Why is 24p so choppy? And why do people keep using it?

Discussion in 'Digital Video' started by ppc_michael, Apr 10, 2008.

  1. ppc_michael Guest

    ppc_michael

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    #1
    I just don't get it. 24p video looks terrible in the choppy/jerky department. It looks very amateur to me. But why? Film at 24 doesn't look at all jerky to me. They both use 1/24 shutter, 24 images a second, I don't get it.

    (Obviously there are huge differences image quality-wise, but I'm just talking about the actual motion)
     
  2. killmoms macrumors 68040

    killmoms

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    #2
    If often depends if what you're looking at from a camera is true 24p or not. Some cameras use a 24p-like mode that attempts to emulate the look of 24fps but they're still capturing internally at 29.97 (59.94). Most cameras that I've seen that ACTUALLY do 24p (truly sampling input at 23.976) and whose footage has been capture correctly at 24p look fantastic.
     
  3. LethalWolfe macrumors G3

    LethalWolfe

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    #3
    Unless you plan and make adjustments for shooting in 24p it will look choppy compared to 29.97 video. Broadcast sporting events, for example, are either shot in 60i or 60p because the fast motion looks better at a higher frame rate.


    Lethal
     
  4. bigbossbmb macrumors 68000

    bigbossbmb

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    #4
    its also a 1/48 shutter, not 1/24... but thats not the issue.

    with 24p, you need to be careful about camera moves and shakiness. yes there can be strobing problems, but that's just with poor camera work.

    Lethal's right, fast action stuff with look a lot smoother in 60i/60p.
     
  5. lfielder06 Guest

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    #5
    you also have to make sure you get it out of the camera correctly. If you don't do this it can look pretty bad.
     
  6. killmoms macrumors 68040

    killmoms

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    #6
    YES. This is also a problem, mostly on standard def 24p cameras like the DVX100A/B, which has to lay down its 24p footage onto a 29.97 NTSC DV stream in either regular 3:2 or in what they call 24pA (2:3:3:2, which lets the middle frame in a five interlaced frame sequence simply be dropped instead of having to recombine two split frames as you'd find in a normal 3:2:3:2 sequence). Capturing in 29.97 will lead to problems.
     
  7. jazpirozleehan macrumors newbie

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    #7
    Why do 24 fps work in a movie theater but not on a computer or TV

    The answer to the question has to do with the psychophysics of vision.
    In a darkened movie theater, with a bright big screen, the phenomenon known as persistence works well. Twenty four flashes of bright light per second appear to our brain as a continuous stream of images. With lower brightness, and interlaced or progressive scan, the limit is higher, about 60 Hz, which can still appear "choppy"
     
  8. killmoms macrumors 68040

    killmoms

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    #8
    Technically, in a theater you're seeing 48 flashes of bright light per second—the shutter clicks twice for each frame to reduce flicker.
     
  9. Krevnik macrumors 68030

    Krevnik

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    #9
    I (personally) don't have any problem with either 23.97 or 29.97 content, I don't really seem to be aware of the stutter that is really present.

    I do have a problem with content was recorded or authored at one rate and then converted into the other and deinterlaced or frame blended in the process. Any real-time sort of deinterlacing or rate conversion in a camera very likely is bare bones and will introduce issues to the content which can be hard to fix. The rule of thumb is: if you want it to keep looking good, then you have to keep it in the frame rate as it was authored or recorded originally. Otherwise you will degrade the motion as frames get badly timed, blended, etc. These sorts of issues are more visible to my eye than just 23.97 fps blanking.

    I think killmoms is probably thinking on the right track here, IMO.
     
  10. killmoms macrumors 68040

    killmoms

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    #10
    Yeah. Never use fake in-camera 24p. Get a camera that actually records 24p, in whatever form that may be (native progressive files on Flash cards a la the HVX200 or some of those new AVCHD cameras, or 24p/24pA w/ a clean cadence onto tape) and always capture in the correct format. Always do your edit on a 24p timeline (23.976 for NTSC, technically). If you need to deliver on videotape, you can ALWAYS get clean cadence from a 24p timeline, and you still can export a clean progressive file for DVD or the Internet.
     
  11. David Schneider macrumors newbie

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    #11
    Interesting. So what's the solution?

    Here's my situation: we've shot a series of short films, using a Canon XL2, in the 24p mode. All captured to a 29.97 timeline (as was my understanding as to how it should be). They all look great, except for one, which, when output to DVD, strobes on any onscreen movement to an unacceptable degree. Definitely not the usual strobing that I know is sometimes associated with shooting 24p, but, "Whoa, what happened there!?" strobing.

