[opendtv] Re: Popular screen aspect ratios

  • From: Ron Economos <w6rz@xxxxxxxxxxx>
  • To: opendtv@xxxxxxxxxxxxx
  • Date: Fri, 07 Jan 2011 05:06:17 -0800

No, I'm referring to the IEC-61880 signal in the VBI.
It's those white lines at the top of the image below
the closed captioning. On Cliff's player, it's being
sent in what looks like two separate lines in both fields.

The first white line at the left of the raster is the reference
bit. It sets the timing of the following bits. The next bit
is always zero and then there are 20 bits of data.

After the zero bit, there are two bits of aspect ratio
information.

00 = 4:3
10 = 16:9
01 = 4:3 letterboxed
11 = not defined

In the first picture (MGM Title displays 1 X 1 aspect ratio
on 4 X 3 screen when BD player is set to pan and scan .jpg),
the aspect ratio is 10 or 16:9. On the second picture
(MGM Title displays stamp sized 4X3 aspect ratio
on 4 X 3 screen when BD player is set to letter box .jpg),
the aspect ratio is 01 or 4:3 letterboxed.

The other 18 bits are used for CGMS-A, APS and a
6 bit CRC. The 2 bit line near the center of the images
is signaling CGMS-A copy never. The CRC is off to
the right of the raster.

In Cliff's pictures, it's on the wrong line. IEC-61880
is supposed to be on line 20 and 283 Here's a
picture from a know good DVD player and capture
method with CC on line 21 and IEC-61880
on line 20 and 283 (signaling 16:9).

http://www.w6rz.net/analogcc1.png

Now for the aspect ratio bonus round question!

If a 720x480 MPEG-2 video bitstream with
aspect_ratio_information set to 4:3,
display_horizontal_size set to 540 and
display_vertical_size set to 480 is received
by a decoder, how should it display the
image on  a 16:9 output?

Ron

On 1/6/2011 5:10 PM, Manfredi, Albert E wrote:
Ron Economos wrote:

Actually, the baseband analog signal can convey aspect ratio
information. You can see it clearly in the pictures Cliff
Benham posted the other day.
I must disagree, or are we using different semantics?

Cliff's posts showed that if the BD player was set to "letterbox," and 
presumably that specified inherently that the image was going to a 4:3 display (otherwise 
why would you have to letterbox), then there was no distortion. The image was correctly 
letterboxed. And you wouldn't expect any under these conditions.

If the BD player was set to pan and scan, again one must presume that this mode 
was intended for 4:3 displays only, then the image was distorted. It was 
compressed, as if the 16:9 signal was sent to the 4:3 display by mistake.

John Shutt concluded, and I have to agree with him, that this was caused 
because the disc being played simply did not support that pan and scan feature. 
So the player could do nothing other than default to 16:9 output instead. And 
as expected, the 4:3 screen showed a compressed image.

The thing is, no matter whether the image is going to a 4:3 or a 16:9 display, 
the entire frame must be scanned in the same amount of time, with baseband 
analog interfaces. Therefore, there is nothing in that baseband signal that can 
give a hint to the display as to what aspect ratio image the display should be 
rendering.

This is what was being confused by the FCC and others, back when they dropped 
the 16:9 requirement.

Bert


----------------------------------------------------------------------
You can UNSUBSCRIBE from the OpenDTV list in two ways:

- Using the UNSUBSCRIBE command in your user configuration settings at FreeLists.org
- By sending a message to: opendtv-request@xxxxxxxxxxxxx with the word 
unsubscribe in the subject line.

Other related posts: