DVD frame dropping.

[videosam]

Hi, I'm dreading asking this question as I'm almost certain there's no way round but you never know... There's some clever sh*t around these days.

THE ISSUE IS: I had to make a showreel and the assets they gave were on DVD and hence had 15 full frames and a load of subframes. I captured as PAL 25p. Showreel looks fine in all the streaming links and prores etc because 25 fps has captured enough of the subframes to make the movement smooth enough. However, when I author back to domestic DVD, the movement look juddery like it has a low frame rate.

THE QUESTION IS: Is there any fancy gizmos or software that will allow me to author playable domestic standard def DVDs but maintain a higher amount of I frames. (in other words, a higher GOP rate)?

I already know that NTSC DVDs have a GOP rate of 18 but i've already tried it and doesn't help.

Any ideas?
Cheers

[filmboss80]

Since you don't list your location, we don't know whether you are in a PAL country or an NTSC country (or a SECAM country, for that matter). Also, you do not mention the format of your source video; i.e. whether the source itself is PAL or NTSC, the aspect ratio, the actual frame rate (NOT the GOP rate), and whether it is interlaced or progressive.

Motion judder is commonly the result of a field mismatch (for example, interlaced video with Bottom Field First playing back Top Field First or vice versa). In your case, there may also be a video standards mismatch. The decision to capture PAL or NTSC is NOT discretionary! You have to know both your source and your playback parameters.

We absolutely cannot give any kind of advice until we know what we are dealing with. First things first: what country are you in, and what country did your footage originate from. That will help us with the PAL or NTSC issue.

[DB83]

Also, you state you have captured at PAL 25fps but then mention NTSC dvds which could well have 29.97(30)fps.

If you did not do that conversion correctly (and it is NOT easy) you could simply have gained 5 frames for every 25 and that will also result in jerky motion.

[videosam]

Ok,
I am a PAL country (England). Imported as 10 bit uncompressed PAL 25p anamorphic 16:9 from a 16:9 PAL DVD through a DVD player and blackmagic card into FCP. Actually, I assumed that because I captured into PAL, you might assume I'm a PAL country but anyway, I'm not a DVD expert so forgive me if I don't know all the tech specs up front. NTSC/ PAL agreed is not discretional but if half your assets PAL and half NTSC but you're asked to deliver a PAL video then you don't have much choice but to encode one to the other at some point.

I think my confusion comes from not knowing what the relationship between a PAL DVD's frame rate and its GOP rate is and how they affect each other. Having said that in this instance it's a bit late as the project is pretty ancient and the assets are long gone and can't be changed or recaptured etc. so I have to work around what I have. So, bearing that in mind, all I have now it the final export which is 25p PAL prores QT file which plays fine, has no juddering or interlaced lines etc. the question remains why then does it show up juddering once compressed back into PAL DVD and is there a way to stop it? I assumed that perhaps because PAL DVDs only have 15 I frames at source, that each frame captured might not sync up with the full I frames.

Anyway, that is the info I have. If I was an expert already I wouldn't need help, so if anyone can decipher my ramblings then I'd be very much obliged!
Cheers.

[jagabo]

There is no relationship between GOP size and frame rate. Using shorter GOPs will not effect the smoothness of your videos unless you are using illegally long GOPs to start with.

If your juddering is very fast (25 times a second), and has a two steps forward one step back, cadence, you just encoded with the wrong field order.

[videosam]

"Using shorter GOPs will not effect the smoothness"
Ah well that's cool although leaves my further perplexed. Excuse the noob question but what's the point of the GOP then if it doesn't affect smothness?

[jagabo]

MPEG 2 encoding encodes some frames (keyframes) as entire images and others as only difference between successive frames. A GOP is a Group Of Pictures consisting of a keyframe (an I frame) followed by several P (forward predicted) and B (bidirectionally predicted) frames. I frames require more bitrate because they encode an entire image like a JPEG file. P and B frames require less bitrate because they only encode parts of the frame which have changed. Regardless of the GOP size, all frames are encoded and played back -- hence, changing the GOP size does not effect the smoothness of playback.

The reason for using P and B frames is to reduce the bitrate requirement ( many parts of the frame don't change from frame to frame so there's no need to include them in every encoded frame, the decoder continues to display the unchanged parts from the last frame, only updating the areas that changed). B frames are also usually encoded with lower quality settings to further reduce bitrate. The theory is that you won't notice a few low quality B frames during realtime playback, the quality will be restored in a frame or two by a P or I frame.

Limiting GOP size is usually only done to make decoding a little simpler (some devices cannot handle long GOPs, this is why DVDs are limited to 15 or 18 frame GOPs, whereas Divx/Xvid often use up to 300 frame GOPs), or to make decoding errors shorter in duration (eg a 25 fps video with 5 frame GOPs will suffer for at most 0.2 seconds when there's a decoding error, vs. 0.6 seconds when using 15 frame GOPs). When a scene change occurs it's best to use start a new GOP with a fresh I frame. Using smaller GOPs will up the bitrate requirement (or lower the overall quality at the same bitrate) because more I frames means more bitrate.

