What kind of framerate is 23.976 anyways?

[DJ Baz]

Hi

I'm having trouble with converting 23.976 fps DivXs to DVD. I can make great quality DVDs from PAL sources, and if I convert my 23.976 ones to 25 fps they work fine also. The reason I don't just convert to PAL everytime is that many of my DivXs have lovely 5.1 AC3 audio, and speeding up the audio with BeSweet means re-encoding - at the expense of quality - and I'd rather keep the original intact.

But in TMPGEnc every time I try to make an NTSC compliant .m2v file from an NTSC sourced Divx I get a horrible intermittent judder.... usually the first few minutes of the DVD plays OK, but then this nasty judder starts, and continues to come and go every few seconds.

TMPEGENc is set to 23.976 fps (internally 29.976), and encode mode is set to 3:2 pulldown when playback. I gather from reading this forum that this is the best way to encode NTSC film to DVD. I have also tried just encoding at 23.976 fps and adding the pulldown flags with pulldown.exe only to get the same result.

Does anybody know what could be the cause of this, or has anyone else had a similar problem? Making an NTSC DVD from an NTSC Divx should be pretty straightforward, right? Although it seems to me, if you're going to use a ridiculous framerate like 23.976 instead of something sensible like 25 you deserve all the problems you get...

[yoda313]

Hello,

23,976 fps with 3:2 pulldown = 29,97 playback fps (NTSC Film, this is only supported by MPEG2 video)
16:9 Anamorphic (only supported by 720x480)

https://www.videohelp.com/dvd

That's from the What Is dvd section found on the left side of the screen.

That 23.9 fps is what hollywood movies are filmed in (I think it's actually 24fps).

As for your conversion issues.... do a forum search for ntsc to pal and you'll find a ton of related material.

Good luck.

Kevin

[adam]

Nothing ridiculous about 23.976fps. Its pretty much what most commercial DVDs are encoded at. Its a technique. The film only starts with 24 frames per second, so why store ~30 every second? You instead store 23.976 and let the DVD player create the frames on the fly. This has several other benefits as well, but all in all, a DVD stored at 23.976fps internally will be much higher quality then if stored at 29.97fps...especially when viewed on a progressive scan tv/dvd player combo.

Such a DVD is naturally compliant on all NTSC DVD players, and most PAL ones too, so there's got to be a little problem somewhere either in your encoding or your authoring.

On the advanced tab in TMPGenc, make sure the source aspect ratio is set to 1:1 and that it is set to progressive.

Other then that, make sure you are encoding to DVD compliant settings. A non-compliant resolution can definitely cause jutters.

Also what program are you using to do the authoring?

[devlynh]

Here is the technical answer:
Video in North America in the days of B&W TV was 30 frames /S (60 fields/s). When colour came along the engineers needed more room (time) in the signal so they slowed the frame rate down to 29.97 frames /s. The difference between 30 and 29.97 is 0.1%.
Most movies shot on film are at 24 frames/s. There is no way to convert 24 fps to 29.97 fps without having a frames dropped at odd intervals. In order to facilitate a correct 3:2 pulldown (method where 24 fps films can be converted to 30fps video) they slow down the film by 0.1%, hence 23.976 fps. This was previously only used for post production work at an intermeditate stage, when transfering film to video for editing. The editing equipment would remove the effect and allow editing of the film at 24fps.
However when the ATSC HD spec was developed it was decided to keep 23.976fps as it meant that video at 29.97 with 3:2 pulldown could be resolved back to 23.976fps non-interlaced frames electronically.
The only actually reason for this is to keep the time correct for the networks. Analogue TV can only be broadcast at 29.97, DTV can solve this issue, the question is will it?

According to the DVD specifications the only valid frames rate are 29.97fps for NTSC and 25fps for PAL.
See http://stream.uen.org/medsol/dvd/home.html for more details on this (good site)
MPEG2 can deal with more frame rates however DVD uses a subset of the MPEG2 spec.

[adam]

I think you mean .1% and not .01% right?

How does the use of 23.976fps internal keep the time correct for networks? 23.976fps has the exact same run time as 29.97fps.

