Frame Rate Conversion and You

There are two basic types of content stored on most DVDs: content that came from a 24 fps source and content that came from a 30 fps source. The most popular is 24 fps source content because all movies are recorded at 24 fps and since the majority of DVDs out there are movies, this is the type we'll talk about first.

Although most motion pictures are recorded at (approximately) 24 frames per second, no consumer television is capable of displaying at that frame rate. In order to get TVs up to the sizes that we all know and love (and at affordable prices), TVs are not as flexible as computer monitors - they are fixed frequency displays, so displaying contents with varying frequencies is not exactly possible. The DVD production houses know this so they have to convert their 24 fps source into something that can be displayed on the majority of TVs out there.

In the North American market, the majority of TVs are interlaced NTSC TVs that display 60 fields per second. As we've just explained, a single interlaced field has half the resolution of a full frame in order to save bandwidth; by displaying 60 of those interlaced fields per second, the human eye is tricked into thinking that each frame is complete. But how can you convert 24 non-interlaced (aka progressive) film frames into 60 interlaced fields?

The first step is to convert the progressive film frames into interlaced frames, which is pretty simple, just divide up each frame into odd and even lines and send all the odd ones to one field and all the even ones to another.

Now we have 48 interlaced fields, but we are still short of that 60 fields per second target. We can't just add 12 more fields as that will make our video look like we hit the fast forward button, so the only remaining option is to display some of the 48 fields longer. It turns out that if we perform what is known as a 3-2 pulldown we will have a rather nice conversion.

Here's how it works:

We take the first progressive frame and instead of just splitting it into two interlaced fields, we split it into three, with the third being a copy of the first. So frame 1 becomes field1a, field2a and field1a again. Then, we take the next progressive frame and split it into two interlaced fields, field2a and field2b, no repetition. We repeat this 3-2 pattern over and over again to properly display 24 fps film source on interlaced NTSC TV.

There are a few movies and some TV shows that are recorded at a different frame rate: 30 fps. The 30 fps to 60 fields per second conversion is a lot easier since we don't need to alternate the pattern, we still create interlaced fields for the sake of NTSC compatibility but we display each field twice, thus performing a 2-2 pulldown instead of the 3-2 pulldown that is used for film. One of the most popular 30 fps sources is Friends (note: it turns out that Friends is incorrectly flagged as a 30 fps source but is actually a 24 fps source) but other sources are sometimes recordered at 30 fps, including some bonus material on DVDs. Because of this, while 24 fps sources are usually categorized as "film", 30 fps sources are usually called "video" (these names will have significance later on).

Remember that the whole point for performing these conversions is that until recently, all televisions have been these low bandwidth interlaced displays. Recently however, televisions have become more advanced and one of the first major features to come their way was the ability to display non-interlaced video. A non-interlaced TV is useless without non-interlaced content, thus manufacturers produced affordable non-interlaced DVD players, otherwise known as progressive scan DVD players.

But if you have a progressive scan DVD player you don't have to buy progressive scan DVDs, the DVD player instead does its best to reassemble the original progressive scan frames from the interlaced content stored on the DVD. Given the two major algorithms we mentioned above, reconstructing the original progressive frames from the interlaced data on the DVD shouldn't be a difficult task. Once the DVD player know if it is dealing with 24 fps or 30 fps content, it simply needs to stitch together the appropriate fields and send them out as progressive frames. The DVD spec makes things even easier by allowing for flags to be set per field that tell the DVD player how to recover the original progressive source frames. No problems, right? Wrong.

It turns out that the flags on these DVDs aren't always reliable and can sometimes tell the DVD player to do the wrong thing, which could result in some pretty nasty image quality. So DVD players can't just rely on the flags, so algorithms were created to detect the type of source the DVD player was dealing with. If the decoder chip detected a 3-2 pattern it would switch into "film" mode and if it detected a 2-2 pattern it would switch into "video" mode. The problem here is that due to a variety of factors including errors introduced during editing, transition between chapters on a disc, and just poorly encoded DVDs, these algorithms are sometimes told to do the wrong thing (e.g. treat 24 fps content as 30 fps content). These hiccups in the 3-2 pattern don't happen for long periods of time (usually), but the results can be quite annoying. For example, if the DVD decoder chip tries to combine two fields that belong to different frames, the end result is a frame that obviously doesn't look right. While it may only happen in a few frames out of thousands on a single DVD those few frames are sometimes enough to cause a ruffled brow while watching your multi-thousand-dollar home theater setup.

So what does all of this have to do with NVIDIA's PureVideo? Although it's not in a set-top box, PureVideo is just as much of a DVD decoder as what you have sitting underneath your TV, it's just in your computer. And to measure its effectiveness, we have to look at how it handles these trouble cases. Remember that your PC is inherently a "progressive scan" device, there's no interlacing here, so the quality of your videos directly depends on NVIDIA's algorithms.

