[opendtv] Re: Popular screen aspect ratios

  • From: Craig Birkmaier <craig@xxxxxxxxx>
  • To: opendtv@xxxxxxxxxxxxx
  • Date: Sat, 8 Jan 2011 09:59:31 -0500

At 4:19 AM -0800 1/8/11, Ron Economos wrote:

It's a trick question. It's just a good example of the
complexity associated with digital video aspect ratios.

The answer is that the decoder display process
(when set for 16:9 output) should just consider it
to be a 16:9 image and not perform any scaling
or cropping at all.
I don't think that this is true given the original question; the use of the display_horizontal_size syntax adds another processing step beyond simply decoding and presenting the samples.
Horizontal scaling is always required when using 720 x 480 as both the 4:3 and 16:9 samples are not square. Square pixel 720 x 480 creates a 3:2 aspect ratio. 





A specification that discusses this is here:

http://www.etsi.org/deliver/etsi_tr/101100_101199/101154/01.04.01_60/tr_101154v010401p.pdf

See section 5.1.3

To see what different decoders do, I've uploaded a
short 720x480 bitstream with 4:3 aspect ratio and 540
display_horizontal_size. The source content is 16:9.

http://www.w6rz.net/artest.ts
Unfortunately I am not equipped to play MPEG-2 TS streams. Would love to see these results to see if the displayed image is indeed only 540 samples in the H axis rather than 640. IF there are no Pan and Scan offsets then the proper method of decoding would be to center cut the 540 from the decoded 640 samples. 

Regards
Craig


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