And just a point of clarification, 29.97fps is the only supported NTSC playback rate for DVD. 23.976fps internal still plays back at 29.97fps and is fully within the DVD specifications.

[Robert Simandl]

So if my capture from an HDTV broadcast is a 23.976fps AVI file, should I set TMPGEnc to do 23.976 or 29.97fps when I reencode to MPEG-2?

And I was thinking the aspect ratio and source aspect ratio should be set to "16x9 display." Can someone tell me is this correct or not?

I'm a newbie at reencoding 16x9 files here, but I've done one. It was a 16x9 24.976fps AVI. I set source aspect ratio to "16x9 display," encoding aspect ratio to "16x9 display," arrange method to "fullscreen (keep aspect ratio)," and frame rate to 29.97fps. It came out beautifully. If I were to change one or more of these settings, could it come out better?

Thanks,

[DJ Baz]

Thanks for your replies guys, although to clarify I'm only talking about creating an NTSC compliant DVD from a 23.976 fps Divx file - making PAL DVDs, either from 25 fps Divxs or speeded up 23.976 ones, is fine.

Also, my little jab at the 23.976 frame rate was only supposed to be tongue-in-cheek (well it's hardly what you'd call a nice round number, is it?).

I'm pretty sure all the settings in TMPEGEnc are OK, at least in that they're the same I use to create PAL DVDs with the exception of the frame rate, resolution, and encode mode - and these only differ from the TMPEGEnc template because I set the aspect ratio to 16:9.

I suppose it could be the authoring, although I have used both TMPGEnc DVD Author and IFOEdit. I also tried NeroVision Express, although there are very few options to fiddle with there, and that produced a DVD with terrible judder and also glitches in the sound.

I'm going to try a Divx of a documentary film from an NTSC video source (29.976 fps). If that turns out OK it might suggest a problem with the 3:2 pulldown, might it?

[adam]

Robert Simandl: If your source is ever 23.976fps and NTSC is your desired output then you should always encode to 23.976fps. Doing a hard telecine to 29.97fps decreases quality and grants you absolutely no advantages. However, if you are actually capturing these broadcasts to your pc then I would imagine you can only capture them at 29.97fps. From there you could encode to 29.97fps or do an inverse telecine (reverses telecine process to get back to 23.976fps) and then encode to 23.976fps. I have no experience with HDTV though so you'll have to ask someone else I suppose, but 23.976fps sources should always be kept that way.

Your 29.97fps encodes would be higher quality at 23.976fps, yes. Whether you would see the difference is subjective, but there is definitely a difference.

As for aspect ratio...If you know your source's aspect ratio and you set it correctly, then you are fine. I suggested using 1:1 for source aspect ratio for an avi file because its fairly foolproof. With mpeg, the actual resolution is rarely going to be 4:3 or 16:9, rather the stream contains a flag which instructs the dvd decoder to resize to the proper aspect ratio at playback. Though some codecs also support aspect ratio flags, most avi's you are going to come across do not use aspect ratio flags, and thus the resolution of the file itself will be either 4:3 or 16:9. By setting the source aspect ratio to 1:1 the source aspect ratio is essentially just determined from the source resolution itself. In short, you don't have to worry that you will mislable the aspect ratio.

DJ Baz, If you also tried pulldown.exe then no, the problem cannot be with the pulldown flags. All DVD players support NTSCfilm (23.976fps) encoded DVDs, and setting these flags in an already encoded stream, ala pulldown.exe, is a trivial thing.

[DJ Baz]

OK, the chances are that there is a problem with the encoding. Why then would TMPGEnc produce a juddery 23.976 stream, while 25 fps encodes come out perfectly smooth?

Lots of people use TMPGEnc, many of them to make NTSC DVDs, so what am I doing wrong?

[adam]

You are just loading the NTSC DVD template, unlocking it, and then setting output to 23.976fps (29.97fps) and changing the output aspect ratio? If so then I don't know what you would be doing wrong at the authoring stage. I'd think it was a decoding problem but since PAL works ok that doesn't seem likely.

Does the encoded mpg appear jerky or only after its been authored? That will at least narrow the problem down a little.

[Robert Simandl]

Oops!!!!!!!!!!

