Audio duration change after transcoding from MP3 to MP2 or AC3

[darklock]

Hi all,

I have the following problem with one AVI file in Xvid format, the audio duration of the original is 1:25:05 and after transcoding to any other format it changes to 1:24:54 whatever bitrate or samplerate i use in my conversion software, i have used like 5 differents software to transcode and all ends with the same results. Here are the details from the AVI file.
* * * Media Inspector 0.7.443 file analysis report.
* * * Media Inspector is ©2011 by Diego Massanti - http://mediainfo.massanti.com
* * * MediaInfoLib by Jerome Martinez - http://mediainfo.sourceforge.net
Created on: 2012-02-23 13:56:10 HNE
Report for file: Phantasm 1.avi

General / Container Stream #1
Total Video Streams for this File.................1
Total Audio Streams for this File.................1
Video Codecs Used.................................XviD
Audio Codecs Used.................................MPEG-1/2 L3
File Format.......................................AVI
Play Time.........................................1h 25mn
Total File Size...................................697 MiB
Total Stream BitRate..............................1 144 Kbps
Encoded with......................................VirtualD ubMod 1.5.10.2 Fr | www.trad-fr.com || (build 2540/release)
Encoding Library..................................VirtualDu bMod build 2540/release
Video Stream #1
Codec (Human Name)................................MPEG-4 Visual
Codec (FourCC)....................................XVID
Codec Profile.....................................Simple @L3
Frame Width.......................................576 pixels
Frame Height......................................320 pixels
Frame Rate........................................25.000 fps
Total Frames......................................127639
Display Aspect Ratio..............................16:9
Scan Type.........................................Progr essive
Color Space.......................................YUV
Codec Settings (Packet BitStream).................No
QF (like Gordian Knot)............................0.218
Codec Settings (BVOP).............................No
Codec Settings (QPel).............................No
Codec Settings (GMC)..............................No warppoints
Codec Settings (Matrix)...........................Default (H.263)
Video Stream Length...............................1h 25mn 5s 560ms
Video Stream BitRate..............................1 007 Kbps
Bit Depth.........................................8 bits
Video Stream Size.................................613 MiB (88%)
Video Encoder.....................................XviD00 47
Audio Stream #1
Codec............................................. MPEG Audio
Codec (FourCC)....................................55
Audio Stream Length...............................1h 25mn 5s 544ms
Audio Stream BitRate..............................128 Kbps
Number of Audio Channels..........................2
Sampling Rate.....................................48.0 KHz
Audio Stream Size.................................77.9 MiB (11%)

I have used MPEG2 Works 4 and a few other to transcode audio under OSX and even windows 7 or windows XP
and got no success .

is there something wrong with my method, i use virtualdubmod to demux my audio fro the AVI file, then try to process audio separately, which is the best i think, because i did it years ago.

i have to say that my file framerate is 25 fps, the video that is. can it change something on the way i process it ?

calculating the final lenght of the audio after conversion it corresponds to a framerate of 25.055 fps. what can cause this drift in the conversion process ?

thanks in advance, and yes i did googled it for a long time before asking here.

[manono]

I think MediaInfo is probably wrong about the length of the audio. Any audio I've ever muxed with XviD AVIs, if the audio is longer it gets truncated to the length of the video. After extracting the audio as MP3, does it still show that length when played in a player?

Are you sure the audio isn't just shorter than the video to begin with? Have you tried muxing your MP2 or AC3 audio with the video to see if it loses synch? And if everything really is as you describe, that's what you get for downloading garbage from the internet (no B-Frames?).

[darklock]

I guess you are right about media inspector, i opened the mp3 file in audacity and it report a lenght of 1:24:54, not 1:25:05. i will try to mux the files together to see if i have any audio sync drift.

could you explain me what does B frames have to do with audio sync, i know that B frames are bi-directional predicted frames but the relation with audio for me is unknown.

Thanks for the help

[manono]

They have nothing to do with the audio, but since they're generally considered an important aid to compression, leaving them out entirely, either purposely or unintentionally, makes me wonder about the skill of the original encoder.