FORUMS: list search recent posts

DNxHD transcode slips audio out of sync

COW Forums : Avid Media Composer

<< PREVIOUS   •   VIEW ALL   •   PRINT   •   NEXT >>
Frederic Lumiere
DNxHD transcode slips audio out of sync
on Dec 30, 2011 at 3:39:00 pm

So after much good advice from the pros, I decided to bite the bullet and re-transcode everything to DNxHD175 instead of ProRes. ProRes looked good but it created too many issues with workflow with other apps like AE & DaVinci.

The audio slips out of sync on the DNxHD material and it didn't on the ProRes (both transcoded via AMA import of H.264 clips).

After studying the issue closer, I discovered that the ProRes encode was adding flash white frames on some scene changes that weren't there on the H.264 clips! (I patch all the takes from one interview into one clip with Streamclip because of the 10 min limit on 5D and I also add timecode).

So the ProRes had extra white frames compared to the AMA H.264 but the audio remained in sync on a 3.5 hr clip.

The DNxHD clips don't have the white frames and it slips out of sync after each scene break. Getting worse with each scene break.

The audio between the DNx and the ProRes clips are in perfect sync all the way to the end.

TC is in perfect sync between the DNx and the ProRes.

The visuals is in perfect sync between the DNx and the H.264.

Very interesting issue...

Frederic Lumiere
Producer
Lumiere Media


Return to posts index

Michael Phillips
Re: DNxHD transcode slips audio out of sync
on Dec 30, 2011 at 5:24:48 pm

I would forgo the interim step to ProRes and link directly to the original H.264 files and transcode to DNxHD from there. It seems that there is a frame rate mismatch in the Streamclip process and frames are being added to maintain an overall duration (driven by the audio). When it slips out of sync, is it a slow drift like 1 frame very ~40 seconds? If so, then it could be a .1% issue not being properly handled in the ProRes step.

Michael

Michael Phillips


Return to posts index

Frederic Lumiere
Re: DNxHD transcode slips audio out of sync
on Dec 30, 2011 at 5:49:25 pm

Hi Michael, thanks for the response!

Actually, there isn't a ProRes interim step. I decided to forgo with ProRes and reconvert everything to DNxHD.

The only reason I mentioned ProRes is because Prores had transcoded with perfect sync where DNxHD didn't. Interestingly enough, ProRes maintained perfect sync by adding a white frame on each scene change.

The weird thing is that DnxHD matches the TRT and the visuals of the H.264 perfectly (unlike ProRes) but is out of sync with the audio while the H.264 has perfect audio sync.

So in summary, the ProRes transcoding maintained sync where the DnxHD didn't. Both from the AMA linked H.264.

Frederic Lumiere
Producer
Lumiere Media


Return to posts index


Michael Phillips
Re: DNxHD transcode slips audio out of sync
on Dec 30, 2011 at 6:21:11 pm

Ah... sorry - didn't make that connection. What is the frame rate of the original H.264? Avid Mix & Match will always maintain original duration as there is no override preference for a frame for frame interpretation. So if you are AMA'ing into project that thinks is a different frame rate, it will get adjusted. When you AMA to the clip and edit into the timeline, is there a green dot on the clip? If so, it thinks it has a different rate between source file and project type.

Michael

Michael Phillips


Return to posts index

Frederic Lumiere
Re: DNxHD transcode slips audio out of sync
on Dec 30, 2011 at 7:04:02 pm

No green dot but here's what's interesting: Although the H.264 clip will play in perfect sync in Quicktime and although MC6 will report it's 23.98 clip in the bin, it doesn't play in sync in MC.

So I did another very interesting test:

1. I trimmed a few seconds out from quicktime toward the end of the 3 hour clip and saved it as a new self contained QT.

2. Plays fine, in sync in QT

3. Brought it in MC with AMA - the H264 clip doesn't play in sync in MC.

4. Transcoded it to Prores: The ProRes clip plays in perfect sync.

5. Transcoded it to DNxHD: The DnxHD also plays in sync.

So it's only where there are scene breaks that the slippage starts.

Frederic Lumiere
Producer
Lumiere Media


Return to posts index

Michael Phillips
Re: DNxHD transcode slips audio out of sync
on Dec 31, 2011 at 3:04:28 pm

Perhaps the long clips are getting offsets introduced during the process. As you said, the original H.264 files aren't that long due to the limitations of the card and the camera. Have you tried AMA'ing to those, transcode, and create you long source reel within Media Composer?

Michael

Michael Phillips


Return to posts index


Bouke Vahl
Re: DNxHD transcode slips audio out of sync
on Jan 1, 2012 at 2:22:18 pm

I can imagine the start / end of the H264 messing things up.
But why make it so hard on yourself and create one long clip? Why not import all the clips as they are, and then transcode?
You mentioned timecode, but what good is that timecode if you have one 3 hour long clip?
If you need Rec run style timecode, you could use QTchange to put it onto the originals, but most people would use the freerun style (as it gives more meaningfull info besides being able to identify each frame, you then also know the time a shot was taken, makes things easier normally.)