Looks like I found a reason to convert to 29.97fps after all.

I imported my brand spanking new 23.976fps mpeg into TMPGEnc DVD Author and got an error message saying TDA doesn't accept 23.976fps files! 25 or 29.97fps only.

Looks like it's time to start over.....

[adam]

No its not. 23.976fps is not DVD compliant. 23.976fps with 3:2 pulldown flags enabled is. So run your mpeg video stream through pulldown.exe to add the flags. It takes a couple minutes. After adding the flags, the stream will be treated by authoring programs as 29.97fps, because that is its playback rate.

The only reason I can think of that you would ever need to convert 23.976fps to 29.97fps output would be if you needed to transfer it to an analogue format where NTSCfilm was not supported.

[Robert Simandl]

Wish I'd read that before I deleted the mpeg....



Oh well, I'll remember it for next time.

Thanks...

[Robert Simandl]

Another question...

I'm starting again in TMPGEnc Plus. In the Video tab, I've selected 16x9 display, 3:2 pulldown while playback, and 23.976 (29.97 internally) fps.

In the Advanced tab, I've selected non-interlace, bottom field first, 16x9 display, and fullscreen (keep aspect ratio).

Do the above settings sound right for a 624x352 23.976fps XviD file? Should I select 3:2 pulldown in the advanced tab also?

Thanks...

[adam]

Those settings sound fine as is. To reiterate what I was saying before about aspect ratios. Take 624 and divide it by 16, you get 39. Take 352 and divide it by 9, you get very close to 39. (slight aspect ratio error but that's ok, 16:9 is sort of a rounded figure anyway...very long story.) Therefore, your avi has a 16:9 aspect ratio. If it weren't this, it would have most likely been 4:3. See how the physical resolution conforms exactly to the aspect ratio? So by just selecting 1:1 as the source aspect ratio it automatically would select 16:9 and save you from making a mistake. So yes, 16:9 is the correct setting for this avi, but it might not always be correct for other avi's. So if your source is an avi then just save yourself the trouble and use 1:1.

As for your other settings, filed order (bottom versus top field) is irrellevant if the source is progressive (non-interlaced.) If its non-interlaced then there are no fields, so there's no reason to worry about field order.

No, do not use the 3:2 pulldown filter on the advanced tab. That is for physically telecining 23.976fps to 29.97fps. Like I mentioned before, the only reason I can think to do this is if you need analogue output. And if you encode to 23.976fps AND use this filter you will most likely get very strange results.

[Robert Simandl]

So if I pick 1:1 for source aspect ratio in the advanced tab, 16x9 display will automatically show up for the encoded aspect ratio in the video tab?

Thanks again,

[adam]

Only on the advanced tab.

[edDV]

Here is the technical answer:
Video in North America in the days of B&W TV was 30 frames /S (60 fields/s). When colour came along the engineers needed more room (time) in the signal so they slowed the frame rate down to 29.97 frames /s. The difference between 30 and 29.97 is 0.1%.
Most movies shot on film are at 24 frames/s. There is no way to convert 24 fps to 29.97 fps without having a frames dropped at odd intervals.'''''''.

devlynh,

Good info there but the reason for 29.97 fps for NTSC was a bit more complicated and involved selecting a compatible color system that would work with existing monochrome TV sets without causing color interference from audio or color beats with audio on monochrome TV sets.

to be precise

"More experimentation revealed that if the frame rate was dropped just 0.10001 % down to 29.97 frames/second then the resulting horizontal scanning frequency would be 15.734264 kHz (29.97 x 525), and the chrominance subcarrier would be 3.579545 MHz (455/2 * 15734264). Thus the audio/color subcarrier difference would be 920.455 kHz (4.5mHz - 3.579545 MHz). Turns out this is very close to being an odd multiple of one-half the scan rate. (15734.264 / 2 = 7867.132 * 117 = 920.4544 (almost an exact odd multiple - the 117th multiple in fact !) So by dropping the scan rate from 15,750 to 15,734.26573 and selecting the 455th/2 harmonic for the chroma subcarrier, effectively interleaved the chroma with the luminance information, which greatly reducing the visible interference."

quote from http://www.videointerchange.com/pal_secam_conversions.htm

[DJ Baz]

=Does the encoded mpg appear jerky or only after its been authored? That will at least narrow the problem down a little.

