Quantcast Encoding is adding duplicate frames - digitalFAQ Forum
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09-01-2010, 05:22 PM
bbartley9 bbartley9 is offline
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Need some help understanding a video conversion issue I am seeing. I'm taking a captured AVI file (HuffYUV encoded, 640x480, 29.970 FR, audio is PCM 48kHz) from PC to Mac, then using Apple Compressor to convert to an editable format for FCP. I choose Apple's ProRes422 HQ, and I see a difference in the encoded output when I use the setting "Frame Rate = 29.97" versus using "Frame Rate = Same As Source". When I use 29.97, I have noticed that some duplicate frames are added to the video (perhaps substituted, not sure yet). This creates some slight jittery spots in some places. In this case, the audio is in sync with the output video.

When encoded with "Same as Source" frame rate choice, I don't see the duplicate frames anymore, but I also notice in that case that the output has about 49 frames less (out of 200K or so) than the original AVI. Also, the output audio is out of sync with the output video in that case, even when I set Compressor to do "pass through" audio (...and I get the same result when audio is set to PCM in compressor).

I am wondering if Compressor is seeing bad frames and dropping them (and substituting a duplicate when I use the 29.97 frame rate setting to maintain the rate?). Is there any way to tell which case it is? Also, what's a good way to make sure the audio is in sync with the video, if I choose to stick with the video output that doesn't add duplicates?

Maybe I should be figuring out if it's bad frames (and fixing?) first? I am not aware of any tools that do that yet.
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  #2  
09-01-2010, 05:52 PM
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And you have a full TBC in the capture chain, correct?

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  #3  
09-01-2010, 06:42 PM
bbartley9 bbartley9 is offline
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Correct, in general (AVT 8710).... I am trying to remember if this particular capture had it in the path, though ... maybe not. The frame that seems to be missing looks OK to me in VLC with the AVI (unless VLC is just not showing the frame I was comparing, around the spot where there is a duplicate).

Last edited by bbartley9; 09-01-2010 at 06:42 PM. Reason: add detail
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09-01-2010, 06:55 PM
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Maybe there's an issue with FCP or the HuffYUV Mac codec?

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09-01-2010, 08:18 PM
bbartley9 bbartley9 is offline
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Could be the HuffYUV, which is done through the Perian package on Mac. FCP is out of the picture, I think, as I see this right after the first encode to ProRes422 using Compressor (before importing to FCP). I'm going to play around some more and see if I can narrow down things to specific frames that seem bad or are getting substituted out.

Can you recommend and PC software that's good at analyzing AVI's and maybe correcting them? I do have GSpot, but will have to run it again to see if it reports any errors.
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09-01-2010, 10:37 PM
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Something just does not sound like it's being decoded properly. And this is something I've seen and read about before. Not with Mac or Huffyuv specifically, but just codecs in general. Although it's often with MP4's more than others.

I'm not seeing anything obvious.

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09-01-2010, 11:05 PM
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Yeah, I was initially wondering if the Apple ProRes codec is making some occasional mathematical errors in the process of trying to enforce a 29.97 frame rate (by adding duplicates), which might explain why it doesn't happen when i set the frame rate to match the source with the other compression settings staying thr same. That assumes the latter mode would shut off any frame stuffing or subtracting algorithms. I don't know, buy I'll have to work around it in this case.
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