[opendtv] Re: Popular screen aspect ratios

  • From: Craig Birkmaier <craig@xxxxxxxxx>
  • To: opendtv@xxxxxxxxxxxxx
  • Date: Thu, 30 Dec 2010 09:15:41 -0500

At 6:26 PM -0600 12/29/10, Manfredi, Albert E wrote:

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.

First, the output modes of all boxes (STB, DVD, etc). have the 4:3/16:9 settings for legacy reasons. Accommodating widescreen sources on 4:3 displays requires additional processing that must be done in the box, as older 4:3 displays DO NOT have internal display processors. So you must tell the box that the display is 4:3 and then use the control in that box to deal with aspect ratio accommodation.

For new TVs you simply choose 16:9 and let the display processor deal with the accommodation issues (usually automatically, but you can override the defaults).

Second, the decoder creates what it is instructed to create via the metadata in the headers. For widescreen movies delivered via SD , it is typical to use the 16:9 format but to only encode the actual number of active lines used. There is a header that can be used to specify the additional number of inactive lines; the decoder only acts on the active macroblocks, then adds (or subtracts) these additional lines when creating the output frames.

For example, please note that there is no 1920 x 1080 emission format. The ATSC format is 1920 x 1088, with 8 inactive lines in some of the macroblocks. These lines are subtracted from the decoded image for presentation.


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.

More or less correct. When the boxes are set to widescreen mode (as opposed to 4:3) they will typically output a 16:9 raster that may have throw away areas. You will then use the display controls to select the mode of aspect ratio accommodation. IF the display has the ability to accommodate multiple aspect ratios, you would always set the source to widescreen; the 4:3 mode is only for legacy display accommodation.


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

It accommodates whatever source it is given, which will typically be 16:9. But there is no reason that this display cannot be fed a full 2560 x 1080 source from a box that has extended capabilities.

The issue is that the TV industry and CE vendors are the ones stuck on "One Format To Rule Them ALL." In the world we are moving into this stuff simply does not matter.

I'm catching on? How does the decoder know "the aspect ratio for which it must produce the undistorted raster," Craig?


The aspect ratio is required in the MPEG headers. The decoder will reproduce an undistorted raster to match the header information. The display can then deal with aspect ratio accommodation and add any distortion it wants. That being said, some device may have integrated decoder/display processors. Thus a device could be smart enough to simply pass the decoded pixels to the display processor, which would then deal with the necessary scaling to accommodate the source on that display.


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?

Any external box will produce wither a 4:3 or 16:9 output. If the source is letterboxed into 16:9, this is what you will see.


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.)

Again, this depends on the integrated display device. IF the decoder is integrated the designer has the ability to do the scaling anywhere they want, based on the metadata that is presented with the compressed sample data.

Regards
Craig


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