Yes, the jerkiness is definately there in the encoded .m2v file. Often the beginning of the film seems to be absolutely fine, but the judders start to appear about 10 or 15 mins in.

Could this be due to using my computer to do other things while it encodes away in the background? Then again, it's nothing too strenuous, only web browsing and word processing, and surely this would affect PAL output too?

[adam]

Short of crashing the program entirely, no do additional tasks during encoding shouldn't have any effect other then to slow down the encoding process. I really can't think of why this would be happening only with NTSC output set, but try taking the usual precautions. Check your avi files in advance for bad frames. If possible, frameserve the avi to TMPGenc using VirtualDub or Avisynth. Finally, if you must use TMPGenc alone, then try increasing the directshow priority. Go into environmental options/VFAPI plugins and right click on the direct show file reader and increase its priority until it is highest in the list. This often solves problems with troublesome divx sources.

[DJ Baz]

I already have DirectShow as highest in the list, following the advice of one of the backup guides I read here. Still, like you said, it does seem unlikely that this would affect NTSC but not PAL output.

As far as I can tell, all of the Divxs and Xvids I have tried have been perfectly smooth, top quality files. Besides even the troublesome 23.976 ones are fine in TMPGEnc when speeded up to 25fps in VirtualDub.

It might sound crazy, but I'm going to try speeding one up to 25 fps, and then slowing it back down to 23.976 fps again to see if that makes any difference. I may also try frameserving in Virtualdub like you suggested.

Alternatively, can you suggest any program that can speed up an AC3 file to match a framerate of 25 without re-encoding? I'm not convinced by BeSweet, the files it produces are noticably much quieter than the original - plus I suspect it introduces some kind of dynamic range compression.

Thanks once again for your help, though...

[adam]

Since AC3 is an encoded audio format there is no way to timeshift it without first decoding it...so no you've got to do a full decompress/recompress.

You could do the time shifting in any non-linear editor like Premiere or could do it in BeSweet but let it export to wav and then you could use the AC3 encoder of your choice. That's what I usually do for these conversions.

But...looking back through your posts you say that not only does the video stutter, but the audio does too? And you just using the original AC3 stream correct? So the problem has got to be with more then just the video stream, or with TMPGenc's encoding of it. How are you multiplexing the audio and video together?

[DJ Baz]

You could do the time shifting in any non-linear editor like Premiere or could do it in BeSweet but let it export to wav and then you could use the AC3 encoder of your choice. That's what I usually do for these conversions.

But what if it's a 5.1 AC3 file? Can you save that to a WAV, or would it have to be six separate mono WAVs? I only have ffmpeggui, and I think that only does 2 channel AC3. Which encoder would you recommend?

But...looking back through your posts you say that not only does the video stutter, but the audio does too? And you just using the original AC3 stream correct? So the problem has got to be with more then just the video stream, or with TMPGenc's encoding of it. How are you multiplexing the audio and video together?

No, sorry... that was only one early attempt with NeroVision Express. I haven't had that problem since switching to TMPGEnc and a whole battery of other programs. For an all-in-one solution, Nero was pretty good for PAL stuff, as long as you were happy with mp2 audio, but with NTSC it produced much worse judders in the video that I get now, plus audio glitches.

Yeah, so I switched to doing it manually so I could retain the 5.1 audio and do NTSC output. So far I've only achieved the first goal...

[adam]

If its 5.1 then yes you have to export to 5 separate wavs and then encode them all to AC3 again. All this can be done with BeSweet, otherwise you'd need to use a commercial AC3 encoder, as far as I know.

Well I'm stumped. Converting a 23.976fps source to 23.976fps obviously should be the least problematic process, as opposed to converting it to something else. I can't figure out why you only get problems when you leave it at its native framerate. I'd still try frameserving, it will probably eliminate the possibility of any problems in your source creating the jitters. Also, perhaps this is a codec pack. If you installed a codec pack then get rid of it and just install the codecs you need.