On the 23.98fps problem...

Zarxrax · Post by **Zarxrax** » Tue Aug 19, 2008 2:49 pm

So as most everyone is aware, there has always been this big complicated thing in the guides about how its so bad to edit in 23.98fps, so instead you should do all this junk where you change the speed of your audio and edit in 24 fps, then convert it all back...

And I'm just thinking, GEEZ! What the hell for?!
Seriously, there's not that much of a difference between 23.98 and 23.976. I think it works out so that there's about 1 frame difference every 4 minutes or so.

So you just skip all the hassle, edit your AMV at 23.98, then ChangeFPS("ntsc_film") that sucker, and maybe, just maybe, one frame of your amv might get deleted. And it's really perfectly fine to distro 23.98 on the web, so even then you would only have to do that on a convention version.

Again, to put this in perspective here. We are talking about, in most cases, one measly frame. Does anyone actually go through all this trouble? Why? Discuss?

/me is currently trying to streamline/simplify the guides where possible, and thinks this section is ripe for chucking.

trythil · Post by **trythil** » Tue Aug 19, 2008 8:24 pm

I do it because it's easier to reason that what you're doing to the video is correct. Obviously playing fast-and-loose with 23.98 -> 23.976 isn't "correct", in terms of maintaing proper audio/video sync; it just happens to work out.

Besides, it's not even that hard to reason through. Especially once you've done it once or twice.

Anyway. Nobody seems to care about correctness anymore, so you might as well just chuck it, and let those of us who do care be all smug about how Everyone Else Is Doing It Wrong.

trythil · Post by **trythil** » Tue Aug 19, 2008 8:25 pm

trythil wrote:I do it because it's easier to reason that what you're doing to the video is correct. Obviously playing fast-and-loose with 23.98 -> 23.976 isn't "correct", in terms of maintaing proper audio/video sync; it just happens to work out.

Besides, it's not even that hard to reason through. Especially once you've done it once or twice.

Anyway. Nobody seems to care about correctness anymore, so you might as well just chuck it, and let those of us who do care be all smug about how Everyone Else Is Doing It Wrong.

I guess I should say that I did it. Obviously, I'm too cool for AMVs these days. Or something.

mirkosp · Post by **mirkosp** » Tue Aug 19, 2008 8:53 pm

Premiere Pro CS3 has a working 23.976 timeline. Telling people to update to it or just to not care about it and edit 23.98 and changefps("ntsc_film") the outputted video is the best thing, yes.

Phantasmagoriat · Post by **Phantasmagoriat** » Wed Aug 20, 2008 12:58 am

maybe just make an obscure reference to it when describing ChangeFPS("ntsc_film") saying that technically, the older method is more correct, but for practicality reasons, it was removed from the guide.

Although, unsynch might be an issue. I don't know how ChangeFPS() determines when to remove/add frames, but I'm guessing it could unsych a video by 1 frame, [if it chooses new frames based on the current frames at 23.976fps timepoints] or by 1/2 a frame [if it chooses new frames based on nearest frames at 23.976fps timepoints]

This is pretty much just speculation, but I think I've reasoned enough to guess that, as a worst-case-scenario, the synchronization would be off by 1 frame, which is enough to be noticeable IMO... especially if the amv has a lot of sync.

For instance, sometimes there's about a 1 frame difference between VLC and MPC+CCCP. I know I'm not the only one that's noticed this, so maybe 1 frame is enough reason to warrant the extra steps. [although, I convert everything to 29.97 fps, so I don't think I would ever do this anyways...]

mirkosp · Post by **mirkosp** » Wed Aug 20, 2008 8:41 am

Phantasmagoriat wrote:Although, unsynch might be an issue. I don't know how ChangeFPS() determines when to remove/add frames, but I'm guessing it could unsych a video by 1 frame, [if it chooses new frames based on the current frames at 23.976fps timepoints] or by 1/2 a frame [if it chooses new frames based on nearest frames at 23.976fps timepoints]

ChangeFPS() keeps the audio sync very well. Then again, if you aren't sure about that, use ConvertFPS() ... but that's only if you don't mind frame blending in order to preserve audio sync.

Zarxrax · Post by **Zarxrax** » Wed Aug 20, 2008 9:14 am

Phantasmagoriat wrote:For instance, sometimes there's about a 1 frame difference between VLC and MPC+CCCP. I know I'm not the only one that's noticed this, so maybe 1 frame is enough reason to warrant the extra steps. [although, I convert everything to 29.97 fps, so I don't think I would ever do this anyways...]

I've actually noticed that difference between VLC and CCCP myself. I even tried to do some testing to find the exact problem, and I saw that its actually as much as 2 frames off sometimes.

Regarding ChangeFPS(), a test I just performed indicates that given:
1) A 23.976fps source
and 2) A 23.98fps source with ChangeFPS

The first dropped frame will occur at frame 2998, which is just over 2 minutes into the video. The 2nd dropped frame occurs at 8994, which is a little over 6 minutes into the video.

Given that there is a 1 frame difference per four minutes, and ChangeFPS makes the first frame drop at 2 minutes, then it would stand to reason that when using ChangeFPS, we never really see more than about half a frame of desync.

Phantasmagoriat · Post by **Phantasmagoriat** » Wed Aug 20, 2008 10:40 am

mirkosp wrote:ChangeFPS() keeps the audio sync very well. Then again, if you aren't sure about that, use ConvertFPS() ... but that's only if you don't mind frame blending in order to preserve audio sync.

I wouldn't want that, so yeah, ChangeFPS() would still be a better choice.

Zarxrax wrote:I've actually noticed that difference between VLC and CCCP myself. I even tried to do some testing to find the exact problem, and I saw that its actually as much as 2 frames off sometimes.

Yeah, that's always bothered me.

Zarxrax wrote:Given that there is a 1 frame difference per four minutes, and ChangeFPS makes the first frame drop at 2 minutes, then it would stand to reason that when using ChangeFPS, we never really see more than about half a frame of desync.

I kinda figured it would be off by either a one frame difference, or a half a frame difference. At least I can rest assured that the makers of ConvertFPS() designed it so only the latter occurs lol

Now I'm just trying to decide how significant 1/2 frame desync is

and where the trouble spots are.
The sync would be perfect at time 00:00, but as we approach the 2 minute mark, the timing would slowly slip, until the 2minute mark. Just prior to that, the video would be half a frame late. Then the frame drop occurs, and the sync would be half a frame early. So that's the point I would be most concerned with. I think I could live with 1/2 a frame off, but the moment when the switch occurs would be slightly exaggerated, and could leave the editor scratching their head [...well... maybe...] and you wouldn't notice it in previews either.

This is of course, assuming that something just happens to be synced at that moment

Zarxrax · Post by **Zarxrax** » Wed Aug 20, 2008 11:13 am

Well, personally I find even around 3 frames desync to be unnoticeable in most cases.
The only reason I can tell regarding the VLC/CCCP issue was with my own videos, where I know for a fact that I have something synced a certain way and have seen the sync repeatedly. I would never be able to tell on someone else's video. At a half a frame desync, I think it would be impossible for *anyone* to tell.

Phantasmagoriat · Post by **Phantasmagoriat** » Wed Aug 20, 2008 11:25 pm

I pretty much agree. On half a frame... you would really need to repeat the video over and over before you noticed it. And editors that are overly concerned with sync would likely use 29.97fps anyway. So yeah, you can probably scrap the section... but for correctness sake, an obscure reference to the old guide might be a good idea.