[opendtv] Re: Popular screen aspect ratios

  • From: "Manfredi, Albert E" <albert.e.manfredi@xxxxxxxxxx>
  • To: "opendtv@xxxxxxxxxxxxx" <opendtv@xxxxxxxxxxxxx>
  • Date: Wed, 29 Dec 2010 18:26:01 -0600

Craig Birkmaier wrote:

> This is true for EVERY display out there Bert. Source must be accommodate
> to the actual screen aspect ratio. 4:3 and 16:9 are common presents, but are
> by no means the only aspect ratios that can be displayed undistorted. You
> can see this all the time with SD DVDs that use different number of active
> lines to support wider than 16:9 source formats.

What these DVDs actually do is, after you tell them whether your display is 
16:9 or 4:3, they create the 16:9 or 4:3 image, and they add in the black bars 
to fill that 16:9 or 4:3 frame properly. Then, after the decoder has done this, 
the user can determine whether to display as is, or zoom in and crop the black 
bars. Or distort the image to fill in tha black bars with over-stretched image.

The important point being, typical DVD players and STBs don't ask you, for 
example "Will this image go to a 3:5 screen?" (Meaning taller than it is wide.) 
If you do have a 3:5 screen, the STB wouldn't know what to do with it. It would 
take extra smarts in the display to take care of the problem, and the 
manufacturer of the display would have to tell you which of the two common STB 
settings to use.

I'll bet you this is exactly what the Philips 21:9 display does, Craig.

> You are catching on. But you do not need to tell the STB anything. The STB
> is going to present the display with a raster that has been decoded to the
> proper aspect ratio. The decoder already knows the target aspect ratio for
> which it must produce an appropriate undistorted raster.

I'm catching on? How does the decoder know "the aspect ratio for which it must 
produce the undistorted raster," Craig? The decoder is in a commodity STB, not 
associated with any particular display. How do you assume such clairvoyance? 
Ever tried to enter the wrong display aspect ratio selection, when setting up 
your STB? Try it and see what happens. More importantly, have you ever had any 
STB that didn't ask you for the aspect ratio of your display, when initially 
setting it up?

Possibly, with a two-way digital interface between STB and display, your 
clairvoyance assumptions might be valid. The display could indicate to the 
decoder that it is some weird aspect ratio, and the decoder could accommodate 
that weird shape. But this cannot and does not happen with common analog 
connections, like composite, S-video, or component. (I don't know whether STBs 
ask for your display aspect ratios when you use the HDMI interface. If they do, 
then clearly they have to have this info a priori.)

Bert
 
 
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