I know nothing about progressive scan!

Discussion in 'Playback Devices' started by Sam Davatchi, Mar 8, 2006.

  1. Sam Davatchi

    Sam Davatchi Producer

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    I have a progressive DVD Player and Plasma TV and I'm using progressive output daily but I just realized I know nothing about progressive scan! I don't know how it works and there are some major plot holes in the scenario for me.

    What I have learned is that when an NTSC film is played by the DVD player, it adds the 3:2 pulldown. Now with a progressive player, it takes this signal and uses an "expensive" chip to reverse the pulldown.

    First, my question is why all the trouble for the progressive player? I know that NTSC movies are encoded as 23.97 fps on a DVD. Why the player doesn't get that directly? Doing that conversion twice is useless.

    Second, my big question is that no matter what, the result is 24p frames. So how that 24 frames are gone through the NTSC signal and shown on a TV? Because NTSC is 60hz!
     
  2. Ronn.W

    Ronn.W Second Unit

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  3. Sam Davatchi

    Sam Davatchi Producer

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    Yes, thanks. I had read that before but it doesn't help. I want to know how a 24p video (constructed by the progressive DVD player) is shown through NTSC signal.
     
  4. ChristopherDAC

    ChristopherDAC Producer

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    Short answer: it ain't.

    To be more specific: the DVD contains material which is encoded for playback at (30×1000/1001) frames per second, interlaced. You are, therefore, seeing two effects.

    The first effect is that, instead of sending each video frame as two 262.5-line scans, at an interval of 1/(60×1000/1001) second, each video frame is transmitted as two 525-line scans during that same time period.

    In order to do this, the player has to figure out which fields [the individual 262.5-line scans] should be "stitched together" to make the original picture. On a properly-mastered DVD made from 24-frame-per-second film, this is easy: instead of taking each film frame and making it into alternately 3 and 2 video fields, only two fields are included per film frame, and the player recieves an instruction to repeat one of them in every alternate frame, in order to keep the output frame rate at [effectively] 30 per second. If that is done, the DVD player need only take the two adjacent fields, stitch them together, and it has a "progressive" frame, which it can use as a 525-line continuous scan. This 525-line scan is then repeated alternately 2 and 3 times per film frame, just as the 262.5-line scans are in the interlaced case, to keep the output frame rate at [effectively] 60 per second.

    The complications come because not every film-source DVD is properly mastered, and many DVDs are from video-source material such as television, in which there is motion or other changes between each successive field. The player in this case needs a sophisticated system to allow it to recognise which part of each field are the same as in the previous field, and so can be stitched together, and which parts are different and need to be geometrically interpolated to back out the missing scan lines. The most complex of them can even work out what kind of motion is occurring, and shift picture-elements in the previous field around for the best results in adding them to the new one.
     

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