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Audio sync issues with MP4, wonder if audio bitrates are to blame?

434 Views 8 Replies Latest reply: Feb 8, 2013 9:52 AM by Tom Wolsky RSS
Bronwyn78 Calculating status...
Currently Being Moderated
Feb 8, 2013 8:14 AM

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)
  • davidsignal Level 3 Level 3 (525 points)

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

  • Tom Wolsky Level 10 Level 10 (104,685 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.

  • Tom Wolsky Level 10 Level 10 (104,685 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.

  • Tom Wolsky Level 10 Level 10 (104,685 points)

    The bit rate won't effect the sync.  

     

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

  • Tom Wolsky Level 10 Level 10 (104,685 points)

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

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