[opendtv] Re: Popular screen aspect ratios

  • From: Albert Manfredi <albert.e.manfredi@xxxxxxxxxx>
  • To: <opendtv@xxxxxxxxxxxxx>
  • Date: Sun, 26 Dec 2010 19:16:26 -0500

Craig Birkmaier wrote:
 
> 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.
 
Not quite, Craig. If you did the numbers, the aspect ratios supported by those 
options are either 1.33:1 or 1.77:1 for most of them. Then, in the cases where 
slightly different widths are allowed, e.g. 720 instead of 704 and 544 instead 
of 528, the unsqueezing is specified to be by the same amount as the slightly 
narrower option. So you get a slightly wider aspect ratio. The final results of 
the oddball options are 1.37:1, 1.36:1, and 1.81:1.
 
Even if receivers were to support these odd cases correctly, it wouldn't make a 
lot of difference. As proof, when you see a 1.85:1 movie transmitted over HDTV, 
or on DVD, you won't notice any appreciable letterboxing, or any at all.
 
Looks to me like the dimensions of the displayed frame are meant to be 4:3 or 
16:9 by convention, or too close to make a difference. So that means that any 
odd-shaped display like 21:9 will be forced to throw out some of the image 
resolution, if it wants to display full screen images undistorted, while being 
compatible with other receivers out there. Transmitting any other squeeze 
ratios would be a risky proposition. It all depends on the receiver's software, 
and what unsqueezing options it can support.
 
Bert
                                           
 
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