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Linedoubling and progressive scan - What's the difference?

Discussion in 'Projectors, Screens & Video Processors' started by Peter Webb, Feb 21, 2002.

  1. Peter Webb

    Peter Webb
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    Though i think i have a reasonable understanding of what progressive scan is,and also what line-doubling is i'm still a little confused.For example is the Sony DRC-MF system,as opposed to so called progressive scan on other CRT televisions the same thing? Is the I-scan unit a line doubler or is it an interlace to progressive scan converter?

    Is progressive scan just another name for line doubling?
    If so what about line tripling or quadrupling?

    Am i right in thinking that line doubling uses some form of interpolation to 'fill in the gaps' and progressive scan simply
    shows all picture lines in one pass?

    Sorry this is a little garbled but if anyone can clarify the differences i would be very grateful.


    :)
     
  2. Ludae

    Ludae
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    The term 'line doubling' has been mis-interpreted and mis-used over the years. It was originally used to describe a video system that doubled the line scan rate, that doesn't necessarily mean that the resulting scan system would be progressive.

    A de-interlacer is, just as the name suggests, a device for converting interlaced video fields into progressive video frames and to do so it must double the number of scan lines in each video field to convert it to a frame. There are many methods used to achieve this function and they are somewhat long winded to describe but suffice to say that the quality of the results can vary significantly depending upon the type of interpolation applied and the sophistication of the algorithms used.

    Really cheap systems use either:

    ...line repetition, filling the gaps with a copy of the scanline above (or below). No motion artifacts (except aliasing) but produces a horrible low detail image with very obvious line structure and 'jaggies'.

    or

    ...interpolation between the scanlines of the current field (scanline averaging) which produces no motion artifacts but reduces detail and gives blurred results

    or

    ...use field insertion, simply combining the odd and even video fields to create a progressive video frame which retains detail but produces terrible motion artifacts

    Most de-interlacers used in progressive devices on the market recently use a system called spatio-temporal interpolation, which is a algorithm designed to balance the need to preserve resolution and reduce motion artifacts, its a compromise but is inexpensive to implement and works quite well.

    More sophisticated systems like the iScan and Faroudja de-interlacers use a variant called motion adaptive spatio-temporal interpolation which actively balances between a spatial and temporal algorithm depending upon image changes and motion.

    For film source material a better system can be applied whereby the de-interlacer detects the source and field sequence (usually refered to as pulldown cadence such as 3:2 for 60Hz systems and 2:2 for 50Hz systems) and simply combines the odd and even video fields to create a perfectly de-interlaced (progressive) video frame. Such methods are thankfully becoming more common now.

    The Sony DRC (Digital Reality Creation) system was designed to process interlaced video and produce an interlaced result with greater resolution (i.e. twice the number of scanlines per video field with twice the number of pixels per line eg. 720x480 into 1440x960 but still interlaced). It is a sophisticated detail enhancment system that can scale the pixel structure of an image by a factor of two or four. The results can be de-interlaced and scaled up or down further by means of any appropriate de-interlacer and pixel scaler for use on progressive displays such as LCD and Plasma. The Sony DRC system is not a de-interlacer it doesn't produce a progressive image itself but it could be described as a line doubler as it doubles the number of scanlines and the scanline rate!

    Line tripling or quadrupling means (typically) taking the original video field scanline rate and either tripling or quadrupling it. This could involve (and most often does) de-interlacing the video and then scaling it up to a greater number of scan lines.

    For example;

    ...60Hz NTSC type video of a 525 line system has 480 active scanlines per video frame (for digital systems) and 240 per video field. A typical tripler would de-interlace the video field to 480 lines progressive and then scale that progressive frame up to 720lines progressive still at 60Hz, that's 3 times the number of scanlines displayed every 60th of a second.

    It's a big subject but I hope this brief explaination helps answer some of your questions.
     
  3. Peter Webb

    Peter Webb
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    Ludae,thanks for the explanation. As you say, it seems to be a rather large subject to explain within the confines of a thread like this!
     

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