAL modulation because it would require an additional "box" and confuse consumers. An additional "box" to receiv... Hi Ed, As a general comment, I have seen that a good high quality multipass encoder can get good results. However my understanding (and please someone jump in and correct me) is that a lot of content is encoded on the fly, and there is no real way to get "best" results in that scenario? I also notice cable system tend to have really bad quality, and since they're paying for satellite bandwidth for the feeds, I can only assume that in doing an economic tradeoff (as a monopoly) they crank up the compression (MPEG4) to keep the cost down and then transcode locally back to MPEG2. I know you were focused on broadcast, but IMHO the only real opportunity is with cable systems as new/better boxes could be given to customers; people are not going to run out and buy new sets for a government mandated ATSC II. However, and there's a snowball's chance in hell of this happening, IMHO any new *public* broadcast standard should use license (patent) free audio and video standards. The headac... I think there may be a misinterpretation. The M/H Candidate Standard A/153 Part 7 (available from the ATSC.org web site for free) states pixel resolutions in the H and V dimensions. The M/H system uses MPEG-4 Part 10 AVC and SVC video coding as described in ITU Rec. H.264 | ISO/IEC 14496-10, with certain constraints. The pixel counts for the AVC being 240 x 416 (wide screen) and for SVC as 360 x624 and 480 x 832. Both are progressive scan systems, and these are 'before compression' ratios. I seriously doubt that even scaling up to 15 Mbps (consuming nearly all of the available video payload) for an OTA 8VSB DTT system would look good without a considerable amount of 'post processing'.... You seem to imply that MPEG-4 gives a better quality of picture then MPEG-2. However most the places I go online to read about MPEG-4 diputes this idea. Instead the web says that MPEG-4 is good for more channels to be pushed out due to it's enhanced compression processing ability. So who is right, you or all the other websites out there that state that a better quality image is NOT what you really get with MPEG-4. Help me understand please. Till then call me confused. :?... More from Ed Milbourn
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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. |
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