    A clue is that, because these are all horror shorts, they're all shot in moody, lower-light situations; at night and/or in dimly lit rooms. The only one that's actually brightly lit, with a lot of natural light, is the one that's strobing.

    So, do you think that would still be as a result of the pull-down situation you mentioned, or something else? And if so, do I recapture differently? Or do you think it might be something else?

    Any help is appreciated!
     
  12. KeithPratt macrumors 6502a

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    #12

    Brighter areas of an image do strobe more. And the greater depth-of-field of a 1/3" sensor will result in more apparent strobing than on 35mm film. Don't know that there's much science I could quote on this, it's just that softer lines appear to move more smoothly/less noticeably than harder ones. This could explain why you see it in the brighter bits but not in the darker bits. I really can't judge without seeing the footage, but if that's the case re-capturing will not solve it.
     
  13. Gymnut macrumors 68000

    Gymnut

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    #13
    "Strobing" like interlacing?
     
  14. killmoms macrumors 68040

    killmoms

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    #14
    If you're only ever going to output to video (broadcast), you should do that. But if you shot in 24p mode (which lays down 3:2 in Normal mode and 2:3:3:2 in Advanced mode) and wanted to do anything else (make DVDs, put it on the 'Net), you should've captured in a mode that would IVTC on ingest and then edited in a 24p (23.976) timeline. Optimally, you would've shot 24p Advanced, as this is the easiest form to ingest correctly.

    I'm assuming you understand the difference between the two modes, of course, and how telecine works in the first place. If not I'll be happy to explain... unless I'm feeling lazy, in which case I'll throw Wikipedia at you.

    The problem is that now you have a complete edit in 30-drop with 30-drop clips and 30-drop timecode. Assuming your shoot tapes have no timecode breaks, you ought to be able to do frame count conversions, ingest the tapes properly, and reassemble your cut into 24p using Cinema Tools, but I have no personal experience doing that myself. I just know it can be done.

    Yes, the key here is to capture your footage in a 23.976 mode that removes the extraneous duplicated fields in a mode that is appropriate for how you shot. Do you know if you shot 24p Normal (3:2) or 24p Advanced (2:3:3:2)?
     
  15. David Schneider macrumors newbie

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    #15
    killmoms, thanks for the help. Yes, I do understand the basic principles behind 24p vs. 24p A, and of the telecine process. I think I'm only just now realizing the ramifications as far as what it means to the end product of what we're doing.

    I'm pretty sure all our material was shot 24p Normal, but I'll have to check with our cinematographer later on.

    I think I understand what you mean as far as reassembling the cut into a 23.976 timeline using Cinema Tools -- I'll have to research that, as I have no experience doing anything like that. All the tapes should have been striped, so there shouldn't be any timecode breaks.

    Thanks again for the help.
     
  16. David Schneider macrumors newbie

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    #16
    I suppose you could say that, but worse than any interlace strobing I've seen.
     
  17. killmoms macrumors 68040

    killmoms

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    #17
    I mean, to be fair, if you're watching the resultant DVDs on a regular standard def tube television and still seeing strobing, that might just be inherent to how it was shot (the conditions and how they impinge on video quality at 24p as shot on the XL2). Doing all the work to get it back into a progressive timeline might not help with that specific issue. But it would be a good idea in general, as a 24p timeline gives you much more flexibility when you're done with your edit. Want a film-out? No prob. Want to make a progressive-scan DVD? No prob. Want to put it on Digi for broadcast? You can always re-introduce a 3:2 cadence into the finished edit for output back to 29.97 (benefit being it will be a clean cadence).

    EDIT: Sorry if I appeared to be talking down... you wouldn't believe how many people hear the buzzword 24p and understand none of the mechanics that underly the term—and thus shoot, capture, edit, or do any combination of the three wrong. You're clearly a step ahead of the game in that department.

    A couple things to consider: I'm not sure how smart Cinema Tools' pulldown feature is, and if it's adaptive or if it demands a clean cadence. I would assume the XL2 resets its cadence every time you cut, regardless of where it is when you stop laying video to tape, so from end-to-end I'd imagine the tape has very broken cadence. This means you'll probably want to capture your clips cleanly (frame accurate) from in to out to expedite the process. Then I'd imagine getting the cut into proper timing would be as simple as exporting a 29.97 EDL and running it through something to convert the frame counts from 30-drop to 24.
     
