8 Replies Latest reply: Feb 8, 2013 9:52 AM by Tom Wolsky
Bronwyn78 Level 1 Level 1 (0 points)

I posted this as part of another question, but didn't get an answer and I'm befuddled by this issue ...

 

I deal a lot with video work that comes from various source files, think montages, and my work is talking head type work so the audio track synching with the video track is of HIGH importance! When I convert my .mov to .mp4 I have found that the audio comes slightly unsynched from the video but usually only in a couple of frames, so this caused me to further dig in the footage I'm using to figure out why some of the frames stay in synch and some fall out. So here's my best example, I have a 1:10:15 clip with approximately 9 different people speaking in it -- 3 of the segments fall completely out of synch while the remaining 6 seem to stay on track. I have examined the clips to see what makes them different in comparison to each other ... here's what I have found:

 

Segment 1 (out of sync audio)

Audio View

Audio Output Channels: 2

Audio Sample Rate: 48kHz

Audio Configuration: 1 Stereo

Audio Bit Rate: 162

 

General View

Frame Size: 710x400

Video Frame Rate: 23.98 fps

Codecs: AAC, H.264

 

 

Segment 2 (in sync audio)

Audio View

Audio Output Channels: 2

Audio Sample Rate: 48kHz

Audio Configuration: 1 Stereo

Audio Bit Rate: 151

 

General View

Frame Size: 710x400

Video Frame Rate: 29.97 fps

Codecs: AAC, H.264

 

 

Segment 3 (out of sync audio)

Audio View

Audio Output Channels: 2

Audio Sample Rate: 48kHz

Audio Configuration: 1 Stereo

Audio Bit Rate: 161

 

General View

Frame Size: 710x400

Video Frame Rate: 29.97 fps

Codecs: AAC, H.264

 

 

Segment 4 (in sync audio)

Audio View

Audio Output Channels: 1

Audio Sample Rate: 44.1kHz

Audio Configuration: 1 Mono

Audio Bit Rate: 97

 

General View

Frame Size: 600x338

Video Frame Rate: 23.98 fps

Codecs: AAC, H.264

 

 

Segment 5 (out of sync audio)

Audio View

Audio Output Channels: 2

Audio Sample Rate: 48kHz

Audio Configuration: 1 Stereo

Audio Bit Rate: 161

 

General View

Frame Size: 710x400

Video Frame Rate: 23.98 fps

Codecs: AAC, H.264

 

 

Segment 6 (in sync audio)

Audio View

Audio Output Channels: 2

Audio Sample Rate: 48kHz

Audio Configuration: 1 Stereo

Audio Bit Rate: 141

 

General View

Frame Size: 460x258

Video Frame Rate: 23.98 fps

Codecs: AAC, H.264

 

 

Segment 7 (in sync audio)

Audio View

Audio Output Channels: 1

Audio Sample Rate: 44.1kHz

Audio Configuration: 1 Mono

Audio Bit Rate: 97

 

General View

Frame Size: 600x338

Video Frame Rate: 23.98 fps

Codecs: AAC, H.264

 

 

And so on ... The only thing I've noticed that seems to be a glaring difference is the Audio Bit Rate, when it's higher than 160 the clip is out of synch in mp4 ... with my limited knowledge of the minute details of things like this, I'm not sure if that even makes a difference? Looking over the info I provided is there any other explanation for why segments 1, 3 and 5 are not in synch like the rest? I'm assuming this has something to do with what happens when the file is compressed into a mp4, as the mov files present no synch issues. Any help or understanding is appreciated. And if further information is needed to help, I'm happy to provide it!

 

Bronwyn


Final Cut Pro X, OS X Mountain Lion (10.8.2)
  • 1. Re: Audio sync issues with MP4, wonder if audio bitrates are to blame?
    davidsignal Level 3 Level 3 (525 points)

    try a different application to convert and remux.  (MP4tools)

  • 2. Re: Audio sync issues with MP4, wonder if audio bitrates are to blame?
    Tom Wolsky Level 10 Level 10 (106,230 points)

    Why are you converting to MPEG-4? That's horrible for editing. Don't convert to a format that includes compressed audio like AAC.

