AC3 audio conversion

[exekutive]

I have a movie that contains errors in the audio track, and it doesn't matter which decoder I use, it always comes out shorter than the video track & out-of-sync.

Is there a good audio conversion utility I can use to externally convert the audio from WMAV2 to AC3, maybe fixing/filling in the error parts so that it ends up the same length and I can mux it back into the converted video?

[tomlee59]

I use Audacity (free and powerful) a lot for exactly this sort of thing, but I don't know if it supports WMAV2. If it does, it will allow you to stretch, shorten, insert, delete, blend, etc. It's not particularly fast, but it gets the job done with grace.

I'm sure others will offer other (probably better) suggestions.

[exekutive]

I'm still having problems with this, guys.
Audacity doesn't open the movie file, and doesn't convert to ac3. I'm also having a hard time separating the audio stream from the movie and preserving its length.

[tomlee59]

If you were expecting Audacity to operate on the muxed video file, that's one problem. Audacity is an audio processing tool. You must feed it an elementary audio stream for it to work. So, demux your movie first, then feed the audio stream to Audacity.

Next, do whatever it is that you need to do to the audio. Don't worry yet about not being able to write ac3 files. After you're done editing the audio, then write a pcm (e.g., wav or aiff) file.

Finally, let ffmpegx (or other transcoding utility) convert that pcm file into whatever format you ultimately want to end up with.

There are undoubtedly less cumbersome ways of accomplishing what you are trying to do, but this is what I use.

Others will hopefully contribute their (better) suggestions.

[exekutive]

Thanks tomlee59

I understand what you're saying, and I was trying to work along those lines, but without any luck.

I was hoping that Audacity might be smart enough to extract the audio stream itself to work on, but it kept crashing, so I don't know.
Next, using Quicktime Pro, I managed to extract the audio track unchanged into a separate file. So now I have a MOV container with the WMA2 audio with original length. I also tried to export the audio using QTPro into various formats (AIFF etc.), but that process just stays frozen @ 0%. I tried opening this file in Audacity and the progress dialog said it was opening an MP3. The resulting audio was just a few minutes of screeching.
I had a bit more success with VLC. Using the stream/export wizard, I managed to convert the file to various formats such as raw a52(ac3), WAV etc. Great, but the total length decreased meaning VLC didn't handle the errors very well, and then Toast wouldn't accept these files as valid elementary audio streams.
I tried some other things that I can't remember, but I do know I tried ffmpegx for audio conversion and could never get it to work, because I kept getting errors that it couldn't find the video stream.

I dunno, maybe I would be better off working with this movie on a PC where the WMA codec is native.

[tomlee59]

If VLC was able to produce a valid aiff/wav file, then you're at a point where audacity might be able to take over to fix up the duration problems. Your strategy from here on out depends on the nature of the length shortening. Is the duration short because there are intermittent errors that are effectively skipped? If that's the case, are we talking about a huge number of randomly distributed errors, or is it in just a couple of places? Or is there a subtle speed change that is more or less constant throughout? And what is the total duration error?

If there are intermittent errors in just a couple of places, you can fix those with a reasonably low amount of manual ad hoc tweaks (e.g., leaving silences, or repeating short segments). If the speed is simply off, you can use the tempo change feature I mentioned before. But if there are huge numbers of randomly placed errors, then you may be SOL.

[exekutive]

Well judging from the log output when I originally had tried to do a simple reencode with ffmpegx, there were probably a dozen errors all close to each other in one spot, and about a dozen more in another spot.

I'm assuming those segments were simply cut out instead of filled in, resulting in a reduction of overall length. I could be wrong though, because the the sound seemed out of sync throughout the duration of the movie. I will have to investigate further into this. I wish I could use a program that would let me edit the audio and also give a video preview to use as reference.

In any case, I would really like to know how to do the AC3 conversion using ffmpegx as you said, because I was having problems doing that step as well.

[tomlee59]

To convert from one audio format into another, just drop the source file into the "source format" area in the summary window. Then in the pulldown options for the target format, select "audio file to ac3." Head over to the audio tab, and set parameters as you wish.

So, after you fix up the file with Audacity (or some similar thing), you can convert the result to ac3 for final authoring.

[exekutive]

That easy huh :P I didn't even think to look there. Sometimes the best solution is the most obvious one eh Thanks.

[exekutive]

Hi guys. OK somehow I managed to get this movie to work, but I'm not really sure how. I redid it so many time it was just a mess of streams everywhere. Didn't have to do any audio editing though. I think I used the VLC export wizard. VLC handles errors on the fly really well.

Anyways, to get to the point, I happened accross a super program in my travels. It's called avidemux. It's a really powerful tool and the best part is it handles Windows Media files really well.