An Interlacing Primer A Brief Look at De-Interlacing Modes
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  • MDme - Monday, December 20, 2004 - link

    Hey Anand, why not suggest to nVidia that owners of 6800 series GPUs be able to download the codec and just have their serial #'s or proof of purchase? I mean, they shouldn't tout a feature that you need to pay extra for?

    They already screwed us 6800GT/U owners with a "broken" feature, now they wanna screw us 20$ more to get the "broken" feature?

    We need a petition site!!!!!
    Reply
  • Guspaz - Monday, December 20, 2004 - link

    I was under the impression that the deinterlacing was done by the DVD decoder app itself. So would not the results differ from WinDVD to PowerDVD to Zoom? Also would have been nice to compare it to other modes of software deinterlacing such as motion compensated.

    Something that I didn't see mentioned in the article is that while the hardware deinterlacing is certainly a win for nVidia, ATI wins the WMV9 acceleration category, as I'm pretty sure that all R3x0 cards support it back to the 9500/9700. That means that ATI's WMV9 market penetration is way higher than nVidia's, since very few nVidia cards can do the acceleration.
    Reply
  • Klaasman - Monday, December 20, 2004 - link

    Nevermind. Found info at Nvidia.

    "The new software will upgrade the decoders in ForceWare Multimedia 3.0 to access the new features in the NVIDIA DVD Decoder. The NVIDIA DVD Decoder will not upgrade NVDVD 2.0 as it was a separate application based upon an old architecture."

    They want me to pay twice for it I guess. Greedy bastards.
    Reply
  • Klaasman - Monday, December 20, 2004 - link

    I got a copy of NVDVD with my eVGA 6800GT. How do I update the DVD codec? When I install the new version from Nvidia, it still asks for credit card info and stuff. Reply
  • jonny13 - 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."

    I know that their DVD decoder has always been $20. But, they never specified that you would need to pay extra for a feature that they touted for months. Thats like making you pay for a special driver to get PS3.0 to work. If they advertise it as a feature on a card I am paying $500 for, it better damn well be free and not another 20 f'ing dollars.

    /rant
    Reply
  • b3roldan - Monday, December 20, 2004 - link

    I'm really disappointed with the 6800s not having the wmv9 decoding... was gonna get myself a 6800U for xmas. I was reading on the X800XT-PE coz there's a Sapphire X800XT-PE relatively cheaper than most 6800Us in http://buycomp.hkgolden.com.

    Then I came accross this webpage that says the nv48 is still coming... http://www.digitimes.com/mobos/a20041220A7039.html

    hehe... I guess I can wait 3 months for the nv48, but then with the R520 just around the corner... man buying a card is starting to become a tough decision.

    neways... I was wonderin if neone could confirm this information on the hardware flaw...

    http://www.animenation.net/forums/archive/index.ph...
    Reply
  • RCIEGR - Monday, December 20, 2004 - link

    You missed one very important feature to me.
    The 6800gt lacks the true hd output formats in component video output. I have a 6800gt and these card didn't include the special 10-pin din connector to the hd-component/s-video/composite adaptor plug. My freind at work bought a 6600gt(Chaintech) for his son for xmas, so I borrowed the card for a test this last weekend. Having a old school sony KDP-57XBR2 HDTV with only firewire and 2-hd component vidoe imputs. The true hd-output of the 6600gt is way more important of a feature to me then the wmv9 decoding.

    The output over component(both true 1080i & 720p; the sony can nativly handle both) looked very good, for both hd video(surferp****.com), hd tapestreams recorded from the firewire output of the mot-6208 hd-cable box and the latest games, once you figure out how n-view works and you get the game resolutions correct.

    anand, take a close look at the reference 6800gt cards and I think you will see the 10-pin din connector.

    I want a RMA for a 6800gt with the true hd-output with component video. eVGA does have a 90-day upgrade program. Maybe a 6900gt will show up quick?

    Thanks for you time
    Robert
    Reply
  • MDme - Monday, December 20, 2004 - link

    Well, my XFX 6800GT didn't come with the NVDVD decoder. it's a shame nVidia sells a very expensive video card and tout's this feature but doesn't include the software to make it work.

    I think nvidia should know how it's customers feel. after all we were the early adopters. it's one thing to have a partially working feature but it's totally a rip-off not to have it at all!!!! $400 for a video card and you need to pay more for software to make it work? :(

    Hope Anand can be a voice for us.
    Reply
  • ViRGE - Monday, December 20, 2004 - link

    Anand, check the final X800 press presentation slide-show/PDF, it was in there somewhere last time I read it. Reply
  • sheik124 - Monday, December 20, 2004 - link

    nVIDIA has really done it this time. #29 mentioned ATI not doing their part, but at least the features they advertised and are slowly deploying work on ALL OF THE GPUs. nVIDIA owes us something for the false advertising, they've really wrecked my day, I paid for something, and I expect to have it, not half of it. Reply

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