     

    The video formats are all completely non-standard. You can't make an FCP project in any of them. The best is to use some standard definition format.

     

    If you're going to convert the media, convert it all to a common QuickTime format, a common I-frame codec, with uncompressed 48k audio.

  • 3. Re: Audio sync issues with MP4, wonder if audio bitrates are to blame?
    Bronwyn78 Level 1 Level 1 (0 points)

    I am using mp4 for a couple of reasons, one is that we use mp4 when posting the videos to our website, which is what we do with 90% of what I create -- and since .mp4 tends to bring the file size down nicely I tend to use it for sharing video in cases like this. A little more background, we used this particular video in a presentation at a national conference, the 300+MB size of the .mov file was far too big to easily share amongst our computers and even more cumbersome to embed in the .ppt that my boss was trying to use.

     

    Is there a better way of keeping this together with a lower file size? The .mov doesn't have the audio synch issues that the .mp4 does. I'm still coming up to speed with working with video, and especially coming to understand formates, etc. -- so to understand your answer a bit better, is your suggestion that I take the original segments and convert them to something more standard and then piece them together? Instead of them being this odd piece-meal montage they are now? What is my best work flow for something like that?

     

    I really do appreciate any advice or information!

     

    Bronwyn

  • 4. Re: Audio sync issues with MP4, wonder if audio bitrates are to blame?
    Tom Wolsky Level 10 Level 10 (106,230 points)

    Doesn't matter what you're delivering. You don't edit in a delivery format. You edit in a format designed for production and then output in your delivery format.

     

    I don't understand why you have such an odd bundle of sizes and frame rates. If this media is all coming from different sources with these weird different sizes and you need to convert them, convert them to QuickTime in a common size, frame rate, codec, and audio spec. Edit your program and export or comrpess to a format that suits your web site or PowerPoint or whatever.

  • 5. Re: Audio sync issues with MP4, wonder if audio bitrates are to blame?
    Bronwyn78 Level 1 Level 1 (0 points)

    I understand your first point Tom, and maybe my issue is fueled by the fact that the montage I created was done using clips of video that already existed for our website, ie they came from various different video shoots we had done and were the products of our final edited clips, and were already .mp4 when I began working with them. I understand the preferred workflow would include using original raw files, but the .mp4s are what is easier for me to get my hands on.

     

    As for my workflow, I put everything as I want in FCP, and then share (export) to master file and generally I then save as H.264, which would accomplish your statement of converting them to a QuickTime common size right? All the individual segments I identified are the raw information of the items existing in the clip I was building, after exporting, I end up with a single file .mov. Then I run the .mov through MPEG Streamclip to get my .mp4 and that's where the audio starts to de-synch. So this begs the question, is it the Audio Bit Rate? Is there anything I can do to fix this problem? It's just very odd to me that 3 out of 9 segments of the clip slip out of synch like that.

     

    Bronwyn

  • 6. Re: Audio sync issues with MP4, wonder if audio bitrates are to blame?
    Tom Wolsky Level 10 Level 10 (106,230 points)

    The bit rate won't effect the sync.  

     

    Is the exported master file in sync before it goes to Streamclip?

  • 7. Re: Audio sync issues with MP4, wonder if audio bitrates are to blame?
    Bronwyn78 Level 1 Level 1 (0 points)

    Yep, the exported master file is fine, then I put it in Streamclip and export to .mp4 and it goes to crap. And I've tried HandBrake to export the final .mp4 and it does the same thing on there.

  • 8. Re: Audio sync issues with MP4, wonder if audio bitrates are to blame?
    Tom Wolsky Level 10 Level 10 (106,230 points)

    Sorry, I can't help. I can't see how that can happen.