  18. David Schneider macrumors newbie

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    #18
    No sweat, I appreciate the help and fully admit I'm only part of the way to understanding all this.

    Obviously, I'd prefer to avoid having to recapture everything and run it through Cinema Tools, but if that's what it takes to make it look its best, I certainly will.

    I guess the thing that bothers me most is that all the other films (there are 15 in total) look so beautiful, it's a little annoying having this problem on just one of them. But I guess the fact that it's more brightly lit than the others, as KeithPratt points out earlier, makes the problem very apparent.

    One other thing, for what it's worth: I know judging the film by how it looks in Quicktime is not exactly the best way to gauge it, but the strobing is completely absent viewing it that way. It isn't until it's on DVD that it shows up, which does make me wonder some if the problem is in the encoding somehow. No, I don't have a studio monitor hooked up to view the footage... I can't afford that setup just yet, but it is down the road.

    Thanks again for your help.
     
  19. aloofman macrumors 68020

    aloofman

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    #19
    He's right. This is the reason that 24 fps looks decent in a movie theater. A motion picture needs an image rate of at least 35-40 per second to be fast enough for persistence of vision to kick in and look like a moving image instead of a series of flashing pictures.

    Early filmmakers and theaters figured out the shutter thing about a hundred years ago, and so 24 fps became the slowest frame rate they could get away with. The creators of the first TV standards used a similar idea principle, but instead of using a mechanical shutter, they electronically split the frames into fields and interlaced them. So TV could be broadcast as slow as 25-30 fps while still looking like a full image.

    Here's the thing about 24p that so many people miss. Unless you intend your final output to be film, there's not much reason to use it instead of a faster frame rate. The great thing about 24p (not the fake kind) is that you can output it to be exactly one video frame per film frame, without any jitter or half-frames. But if you're shooting for video or the web, then there's no special reason to use it.
     
  20. bigbossbmb macrumors 68000

    bigbossbmb

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    #20
    I have to disagree. There is a reason to do it and that is so that the video looks more like film. There is a tangible difference in the look of 60i and 24p and most filmmakers will always choose 24p.

    The strobing issue almost seems like a problem with the cadence combined with improper camera moves (ie 7 sec rule for pans). I would agree with killmoms that you should remove the pulldown prior to editing whenever you shoot 24p/24pA. I always remove the pulldown before editing no matter what my output is.
     
  21. bigbossbmb macrumors 68000

    bigbossbmb

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    #21
    to the OP,

    you may want to post a sample or two so that we can tell exactly what symptoms you are getting in your video.
     
  22. David Schneider macrumors newbie

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    #22
    I assume you're talking to me, since the OP didn't actually mention any specific problems. I will post some samples of where I'm having issues, but as I said earlier, the strobing really isn't showing up in the Quicktimes... it's not until it's viewed on a dvd on a television screen that it's showing up. But maybe someone will see something that helps explain why it's happening... I'll post a few clips in a bit.
     
  23. David Schneider macrumors newbie

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    #23
    The thing that's killing me, because it doesn't fit in with anything I'm reading here or elsewhere, is that this strobing isn't happening during heavy movement... you mention pans, but there are no pans at all during any of my problem areas. It's just simply the actress walking (walking slowly, I might add) through the shot, or arm movement... not exactly action-scene type movement.
     
  24. madmaxmedia macrumors 68030

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    #24
    If the video looks great on his computer and then looks bad on DVD, then there's probably something that can be fixed. From the way OP describes it, the difference seems pretty significant.

    I'm not sure about the XL2, but I just bought a HV20 and found these directions on the Apple site on how to reverse telecine HV20 24p footage in FCS 2-

    http://docs.info.apple.com/article.html?artnum=306389

    The HV20 24p footage is real 24p with pulldown, I guess the camera doesn't add pulldown flags which makes footage from it hard to convert back. So if a method can handle the HV20, it can probably handle the XL2. (if you have FCS2 !)

    The other thing you might want to try as a troubleshooting step is to reverse telecine footage from that scene with JESDeinterlacer and then burn to DVD, just to see what happens. It's a free program that many people use with the HV20 as it automatically detects cadence breaks. And it's free- :)
     
  25. aloofman macrumors 68020

    aloofman

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    #25
    How does the same frame rate make it look like film? Just because it's progressive? Because 24 fps just seems like film to you? If it's never going to be output to film, why not just shoot it in 30p instead? There are at least a dozen other reasons that your video camera can create a film look that have nothing to do with the frame rate.
     

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