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PAL D1/DV Widescreen square pixel settings in After Effects (CS4 vs CS3)

Posted by in broadcast graphics

Seems the latest version of After Effects from Adobe (CS4) has changed the PAL D1/DV Widescreen square pixel preset.

In CS3, compositions using that preset would be set to 1024 x 576 pixels. The new version (CS4) uses 1050 x 576.

So which is right? 1024 or 1050?

Well, to begin with, it’s all a bit complicated. I can still remember when it was first explained to me many years ago when I was still at the BBC, and it’s the kind of thing that’s quite difficult to get your head around without drawing little pictures on the back of envelopes. Anyway, there’s a few resources out there that try to explain this (links included below) – but I thought I’d have a go myself.


Oh, and before we get too bogged down, I’m talking about standard definition PAL here. The following explanation assumes 576 visible TV lines (out of the total of 625). So when I talk about aspect ratios and pixel dimensions the vertical height is always 576 regardless.

OK, here we go.

TV pixels aren’t square

A square pixel device (like a computer) would need an image to be 768×576 to maintain a 4×3 aspect ratio.

But TV pixels are rectangular – instead of 1×1, they’re more like 1×1.094.

So if we rewind a bit to the old analogue days – you can see from the diagram below that our TV set with its wider pixels is effectively only 702 pixels across.

television pixels aren't square

television pixels aren't square

Here’s the crucial part – digital TV pictures are wider

The key to all this to remember that digital TV pictures are wider than analogue.

Digital TV pictures are 720 pixels wide.

But the 4×3 image (702 pixels wide) sits inside those 720 pixels.

There’s an extra 9 pixels each side.

digital TV pictures are wider than analogue

digital TV pictures are wider than analogue – the 4×3 bit sits inside

The ‘BBC guide to picture sizes’ (which used to be at www.bbc.co.uk/commissioning/tvbranding/picturesize.shtml but now seems to have been removed) said that these extra pixels are ‘required for digital processing’.

But, they might show up as black strips each side if the whole image is shrunk down via a DVE, or maybe these days if you are running your video in a window on a computer screen. I know that converting a 1024-wide movie from a Quicktime .mov to .mp4 format using Mpeg Streamclip does introduce black lines each side. Or at least it did for me the other day.

So if you’ve ever wondered ‘Why has my video got black strips each side?’ – this is probably why.

The reason for making your original After Effects composition wider is to guarantee that the whole image is filled with picture information. This will avoid either getting black lines each side, or having the application stretch the source material to fit, ending up with an image that’s ever so slightly distorted.

Adjustment for 4×3 compositions

For a 4×3 image that means your computer (square pixel) version would need to be 788 pixels wide.

Notice we now have allowed 10 extra (square) pixels each side on our source image. These equate to the 9 pixels (non-square) TV pixels we need.

To fill the entire (4x3) image area, build at 788 pixels wide

To fill the entire (4×3) image area, build at 788 pixels wide

And the same is true for 16×9 widescreen images too.

16×9 widescreen adjustment

Remember that your original 16×9 image (1024×576 pixels) is only going to occupy the middle part of the TV screen. You still need to account for those extra pixels each side. Again, taking into account that the TV pixels aren’t square, this time we’ll need 13 (square) pixels on our source material to fill in the missing areas.

Widescreen composition should be 1050 pixels wide[/caption]

Widescreen composition should be 1050 pixels wide

So your original (square pixel) After Effects compostion needs to be 1024+13+13 pixels wide. Namely, 1050×576 to guarantee that it maps correctly on to the 720×576 digital output.

I don’t know whether Adobe, or makers of other applications were doing something clever behind the scenes to compensate in AE CS3 for 1024px projects to eliminate black edges. I suspect that what was really happening is that the end results when viewed on a real TV were just stretched very slightly, or any missing picture was hidden by overscan.

Anyway, it looks like Adobe have decided to use 1050 from now on.

And to be fair, nearly everyone I knew at the BBC was working to 1024. A few geeky designers like me tried to keep the 1050 flag flying, but as you can see, it’s not really the sort of thing anyone wants to keep explaining, especially if (like me) you don’t really understand it all in the first place.

Anyway. I hope that’s helped.

But, one last thing. I can remember MANY years ago doing a test at the BBC. I prepared two Photoshop images, each with a perfect circle in the centre of the screen. One was 1024 wide, the other 1050 wide. I scaled them both to 720 and laid them both off digitally and viewed them both on our broadcast-quality monitors. The 1050 one looked perfect, and the 1024 one looked very slightly distorted. Which is why I used 1050 for the BBC Weather animations a few years ago which featured a big glass circle fairly prominently.

Please feel free to comment here if you have anything useful to add. Worthwhile comments are ‘no-follow’-free!

Much love,
Mike

Posted in categories: Tags: This entry was posted on Friday, March 6th, 2009 at 5:03 pm

54 Responses to “PAL D1/DV Widescreen square pixel settings in After Effects (CS4 vs CS3)”

  1. Dejan Says:

    Good explanation and reasonable from some point of view. To my opinion Adobe made a mistake.
    When converting analog video to digital 13,5Mhz sampling frequency is used. Analog video line is 64&181;s “long”. Only 52&181;s is active (just picture, without blanking). Therefore digitized video would be 702px “long”. According to ITU-R 601 digitized video is 720px, which is 53,33&181;s and 18px more. So we are already digitizing more then just active line. Those extra 1,33&181;s (18px) represents a part of blanking line time. I don’t see any sense to add some more pixels. And if you do add 20px to “square-pixel” width (768 -> 788) then pixel aspect ratio would be 4.1026:3.

    My question here is why did Adobe did this? Is it because of mistake equipment manufacturers made and it’s some kind of compensation?

    How I’m dealing with this issue in “square-pixel” application and Adobe workflow? Simply adding this extra 20px on left and right and 15px on top and bottom so my frame is 788×591 px (4:3) and 1050×591 px (16:9) to stay with my perfect shaped circle. I know some clipping occur on top and bottom and some KB or MB are added to my file, but it’s better than black stripes on left and right or even streching.

  2. Dejan Says:

    &181;s – should be micro seconds

  3. Mike Says:

    Thanks for the comments Dejan.

    You say the 18px “represents a part of blanking line time”. But remember those are TV (non-square) pixels. We’re not really adding anything more, just making the appropriate adjustment for square (ie computer) pixels.

  4. Vikrant Batra Says:

    Dude,

    Thanks a ton, I know now that I wasn’t crazy that I noticed , 720*576, 768*576, 1024*576 and 1050*576

    Vik

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