[opendtv] Re: Popular screen aspect ratios

  • From: Craig Birkmaier <craig@xxxxxxxxx>
  • To: opendtv@xxxxxxxxxxxxx
  • Date: Mon, 27 Dec 2010 08:03:47 -0500

At 7:16 PM -0500 12/26/10, Albert Manfredi wrote:
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.
Not really. These are simply accommodations for differences in blanking. But the important point is that you can squeeze a little or squeeze a lot for any family of formats using the same number of lines.
And remember, the ATSC table is just a subset of what is actually being used in the real world. Cable and DBS use a variety of line lengths, and for Over the Top almost anything goes.
Bottom line, the numbers (raster size and aspect ratio) really don't matter - they are just variables that are identified in the file/stream headers. The decoder and display are expected to deal with ANYTHING.
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. 

On a 16:9 display.
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. 

This is all relative.
If a display is accessing content that uses a small subset of available formats (aka ATSC) then it must do the best it can with the source relative to the actual display resolution and aspect ratio.
But there is nothing to say that all sources must be optimized for 4:3 or 16:9. It is quite easy to optimize source for any display resolution and aspect ratio. This happens all the time via the Internet, and is a menu option for any professional creating content. 

You just need to take off your ATSC blinders.

Regards
Craig


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