[DB83]

So the problem does appear to be the conversion of the NTSC assets or , as you now have different sources, a mixture of field-orders in your captures. How were the NTSC assets acquired into your PC ?

[poisondeathray]

Well, he says his final prores 25p export is fine. So assuming he checked it thoroughly , the error cannot have occured during capturing or standards conversion before the editing stage. This suggests it's a MPEG2 encoding or authoring error

all I have now it the final export which is 25p PAL prores QT file which plays fine, has no juddering or interlaced lines

[videosam]

Ok that's cool. Thanks people!
I imported as 10 bit uncompressed PAL 25p anamorphic 16:9 from a 16:9 PAL DVD through a DVD player and blackmagic card into FCP. It seems though there's nothing that can be done now but could someone tell me their preferred method of capturing DVDs onto mac i.e. best software/ best codec/ best specs to capture as (noting that it will need to over-ride any anti-copy encryption).
Thanks

[videosam]

Thanks poisondeathray too- I'm using DVD studio pro and haven't had problems before so I'm a bit stuck I think!

[DB83]

Sorry to ask again but.....

You imported a PAL dvd as PAL 25p. That plays fine.

But did you import the NTSC assets in the same way ? And at what fps ?

[poisondeathray]

I'm inclined to say that you've made an error somewhere and that the prores export does exhibit a cadence problem , because there are so many potential issues in your workflow.

But if you're certain it's ok , then there is no reason the final DVD should exhibit those issues. If everything you say is true, you have to check your MPEG2 encoding and authoring settings, maybe post them here

Maybe if you can post a sample demonstrating exactly what is going on , people will have better suggestions. For example , 25p "judder" is almost the same as 24p "judder" . And this is normal . What you may be identifying as "judder" maybe something else

I can't advise on mac workflow, but I can say for certain PC workflow is much less complicated and less room for error. For instance you don't need to "capture" DVD assets

[jagabo]

Upload a short sample of the DVD VOB/MPG.

[videosam]

No probs DB83! Actually I never had any NTSC assets, it only got mentioned by that boss80 guy. To try and isolate the problem, I authored an NTSC dvd just as an experiment to see if the increased 'I' frames improved it- which it didn't.
'Tis a mystery. Next time I'll just insist on beta's for any projects that require authoring to DVD.

-poisondeathray, I'll post the mpeg settings in a sec.

Thanks guys!

[poisondeathray]

Another way you could try to narrow down the problem is encode that prores export to something else, like a quicktime h264/aac/mov as if you were going to export for internet . If that plays fine, that strongly suggests a MPEG2 encoding/ authoring issue.

And have you checked the final DVD in another DVD player , or software player ? Maybe you have burning or bad media issues ?

[videosam]

Hmm... when I encode it specifically to anything else it has the same effect too. It must be inherent in the prores after all. I can't think why but the problem is far reduced in the prores version, perhaps QT handles just it better? Have tried one hardware player and a couple of PC players.

Here's the MP2 it might not seem obvious at fist but check out the first right pan on to the cowboy and then some of the free falling action in Avatar.
http://rapidshare.com/files/410818599/SAMPLE.m2v.zip.html

Got to run now though- thanks again for your help although I suspect it is what it is.

[poisondeathray]

Yes, serious issues with that .m2v elemenatary stream. There are repeated frames, skipped frames, aliasing problems.

If on closer examination , the prores export has those same issues, you have to go back and find out where you introduced the errors. I doubt there is much you could do to salvage it at this point if you were not willing to fix earlier mistakes - if the original assets were fine, then it is possible to do this correctly (or at least improve on it).

[jagabo]

Yes, the m2v file had lots of duplicate frames and missing frames. I suspect it was drop field deinterlaced from a 29.97 fps NTSC telecined source then converted to 25 fps by dropping every 6th frame.

[DB83]

I will not make an issue of this but...

It was you who specifically said "I'm not a DVD expert so forgive me if I don't know all the tech specs up front. NTSC/ PAL agreed is not discretional but if half your assets PAL and half NTSC but you're asked to deliver a PAL video then you don't have much choice but to encode one to the other at some point."

Which led me to think that you had a dual source and were thus combing it.

In truth, this really is a topic for the Mac forum as they will have the knowledge of the hardware/sofware that you are using. I may be well off the mark here but you mention QT so is FCT producing that or is it the capture device. QT is, I believe, a frame-based format and somewhat highly compressed and if you prepare a field-based movie from that - without precautions ..........

Can your capture device capture Mpeg2 ? Can FCT import Mpeg2, edit it AND export it to DVD-compliant Mpeg2 ?

[videosam]

Assets are long gone- looks like I'm stuck with what I have.
Don't worry about the QT reference, FCP exports into a file with a QT logo on it is all. FCP does allow you to edit Mpeg2 and exports through compressor into DVD compliant Mpeg2/Dolby however i'm not entirely sure the blackmagic card I capture through will allow Mpeg2 capture. Never mind- I'll just have to live with it.
Thanks guys.