New 23.976 Pitfalls (Long)

[Sean McCoy]

There was some discussion about this HD framerate last year that led to the addition of the 23.97 option in Studio. Last week I started work on an HD project that was edited in Avid. Knowing that I would need to render and burn reference DVD's for the client, I asked the editors to provide me with a video file converted to 29.97 on their end, and an OMF with handles for the audio.

They sent me a QT video with Sorensen compression, which I rendered to DV AVI in Vegas Video. The two video files are only two frames different when compared on the timeline in Studio, but while the Viewer window shows the AVI framerate as 29.97, the QT rate is 31.578 (?), and the total frames are quite different. Vegas and QickTime Pro, OTOH, do show the QT Movie as 29.97. Can anybody shed any light on this wierdness?

The OMF file converted fine from EDL Convert Pro, but the timeline is totally whacked out. All the tracks are shorter than the video, but the sync problems seem to be inconsistent even from track to track. EDL Convert shows the OMF to be at 23.97, as opposed to the 29.97 rendered video, which is my guess as to why they don't sync. Can anyone confirm my suspicion? And is anyone familiar enough with Avid to know if it is capable of exporting an OMF with a converted framerate?

I got consolidated tracks from them to work with until we can sort out the OMF issue, but even those were slightly out of sync---and not exactly by the .1% pulldown differential I've run into in the past. These are not quite .11% off, so I can't quite varispeed them into exact sync. All this can be worked around, of course, but I have to deliver stems to another post house later and am very concerned about stuff being screwed up down the line. In retrospect I probably should have kept everything at 23.976 and done my own video conversion for the DVD. Too late now!

[MMP]

I recently had a project with all kinds of frame rate issues. I tracked the problem to use of the Sorenson codec. Have the video output as motion jpeg, and my guess is the issues will disappear.

Regards,

MM

[Sean McCoy]

I recently had a project with all kinds of frame rate issues. I tracked the problem to use of the Sorenson codec. Have the video output as motion jpeg, and my guess is the issues will disappear.

Regards,

MM
Interesting. The audio that was embedded in the Quicktime Movie is full of periodic dropouts. I wonder if that's related to the Sorensen encoding? I've had many Sorensen encoded files sent to me in the past without a problem--- but those were usually for 30 second spots, while this is an 80 minute movie.

The codec could be yet another problem, but since the video does time out to the proper overall length, while the OMF file is way off, I'm still more suspicious of the frame rate difference. The fact that the audio in the OMF plays at proper speed, but the regions are improperly spaced, would imply to me that the OMF file is calculating region start times based on the lower frame rate. Logical?

This stuff can be mind boggling. :(

[MMP]

Well, there is certainly a chance that you have multiple problems occuring. The Sorenson one for me was mind boggling, and I believe possibly new since the latest version of QT. The length was correct, but internally there were errors of plus or minus 3 or 4 frames for specific frames. Hard FX hits were moving around on each output and lip sync was wrong a lot.

So, I would still suggest getting a motion jpeg output to eliminate that possible problem, then move to the next one on your list. For me, Sorenson is now suspect.

What if you were to digitize the video?

Regards,

MM

[MMP]

Also, I often have a two frame error when doing any transcoding that involves QT. I usually require a count down with a two pop, and this helps keep things right in the real world.

The header frame rate seems to often misreport the real frame rate of the video file in Studio, so I wouldn't pay much attention to that, not a reliable debugging tool.

I haven't done any sessions at 23.976 using EDL Convert. I wonder if there are settings there that could pull the OMF into line.

Maybe Chris Conlee can chime in and help...he probably has the most experience on the board.

Regards,

MM

[conleec]

Hey all,

It's been a while since I contributed; been mostly working on picture editing this past year. However, it was me who originally talked to Bob about implementing a 23.976 framerate in SS. The cool thing about his video engine is that plays back 23.976 material natively, and using an appropriate converter such as a Canopus or even a mini DV camera you can get playback to a client monitor without doing a conversion to 29.97. You can also (using the same convertor or camera) get a signal sent to a DVD recorder for your clients.

The cool thing about 23.976 and 29.97 footage is that they are mathematically connected, and if you mix to one and do a mixdown without any pull-up or pull-down it will sync with the other. This is to say that technically you don't have to do a conversion on the material at all -- simply work on the 23.976 material through-out. This is why I was so excited that Bob was able to implement that framerate in SS.

This is how I typically work. Do an OMF export from a 23.976 project, and do a QT reference export of a 23.976 video from the Avid (the QT Ref export from Avid is nearly instantaneous, 'cause it simply references the original media files already on your drives, but because of that it can't be used by anybody else without copies of all your media in identical locations). Then I use After Effects to do a straight conversion of the QT reference file to a 23.976 AVI with a timecode window burnin; this file is transportable and can be sent to another machine without problems. Next, I open the OMF in SS with EDL Convert Pro, mix in SS, and voila!

In your case, you'd have to hope your clients have AE in order to do the conversion to AVI. If not, they could render the project to a 23.976 DV stream straight from their Avid (preferably with a Timecode window burned in), and you could convert it to an AVI in your Vegas. Unfortunately, Avid doesn't have a way to export useful AVIs directly, so a conversion from some flavor of QT to AVI is always part of the workflow.

The conversion of 23.976 footage to 29.97 is fraught with peril if it isn't done exactly correctly. I'm not sure that Sorenson correctly adds the appropriate pulldown frames. I'd say that's most likely where your problems occurred.

However, just because a show was shot in HD doesn't mean it was edited as a 23.976 project either; it could have been cut as a 30i project with pulldown frames inserted, or it could have been cut on an older Avid as a true 24 fps project, in which case you'd need to deal with the approriate .1% pulldown. Did they remove the pulldown frames, or was it shot in P2 with a 24p native framerate? If you let me know all the particulars I can try to give you a guide for getting things done correctly. Sorry to make it sound complicated, but truth is, there are quite a few variables, and they all have to be considered to get the equation just right...

The Avid does not have the ability to generate an OMF at a framerate other than what the project was cut at, so the OMFs you receive actually fairly well dictate how you need to set up SS and what you need to do with your video files.

Chris
818-694-2112

[Mark Stebbeds]

Good post Chris...thanks.

I'm just starting to get requests to create ref DVDs, and export QT movies with sound embedded. Good info.

Mark