How many pixels on a line?

Discussion in 'Projectors, Screens & Video Processors' started by JohnAd, Oct 21, 2002.

  1. JohnAd

    JohnAd
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    I've finally got round to reading the Snell and Wilcox Video Standards books I got from the Event last year and noticed something that I've been confused about for a while.

    I wonder if one of the experts out there can help me out with this.

    I was looking at the page covering what looks like the CCIR definition. This is what I think DVD's are decoded to and what passes along SDI. Anyway the S&W books says there are 720 digital pixels per line, OK no surprise there but there is also mention of 714 (702 for PAL) pixels in an analogue line. Now this is where I'm getting confused.

    I'm just trying to add support for some new cards in DScaler and these allow you to set the decoding frequency, I have set this to 27Mhz in an attempt to get CCIR type output of I assumed 720 pixels. When I use 720 I get a lot of black space at the right of the picture off my set-top box or DVD player. If I only ask for 702 pixels I get no extra black at the sides.

    Also when I recorded Star Wars using my Nova-t card I noticed that the MPEG stream was 702x576 rather than the 720x576 I was expecting.

    So finally my question, when working out the aspect ratio for PAL CCIR/SDI should I use 720 or 702 and how are DVDs encoded for PAL, is the black space just stored as black or should this extra information be ignored?

    Sorry if this is slightly confused...

    John
     
  2. RichardA

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    Hi John,

    I wondered how long it would be before someone asked a question about that book!

    As far as I can tell, all DVD's should have 720 active samples per line, however it depends on the original source as to whether they are all used or whether some pixels are black.

    In theory, a source from a true digital source (Digital camera, recorded on some form of digital tape, e.g. Digital Betacam, and so on) should have 720 pixels all with picture on them.

    If the source has been back to analogue at some stage, there is usually additional blanking used to eliminate the garbage at start and end of line when going back to digital, hence the 702 pixel part.

    This is not the only possible scenario though, as 696 and 688 pixel active lines are possible as well and poor telecine work can have the framing set incorrectly in the original transfer.

    The defining part is that what you get off DVD is always 720 pixels in the 'active' line time - that is what will determine the sample clock (and I believe this is the correct number to calculate aspect ratio as well)

    However for calculating blanking and scaling it is probably safer to assume 702/714 pixels for usable active picture.

    By using 720 pixels for Aspect Ratio, and then zooming in to make 702 or 714 pixels fill the width there will be loss of scan lines top and bottom, but this is well within tolerances for oversan.

    Hope this helps!
     
  3. JohnAd

    JohnAd
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    Richard

    Thanks for the very fast response ( got to love this forum :) )

    I think I understand the answer, so hopefully the following will be true, if it isn't I'd appriciate any corrections.

    What goes on the DVD is 720 pixels wide and every pixel may have a value. (Sometimes all 720 pixels won't be used and sometimes they will, it just depends on the source, mastering etc, if at any time the video has gone to analogue before it was put onto the DVD)

    If you use SDI or PC you can see every pixel that is on the DVD and so could use 720 for the width. If the DVD doesn't have proper values ouside the analogue active region then you may see black borders unless you overscan.

    If you use a STB or DVD player the video does go into the analogue world so is "blanked" to 714/702 so any information in the pixels at the side is lost. The 720 pixels are not squeezed into the smaller space of the timings for analogue video.

    So if you're reading back from analogue and you know it is a digital source you should decode using CCIR timing but only expect to find 714/702 pixels of information even if the source contained more.

    Also if you want to fill a PC screen with picture you should possibly recognise that PAL DVD's might well have blanking on the right and left and adjust accordingly. I'll check again but I seem to remember that a lot of R2 DVD's have quick large black bars on right and left so this is an explaination of why that is.

    Thanks again.

    John

    [edit with corrections from Richard A]
     
  4. RichardA

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    John,

    I think you are understanding correctly, the only 'correction' is that the 'going to analogue' bit may well occur earlier in the mastering chain, and therefore there may not be 720 used pixels on a disk in the first place, so using SDI may not produce extra picture.

    I think I'll have to have a play around with our Proceed PMDT and Meridian 800 next time I'm in the Home Cinema at work and see what's going on!
     
  5. JohnAd

    JohnAd
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    Richard

    Thanks again, I think I've got it now.

    Cheers

    John
     
  6. Rob.Screene

    Rob.Screene
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    Hi John,
    The DVD-Video spec states that 50Hz discs (for PAL) can be 704 wide (x576). I remember in DVD Demystified that this was to make more efficient for storing previously analog material. I can find the chapter and quote it if that will help.

    regards,
    Rob.

    p.s. A problem with DVD authoring at 704x576 was that TheaterTek 1.2 didn't play it back properly, giving flashing green bars on the right. I must find my test disc and re-test with TheterTek 1.5.
     
  7. JohnAd

    JohnAd
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    Rob

    704 is the nearest possible MPEG sixe for 702 pixels and seems to be used for DVB-T and probably sky as well. The odd thing about this resolution is that the pixels are the same aspect ratio as 720x576 which will probably confuse a lot of players.

    John
     

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