[opendtv] Re: Finally anamorphically compressed 480i

  • From: "John Shutt" <shuttj@xxxxxxxxx>
  • To: "OpenDTV" <opendtv@xxxxxxxxxxxxx>
  • Date: Tue, 25 Jan 2011 21:54:44 -0500
----- Original Message ----- From: "Manfredi, Albert E" <albert.e.manfredi@xxxxxxxxxx> 


Case 1. Assume a 1920 X 1080i source program sent by PBS, which contains
4:3 material. This will incorporate the pillars as part of the data. This
is how 4:3 material is always sent on HD channels, as far as I can tell.
(Forget this 1440 X 1080 stuff. That 1440 X 1080 is simply the effective
resolution of the 4:3 image in the 16:9 container, that's all.)

Now PBS wants to create a SD version of that stream. Or maybe the station
does this just before broadcasting, for SD multicast.

Better way:

Forget for about AFD. Take that 1080i stream and create an exact copy in
704 X 480i. Send it out as SD, in a 16:9 container. No AFD should be
necessary. Just a dumb copy, 1080i wide container to 480i wide container.
The pillars will be there, because it is an exact copy of the 1080i, so
they are part of the data. All ATSC receivers will decode it correctly, by
stretching to 16:9. The loss in horizontal res is unobjectionable. Even
4:3 screen will get this correctly, because they will crop out the sides
(using pan and scan).



As Meatloaf's girlfriend said in the song "Paradise by the dashboard
lights:"  STOP RIGHT THERE!

You wrote "Even 4:3 screen will get this correctly, because they will crop
out the sides (using pan and scan.)"

Absent AFD, there is no way for the receiver that is spitting out 4:3 to
determine if the source material is safe to center cut, so it's default
action is to letterbox.  The 4:3 viewer will see a postage stamp of 4:3
material surrounded on all four sides with black.


Case 2. Now assume a 1920 X 1080i source program with 16:9 image. Same
deal, either PBS or the station wants to send it out as a SD multicast.

Better way:

Forget about AFD. AFD wants you to postage stamp the blasted thing (i.e.
AFD wants to letterbox and send as a 4:3 SD stream.) INSTEAD, send this
out just as you did in Case 1 SD multicast. Straight downconversion of the
16:9 1080i image to a 16:9 704 X 480i image. Dumb copy, just like Case 1.



So, what do you do, Bert, when some of the programming is 16:9 full screen,
and some of it is 4:3 pillarboxed?  What does the 16:9 viewer see?  What
does the 4:3 viewer see?

AFD lets each display type (widescreen or conventional) make the best choice
on how to handle the material for it's own display.  If there were just one
universal
"dumb copy" approach that would satisfy both types of viewers, we wouldn't
have needed AFD in the first place.

Now, here's one for you:  A 4:3 SD channel is showing a letterboxed program.
Absent AFD, a 16:9 display will pillarbox that material, presenting the
viewer with a 16:9 image with black surrounding all four sides.  With AFD,
the 16:9 display will know to activate it's zoom and fill the screen with
the program.

So pretend that there is no AFD, and the viewer grabbed her remote and hit
the 'ZOOM' button to get rid of the postage stamp effect on the letterboxed
SD program.  When the next SD 4:3 program comes on, it is full screen
material,
and the 16:9 display will cut off the top and bottom of the frame, the exact
situation you complained about in a previous email.  This will continue
until Ms. viewer grabs her remote again and turns off the 'ZOOM' feature.

With AFD, the display knows that the program is 4:3 full screen, and will
automatically turn off the 'ZOOM' all by itself.  And turn it back on for
the next letterboxed program.

John




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