[opendtv] Re: Popular screen aspect ratios

  • From: Craig Birkmaier <craig@xxxxxxxxx>
  • To: opendtv@xxxxxxxxxxxxx
  • Date: Sun, 26 Dec 2010 10:45:46 -0500

At 6:27 PM -0500 12/23/10, Albert Manfredi wrote:

Craig Birkmaier wrote:


 As with ANY digital compression algorithm, it is possible to use
 anamorphic squeezes to conserve emission bandwidth.
As of today, anamorphic squeeze for TV is defined as a squeeze of 16:9 material into a 4:3 frame (by convention, anyway). 

Whose convention?
There is a video production format that uses this specific anamorphic squeeze; ITU-R BT.601 defines both 4:3 and 16:9 aspect ratios using the same 704 x 480 or 704 x 576 rasters. I use the 704 number rather than the full 720 because the other 16 samples are used for the transition to blanking. I would also note that the ATSC standard uses 704 samples per line for SDTV.
But there are an almost infinite number of squeezes being used with both MPEG-2 and h.264 today to conserve emission bandwidth. The list of ATSC formats that Ron Economos posted 

http://www.atsc.org/cms/standards/a_72_part_1.pdf
has all kinds of variable line lengths that represent anamorphic squeezes of the original source.
In truth, any multiple of the 16 x 16 macroblock is fair game, and ANY conformant MPEG decoder should be able to handle any raster size - however, all bets are off for commercial ATSC receivers.
Squeezing 21:9 material into that same 4:3 frame would result in all existing sets showing tall, skinny people, when receiving the 21:9 transmissions. It would require a separate broadcast stream or a separate BluRay or DVD to do this, or STBs and disc players designed to unsqueeze correctly both 16:9 and 21:9 aspect ratios.
Any conformant MPEG decoder is required to decode any size raster to any aspect ratio defined in the MPEG headers. The local display processor may take the decoded raster and scale/crop it in any way it wants to fill the display.
There is absolutely no reason that a 21:9 (2560 x 1080) source raster cannot be scaled to 1920 x 1080 and distributed using MPEG-2 today. The alternative for a 2.39:1 source would be to encode it as 1920 x 810. There is some question what some TVs (especially ATSC), would do with the anamorphic 1080 source (2.39:1), however, the Phillips TV should handle this without any problems. The safest bet to work with all existing TV displays would be the 1920 x 810 line encoding. Perhaps some day the TV guys will figure out what the computer guys have been doing for years - using the metadata to properly scale ANY source to fill one axis of the screen without distortion.
If, back in the 1990s, a 2:1 aspect ratio had been established for DTV, instead of the 16:9 ratio that was set (in spite of FCC inaction), then the anamorphic squeeze would have been standardized as 2:1 into the 4:3 frame. And if that had been the case, Jeroen's super wide TV would have been a little better off today.
If on the other hand we had accepted the computer industry suggestion to "just deal with it," then every TV would be able to display ANY format in ANY aspect ratio properly. THis is trivially easy - you really have to muck things up to limit the choices...
Instead of achieving 807 or so effective lines of vertical resolution (1.77/2.37 * 1080 = 806.6), as he is doing now on 1080p material, he'd be getting 911 lines vertical. Not bad.
It is easy to use the entire 1080 lines. TVs that only understand certain formats ARE the problem. 

Regards
Craig


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