720x576 PAL DVD not cropped to center-most 704x576 (4:3) part

[Gew]

Today I noticed something funny when I was taking a backup of a DVD of mine. On the computer, I noticed that about 16 pixels was padded blacked on the left side of the screen, instead of ~8 pixels to the left, and ~8 pixels to the right. I thought to myself that it would be logic that when played on a stand-alone DVD player, you would be able to see some (~8px) black padding to the far left of the screen, because [I thought that] DVD players perform a sort of "standardized" crop from center-most position. I was surprised, since there turned out to be no visible padding on the television screen at all.

This kind of ruined my whole conception about how DVD players work. Could someone point me in the right direction here? The DVD player I tried was a kind of expensive Pioneer. I thought to myself that perhaps this high-end stuff had the ability to actually "sense" padding and do the crop accordingly, instead of just do the "legacy" 8px to the left / 8px to the right crop. As we have a cheap Walmart DVD player and a ancient CRT television in the bedroom, I thought I'd have a look there. I popped the DVD. Same thing there, no padding at all visible on the screen during playback. Really strange. Again, please, could someone get me back on the right track?

What really happened here?

[jagabo]

The DVD spec refers to the MPEG 2 spec with regard to aspect ratios. The MPEG 2 spec is very clear: the entire frame comprises the DAR (unless overridden by a "sequence display extension"*). That is at odds with the ITU digital video spec which says the inner 704x480 (~702x576) subsection comprises the DAR and the extra 8 columns left and right are for padding in case the analog signal or capture is off center.

In my experience, most DVD players scale the entire frame to the output resolution when putting out high definition HDMI. When putting out a standard definition analog signal they follow the ITU spec.

Individual DVDs vary in whether they follow the ITU spec or the MPEG 2 spec. Most analog sourced DVDs follow the ITU spec. -- presumably because they used ITU based equipment to capture and left the digital video as-is. Many (most?) direct film-to-digital transfers follow the MPEG 2 spec. In general I don't think anyone in the industry cares about the conflicting specs. Nobody can see the AR difference just watching the screen.

* The Sequence Display Extension is a mechanism for specifying what portion of the frame comprises the indicated DAR. I've only ever seen this used for pan-and-scan purposes. And even there the DAR is set to 4:3 and the subsection is always 540x480 (or 540x576). That implies the full frame is 16:9.

[AlanHK]

I think that there is no padding in the DVD. That may be added by whatever displays it on your PC or DVD player. And they do it in different ways. If you look through the settings of your software maybe you can find a way to change this.

[Gew]

The DVD spec refers to the MPEG 2 spec with regard to aspect ratios. The MPEG 2 spec is very clear: the entire frame comprises the DAR (unless overridden by a "sequence display extension"*). That is at odds with the ITU digital video spec which says the inner 704x480 (~702x576) subsection comprises the DAR and the extra 8 columns left and right are for padding in case the analog signal or capture is off center.

In my experience, most DVD players scale the entire frame to the output resolution when putting out high definition HDMI. When putting out a standard definition analog signal they follow the ITU spec.

Individual DVDs vary in whether they follow the ITU spec or the MPEG 2 spec. Most analog sourced DVDs follow the ITU spec. -- presumably because they used ITU based equipment to capture and left the digital video as-is. Many (most?) direct film-to-digital transfers follow the MPEG 2 spec. In general I don't think anyone in the industry cares about the conflicting specs. Nobody can see the AR difference just watching the screen.

* The Sequence Display Extension is a mechanism for specifying what portion of the frame comprises the indicated DAR. I've only ever seen this used for pan-and-scan purposes. And even there the DAR is set to 4:3 and the subsection is always 540x480 (or 540x576). That implies the full frame is 16:9.

Thanks for a really detailed answer. Unfortunately, I still can't figure out what happened in my case.

I think that there is no padding in the DVD. That may be added by whatever displays it on your PC or DVD player. And they do it in different ways. If you look through the settings of your software maybe you can find a way to change this.

