An Interlacing Primer

A big part of the PureVideo feature set are its de-interlacing capabilities, but before we explain what de-interlacing is we have to explain what interlacing is and why you would want to de-it. Let's say we wanted to display an animation and here we have one frame of that animation:

If the world were perfect we would just broadcast as many frames of our animation as we had, at a constant frame rate, and we would have accomplished what we set out to do. Unfortunately the world isn't perfect and when we first wanted to broadcast this animation there were significant bandwidth limitations both on the transmitting and receiving side, preventing us from sending one complete animation frame at a time. One solution to this problem would be to divide up each frame into separate parts and display those parts in sequence. If the sequence is fast enough, the human eye would be hard pressed to notice the difference. So let's do it, we take our original frame and produce two separate fields, each with half of the resolution of the original frame:


Field 1


Field 2

And we're done, what we've just briefly described is how interlaced television came about. Interlaced NTSC TV (the North American standard) works by displaying 60 interlaced fields per second relying on the human eye to do a bit of blending work on its own, making two half resolution fields appear to be a single full resolution frame. More recently there has been a push away from interlaced TVs to non-interlaced displays, which is a wonderful step towards improved picture quality but not without creating a whole new set of problems. Keep this basic introduction to interlaced TV in mind as we look at converting non-interlaced (progressive) content to an interlaced format and back again.
Index Frame Rate Conversion and You
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  • Rand - Monday, December 20, 2004 - link

    Anand- Could you provide the details on what the test platform in use was?
    You mentioned the processor of course, but it would be appreciated if you could disclose the other components in use.

    Thanks.
    Reply
  • Spike - Monday, December 20, 2004 - link

    But the 6800's come with NVDVD (at least mine did), isn't that basically their decoder? Other users are reporting their new (just recieved today) 6800 GT's coming with the DVD decoder on CD.

    -spike
    Reply
  • Anand Lal Shimpi - Monday, December 20, 2004 - link

    NVIDIA's DVD decoder has always been $20, unfortunately I don't know of a single manufacturer that gives away their DVD decoders for free.

    The WMV9 acceleration can be had without the DVD decoder, however that you will have to wait on Microsoft for as WMP10 needs to be updated.

    The features right now are mostly for HTPC enthusiasts who want the image quality benefits offered by PureVideo.

    Take care,
    Anand
    Reply
  • Spike - Monday, December 20, 2004 - link

    I am still confused on the "free" comment. If I have to pay $20 to enable a feature that was supposed to come with my 6800 GT, how is that free?

    Thanks,
    spike
    Reply
  • Anand Lal Shimpi - Monday, December 20, 2004 - link

    How to enable WMV9 hardware acceleration on ATI cards:

    Note: WMV9 acceleration has been disabled until Microsoft issues a new patch for WMV9. To enable this with other versions of Catalyst (with some rendering errors), RUN regedit -> HKEY_LOCAL_MACHINE -> SYSTEM -> CurrentControlSet -> Control ->Video and find your ATI reg value. The key to update is DXVA_WMV = 1

    Take care,
    Anand
    Reply
  • gordon151 - Monday, December 20, 2004 - link

    #16, It does. It just doesn't have the full capabilities that it was advertised as having. If you look at the descriptions you'll see only the 6600 AGP/PCI-E and 6800 PCI-E have use of the full capabilities of the encoder. Reply
  • Klaasman - Monday, December 20, 2004 - link

    I don't see on Nvidia's website where the 6800GT does NOT have PureVideo capability. They say all Ge Force 6 series. Reply
  • OriginalReaper - Monday, December 20, 2004 - link

    why couldn't *those* sites shut down next week instead :-\ Reply
  • Gatak - Monday, December 20, 2004 - link

    There are some facts missing when it comes to differences between interlaced and progressive video.

    NTSC TV broadcasts are recorded at 60fps, not 30fps. (PAL is 50fps). Each field is recorded after eachother in time. There is a 16.7ms delay between each field.

    If you were to de-interlace two fields into one frame you would loose half of the temporal resolution! This is a big mistake - especially for fast moving things like a football or hockey game.

    A proper de-interlacing method would render each field as a separate frame (like a TV does!), not blend or discard fields.

    24fps content, on the other hand, need to be converted to fit the 60fps NTSC (or 50fps PAL). It is nasty and should be banned. A DVD/MPEG-2 video is perfectly capable of storing 24fps progressive.

    So, what we want is to render 24fps content as 24fps progressive and TV content as 60fps progressive.

    Remember, a TV is already interlaced. There will be no problems with interlaced content because each field is rendered consecutively. It is only on a computer monitor which is progressive where we need to do field deinterlacing.
    Reply
  • SlinkyDink - Monday, December 20, 2004 - link

    How can we enable hardware acceleration on our ATI cards now? (I believe its a registry edit) Reply

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