Audacity Forum

I have a damaged MKV video. I've de-muxed the .264 and .ac3 files out of the MKV.
The .264 file is 49:16 long and appears to be ok.
After repair using ac3fix.exe the .ac3 file is 49:21
Using headac3he.exe I converted the .ac3 to .wav and opened the .wav inside Audacity.
I'm trying use the Time Warp feature to compress 49:21 into 49:16 (5 second reduction).
If this can be achieved I can convert the .wav back to .ac3 and mux with the .264 to create a cleaned up MKV with video and audio that's in sync.

After Time Warping the data and saving the output file as .wav the play time is unchanged at 49:21.
I can't seem to reduce the play time down to 49:16

What am I doing wrong ?
or maybe Time Warp is the wrong function

I think the TIME WARP just speeds up or slows down the playback rate - it doesn't make the time shorter or longer.
What I need is a data reduction function to strip out samples and make the file 5 seconds shorter.

JungleBoy wrote:I think the TIME WARP just speeds up or slows down the playback rate - it doesn't make the time shorter or longer.
What I need is a data reduction function to strip out samples and make the file 5 seconds shorter.

Time Track would shorten the audio if you dragged the time warp line above the horizontal, then exported the audio, but it is not the easiest way to make a pre-determined length change.

If the demuxed AC3 you converted to WAV is actually at the wrong playback speed, use Effect > Change Speed. You want to apply a percentage change of 2961/2956 seconds =1.001691, so a percentage speed change of 0.1691 (meaning you are speeding the track up by 0.1691 percent). Change Speed only works to three decimal point accuracy, so you will actually get 49 minutes 16.001 seconds, but that won't be a visible desynchronisation.

If the audio file actually always was 5 seconds longer than the video file and was not desynchronised (for example, it was just silent audio at the end) then you don't need to do anything, unless you really want to find out where the excess is and delete it. You might want to simply mux the recovered audio and video together to see if synchonisation is satisfactory.


Gale

The capper re-issed the file and all is ok now.

As an exercise I cut 10 x 0.5 second quiet pieces equi-spaced from the 49:21 .wav file and then played the .264 video file and .wav together. The video and audio stayed within +/-0.5sec sync which was better than the 5 second drift but 0.5sec is still very noticeable.

Thanks for the reply.