Thing is, the padding is there. I ran the DVD through DGIndex and popped the D2V project file through an AviSynth script just "as is", and there was this ~16 pixels of black padding to the far left of the frame, but no padding at the right edge of the frame. But still, when I put the disc in my stand-alone SCART-connected DVD-player, it does not show the ~8 pixels that would remain on the left side after ITU/overscan procedure. Really weird.


Alongside (of great importance):
A couple of years ago I ran some tests with the very same DVD players. I made a dummy (720x576) DVD where the far-most 10 pixels of each side (horizontally) contained a padding, split into 5 times 2 pixels sections, which each was of a different color. I did this because I was curious on how my stand-alone DVD player would present the frame. As expected, four out of five "colored padding blocks" (4x2 on each side = 8 pixels left; 8 pixels right) was not present during playback on the stand-alone DVD player. The fifth colored padding block (2px on each side) was visible on the far left/right side of the television. Now, this was all very logic, the center-most 704x576 pixels was presented on my old Cathode ray tube television, using a PAR of 12:11.

Now, this is why I'm really confused now that suddenly all 16 pixels of the left side of this DVD is being cropped when watched on the television, even though the padding indeed is part of the raw MPEG-2 frame. Some strange hocus-pocus? Or could it be as simple as that the two DVD players I have tested both have the "extra feature" that is scans for extra padding, and crops this away as well? That would be a rather simple explanation.


Sum up á Too Long / Didn't Read:
I have earlier read (Wikipedia, etc) that the 720x576 (native MPEG-2 frame) -> 704x576 (what's being presented on the television) crop is regulated(?) to always be done with a center offset, i.e. 8 pixels on each side. Now, I have found not only one but two different stand-alone DVD players where all these 16 pixels are cropped from the far left side of the frame during presentation on my television. Now, is my DVD player that is playing mind tricks on me, or what else is up?

[jagabo]

Thing is, the padding is there. I ran the DVD through DGIndex and popped the D2V project file through an AviSynth script just "as is", and there was this ~16 pixels of black padding to the far left of the frame, but no padding at the right edge of the frame. But still, when I put the disc in my stand-alone SCART-connected DVD-player, it does not show the ~8 pixels the would remain on the left side after ITU/overscan procedure. Really weird.

TVs normally overscan so you never see the edges of the frame. Different TVs overscan by a different amounts. With old CRTs it was typically around 5 percent at each edge. But since CRTs couldn't keep the frame centered and the same size all the time, it could be as much as 10 percent on any one side. With modern fixed panel displays it usually 2 or 3 percent.

[Gew]

With old CRTs it was typically around 5 percent at each edge. But since CRTs couldn't keep the frame centered and the same size all the time, it could be as much as 10 percent on any one side. With modern fixed panel displays it usually 2 or 3 percent.

Really, 5% on each side? So you mean that actually 36 (720*0.05) pixels would be cropped from the far-most left and right? That's pretty severe, if you ask me. Also, as I mentioned above, I ran a sort of a test project a couple of years ago, and then came to the conclusion that my DVD/television set cropped exactly 8 pixels to the left, and 8 pixels to the right. Now, this is why I'm so highly surprised that this particular DVD film is being cropped only (roughly 16 pixels) on the left side of the frame, when played in the very same combination of television and DVD player. Really weird. I just hate it when I can't seem to find a logic answer to an occurrence.

[jagabo]

Is this a CRT TV? Or an LCD/Plasma? As CRTs age (even as they heat up) the amount of overscan will change and the picture may move around.

In my experience, DVD players don't crop the frame when putting out an analog signal. If you have a studio monitor which doesn't overscan and shows the front and back porch you will see the full 720 pixel wide frame. It's just that the inner 704 pixels conform to the ITU spec for the 4:3 DAR image. Here's an example:



The top half of the frame is from the test DVD. The bottom half is what was captured from the player by a capture card on a PC. You can see that only a pixel or two is missing in the bottom half.

[manono]

Now, this is why I'm so highly surprised that this particular DVD film is being cropped only (roughly 16 pixels) on the left side of the frame, when played in the very same combination of television and DVD player.

And you're positive nothing is being lost from the right side of the picture? Not only is there the overscan to take into account, but maybe half of the DVD players out there do some cropping of their own before ever sending the picture to the TV set.