And, you can import a few random clips to check instead of having to wait half a day before you know you got something unusable...

Bouke

http://www.videotoolshed.com/
smart tools for video pros


Return to posts index

Frederic Lumiere
Re: DNxHD transcode slips audio out of sync
on Jan 2, 2012 at 2:06:23 am

The reason I combine them is because it is 1 interview subject and it's essential to get one long clip vs. dozens of clips for the transcription service. It also makes it a lot faster to find what you are looking for (only 1 clip to pull up to find the byte you need).

Finally, it's also a lot easier for the writer who doesn't have to write a new filename every time he pulls or combines a byte which is done frequently when helping people sound 'a lot more intelligent'.

Frederic Lumiere
Producer
Lumiere Media


Return to posts index

Frederic Lumiere
Re: DNxHD transcode slips audio out of sync
on Jan 2, 2012 at 2:09:04 am

[Bouke Vahl] "You mentioned timecode, but what good is that timecode if you have one 3 hour long clip?"

That's when I find timecode the most useful!

Frederic Lumiere
Producer
Lumiere Media


Return to posts index


Frederic Lumiere
Re: DNxHD transcode slips audio out of sync
on Jan 2, 2012 at 2:07:49 am

That's a great suggestion for the future. Basically AMA to all the takes, drop them in a sequence and output to DNxHD?

Frederic Lumiere
Producer
Lumiere Media


Return to posts index

Bouke Vahl
Re: DNxHD transcode slips audio out of sync
on Jan 2, 2012 at 3:21:03 pm

That works, but do not output a long DNxHD file, just consolidate/transcode the long timeline.
Then export that timeline to a file for the transcription service,
and use that timeline as a source when you start editing the interview.

Reason: I personally like to work 'reproducable'.
Meaning, keep links to the originals in a way you can reconstruct your work in case your media dies. (Yes, this WILL happen on one occasion!)
Backing up the Avid project and the original H264 is easy, this way you can always step back / redo without a lot of manual work.

For the transcription, i happen to have written transcription software that does just like you need, without the need for a long file.
It has a function to create one long clip from several short clips, while remembering the timecodes / filenames from the originals.
(it makes a long QT ref, and a list with timecodes / reelnames / filenames)
The software can be set up to paste timecode and or filename in a custom format in the transcription document.

That makes for a transcript that matches back to the originals without any effort for the transcriber.

have a look here:
http://www.videotoolshed.com/product/27/transcriber-2-dvd

(but the option mentioned above obviously also works.)

Bouke

http://www.videotoolshed.com/
smart tools for video pros


Return to posts index

Frederic Lumiere
Re: DNxHD transcode slips audio out of sync
on Jan 2, 2012 at 3:51:23 pm

[Bouke Vahl] "That works, but do not output a long DNxHD file, just consolidate/transcode the long timeline.
Then export that timeline to a file for the transcription service,
and use that timeline as a source when you start editing the interview."


I see, so basically use the timeline's TC for transcription (not the individual files) and use the timeline to edit the interview, not a large DNxHD file?

Editing timeline is new to me since I'm a new AVID convert but it is a lot more feasible in AVID than it was in FCP. I just wish you could individually set a timeline to automatically open in the source when double clicking on it...

If only Nexidia would develop good automated transcription software, I would save thousands of dollars per production! I would probably pay up to $5,000 for such software... based on the track record of Phrase Find, I think the results would be good enough.

Frederic Lumiere
Producer
Lumiere Media


Return to posts index


Frederic Lumiere
Re: DNxHD transcode slips audio out of sync
on Jan 2, 2012 at 4:50:09 pm

[Bouke Vahl] "...use that timeline as a source when you start editing the interview."

Do you know if phrase find will find hits in the sequence and not only in the individual clips? This could prove problematic.

Frederic Lumiere
Producer
Lumiere Media


Return to posts index

Frederic Lumiere
Re: DNxHD transcode slips audio out of sync
on Mar 1, 2012 at 4:20:23 pm

Hey Michael,

Finally back in the edit (long shoot) and ready to tackle this issue again. So I'm taking your advice and I have transcoded the smaller AMA'd H.264 individually. Have them all lined up in a sequence, now what is the best approach to creating a long source reel from there? I know I can make a subclip from the sequence but would prefer to make a source file if possible (in FCP I would simply export as movie).

Thanks!

Frederic

Frederic Lumiere
Producer
Lumiere Media


Return to posts index

<< PREVIOUS   •   VIEW ALL   •   PRINT   •   NEXT >>
© 2017 CreativeCOW.net All Rights Reserved
[TOP]