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How does 1080i scale?

Discussion in 'Televisions' started by PioRow, Oct 21, 2005.

  1. PioRow

    PioRow
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    How does a 1080i signal get scaled to a 720p display? Say you have a 768X1366 LCD how does it display a 1080i imagine.
    Am I correct in saying that a 1080i signal is 1080X1920 interlaced? So 540 lines are displayed on one cycle and the other 540 on the next cycle and so on?
    I'm a bit confused on how a 1080i picture can be displayed on a lcd tv that only has 768 lines in the vericle display? If you had a display that had 1080 verticle lines it would display on lines 1,3,5,7 etc on the first cycle and then lines 2,4,6,8 etc on the second cycle. ON a display with 768 lines it has 540 lines to display first cycle and then 540 lines to display second cycle, there is going to have to be some over lap so lines of the imagien that were supposed to be on different liens will be displayed on the same line, wont this mess the picture up?
    How does a 1080i imagine get displayed on a 768 display?
     
  2. Stephen Neal

    Stephen Neal
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    Two ways :

    1. De-interlaced the 1920x1080/50i video sequence to 1920x1080/50p and then filter and scale vertically (and horizontally)
    2. Treat the 1920x1080/50i signal as a 1920x540/50p (i.e. don't de-interlace) and up-scale this vertically (and filter and scale vertically)

    1. is the high quality solution - as it utilises as much of the approx 800 lines of vertical information present in a 1080i signal (1080i doesn't have full 1080p resolution - as interlaced sources have to be pre-filtered to remove HF vertical detail that would flicker at frame - not field - rate) when converting to 720p.

    2. is the cheap solution as it doesn't require a decent de-interlacing algorithm, or a frame store of a higher resolution than the display. However it only displays 1080i sources at 540p resolution - so effectively chucks away 180 lines of vertical detail (if the display is 720p native)
     
  3. Jeff

    Jeff
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    Option 2 may actually look better because it shouldn't show any deinterlacing artefacts.
     
  4. richard plumb

    richard plumb
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    surely option 2 doesn't lose any resolution? You are upscaling each 540 line field to 720, then updating it 50 times a second. So each refresh you are seeing all the discrete 540 lines coming from the 1080i image (they are just laid over each other)?
     
  5. StevenBagley

    StevenBagley
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    You lose a heck of a lot of resolution. In fact you basically halve the resolution. Take a look at the following image, on the left is a segment from a 720 cycle per picture height 1080i image -- which is near enough the maximum you are likely to get out of a 1080i. In the centre and right-hand columns is the odd field and the even field as would be rescaled up to the panels resolution.

    [​IMG]

    As can be seen, the 720 line cycles has been halved to a 360line cycle in each field and this would be what is seen (this is actually an alias signal and should really be filtered out before you scale down -- something that is very hard to do in interlaced signals). However, what you'll also notice is that the even field is the inverse of the odd field and so the image will flicker (or appear as if it is bouncing up and down) when displayed. To get around this you could drop one field down one line before rescaling, but then you'd get both fields aligned and all you would see is the 360-cycle alias signal.

    If you properly deinterlace the signal to 1080p50 and then rescale that to 720p50 -- you'll get much better results.

    Steven
     
  6. Stephen Neal

    Stephen Neal
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    That's the problem. They aren't offset, they are overlaid. The overlaying process nullifies the extra resolution present over 540 lines, as you aren't interlacing the two fields, you are overlaying them. Effectively the lines could be thought of as being "fatter" ?
     
  7. Stephen Neal

    Stephen Neal
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    It may look better on 50i sources containing moving information - after all 1080i drops to 540p effectively on fast moving objects. However on 25p sources (like films) or static or slow moving elements of a 50i source you will get noticably poorer resolution.

    You are right that a poor quality de-interlacer may introduce artefacts though. This is presumably why some displays use the 540p to 720p scaling, rather than the greater expense required for a decent de-interlacer.
     

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