1/2fr sync errors

[idris]

I have a project which was edited on Avid MC (I believe) and exported as an AAF.
When I have later imported rushes WAVs (to find alt takes etc) and dropped these onto the timeline in (absolute) grid mode set to 1fr, the sync between the audio from the AAF and the rushes WAVs does not match.
I've not had time to check but it seems to be consistently about half a frame out.

As far as I know the frame rate has been 25fps throughout the entire process.
Some footage was shot on pro cameras, some on domestic handicams: I've not had time to thoroughly check, but I'm sure this problem is with the pro cam footage. All the audio was recorded on the cameras, so there shouldn't be an issue with audio and video devices not being locked.

As far as I can think, the offset must either occur when the media is loaded into MC, during the edit process (which seems unlikely with video editing being limited to 1fr resolution) or as part of the AAF import/export process.

I've subsequently had an additional section of the same project that has come from a different edit suite (I don't know anything about this one) where the AAF has sync inconsistencies between the guide audio extracted from a DNxHD32 QT file and the AAF. In this case the sync errors range from about 0.5fr to 2fr.

Any ideas on the cause?

[SBP]

Hi have seen this issue often;
1- Sound rushes loaded into Avid + same sound rushes loaded into ProTools.
2- AAF created in Avid opened in ProTools -proTools reads Avid .mxf media
3- changing from the .mxf to original sound rushes held in ProTools clips bin results in sound being out of sync.

In my experience;

This sync offset error is not consistent and can be well over several frames either way.

This issue seems to be caused when the Avid makes the AAF and creates consolidated files, or files with new bit rates or files with new sample rates.

If you un-tick all the options to re-render files and get the Avid to make a simple 1to1 copy of the required .mxf media then it is possible to change back to the original sound rushes and keep sync.

Another contributing factor can be if the original production sound recordings were started within a frame -many production sound mixers now are able to set their recorders to always start recording on a frame boundary. How ever most likely cause will be the aaf export issue mentioned above.

hope this helps

Bruno

[Fokke van Saane]

And to make it more complex; not all camera's record sound in proper sync. No, i have to say it differently; they don't record the image in proper sync! Is my guess. Usually image is late.
A collegue of mine did a test with a Canon C300; with both sound on camera or on a dual system with a gen-locked Lockit on it, each take had a different offset. I am talking in terms of single frames.

[mfranken]

I've seen a sub-frame sync error as well between an AAF and location WAVs when expanding tracks. A show I was working on had this error on the main unit WAVs and not the second unit. I noticed that the main unit WAVs weren't starting on a frame edge and the distance from the frame edge was the amount the WAVs were out-of-sync by when expanded from the AAF.

To me it looks like a MC problem where it's not seeing that the WAVs aren't starting on a frame edge. I get that location recordist may be able to select an option to record WAVs that start on a frame edge, but MC should still check or fix this on ingest.

Also, if MC isn't syncing these WAVs correctly to a frame edge on ingest, to me the AAF could be up to a frame out-of-sync.

Regards,

Mark

[Brandonx1]

I always see sync issues between camera audio and the multitrack waves. Their is no way to 100% sync the production audio and the camera audio so they drift. Lockit boxes aren't 100%, they are just the tightest way to sync the 2 audio/video clocks. I do tons of reality tv and after 2 hours of shooting, I see huge drift. If the production company doesn't jam sync every few hours, I get totally screwed.

[idris]

Interesting thought on the Avid MC render issue.

I don't think this is necessarily to do with locking recording devices though. As stated, I'm getting sync errors between the audio in a QT file and an AAF, both generated from the same MC project. There's is obviously something that's going seriously wrong in either MC or in the PT AAF import process.

Interestingly I've compared the audio from an mp4 (H.264) and a QT (DNxHD32) created from the same MC session. Their sync didn't match each other, but they were closer than the AAF, and consistent, whereas the AAF sync varied from clip to clip.

[SBP]

Originally Posted by SBP

If you un-tick all the options to re-render files and get the Avid to make a simple 1to1 copy of the required .mxf media then it is possible to change back to the original sound rushes and keep sync.

I should have added the words 'with the audio from the avid' at the end of this para.

whether the audio is actually in-sync is a whole other ball of wax.

Bruno

[Frank Kruse]

just a side note:

for various reasons the sound devices recorders all start their recordings on a full 00-frame. One is to enable cross stamping tc-rates after recording without altering the start-time.

so if your recordings come from an SD machine the sync error must be due to some weird thing either PT or MC do.

there is no obvious reason why the sync shouldn't be sample accurate when expanding to tracks since the source data should be identical bit by bit and so shlould the tc-stamp.

very strange but I have seen this also on some shows. but not on others. and I copy the audio media on finder level and don't even let the avid donthe copying.
sometimes it's just off by a few samples sometimes by 2 frames on a random clip by clip basis.

frank.