DNxHD transcode slips audio out of sync

[Michael Phillips]

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

[Frederic Lumiere]

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

[Michael Phillips]

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

[Frederic Lumiere]

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

[Michael Phillips]

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

[Bouke Vahl]

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

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

[Frederic Lumiere]

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

[Frederic Lumiere]

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

[Frederic Lumiere]

[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

[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.

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:
https://www.videotoolshed.com/product/27/transcriber-2-dvd

(but the option mentioned above obviously also works.)

Bouke

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