[FFmpeg] Unsynched audio in MP4

[Klagar]

Hi all !

I've got this mp4 from which I need to extract a 2 minutes sample, while keeping the exact same quality. I use FFmpeg with a very simple command line :
-acodec copy -vcodec copy -t 120 -ss XX:XX:XX

Problem is, it yields an unsynched audio. It seems to be doing that exclusively with mp4's ; I use the same line with m2t's, avi's, wmv's, no problem.

Here's what MediaInfo tells me about the source :

General
Format                           : MPEG-4
Format profile                   : Base Media
Codec ID                         : isom
File size                        : 478 MiB
Duration                         : 16mn 4s
Overall bit rate                 : 4 155 Kbps
Writing application              : Lavf52.32.0

Video
ID                               : 1
Format                           : AVC
Format/Info                      : Advanced Video Codec
Format profile                   : Main@L3.0
Format settings, CABAC           : Yes
Format settings, ReFrames        : 1 frame
Codec ID                         : avc1
Codec ID/Info                    : Advanced Video Coding
Duration                         : 16mn 4s
Bit rate mode                    : Variable
Bit rate                         : 4 042 Kbps
Width                            : 720 pixels
Height                           : 480 pixels
Display aspect ratio             : 3:2
Frame rate mode                  : Constant
Frame rate                       : 29.970 fps
Standard                         : NTSC
Color space                      : YUV
Chroma subsampling               : 4:2:0
Bit depth                        : 8 bits
Scan type                        : Progressive
Bits/(Pixel*Frame)               : 0.390
Stream size                      : 465 MiB (97%)
Language                         : English

Audio
ID                               : 2
Format                           : AAC
Format/Info                      : Advanced Audio Codec
Format profile                   : LC
Codec ID                         : 40
Duration                         : 16mn 4s
Bit rate mode                    : Variable
Bit rate                         : 108 Kbps
Channel(s)                       : 2 channels
Channel positions                : Front: L R
Sampling rate                    : 48.0 KHz
Compression mode                 : Lossy
Stream size                      : 12.4 MiB (3%)
Language                         : English

The source is synched. It looks pretty usual to me, nothing extraordinary like some weird codec or unusual frame rate or whatever. I don't understand whence this unsync might come...

I've tried to use some filters, like -vf "null" or -vf "copy", to no avail.
So if anyone has an idea how to possibly fix it, or at least what exactly could cause this, that would be great !

Thanks a lot for your time !

[shamus252]

I get same issue with multiple encoder's when tring to go to mp4. On the re-encode your tring to do are you keep the same Frame rate? Does audio keep same with it's sync over time or does it get worse.

[Klagar]

Yes, it's the same frame rate, stays 29,97 input and output.
And it does stay the same over time.

[shamus252]

I don't know alot with FFmpeg but if the sync isn't getting worse over time you could try using avidemux and doing a time shift of the audio.

[Klagar]

Sure that would be a solution for this time... I just hope it doesn't happen too often.

Anyways, I found another solution by using MPEG Streamclip. It does exactly what I need, and sometimes faster and better than FFmpeg. I'll see where I can go with that.

Thanks !