Add 2 sec more to an AC3 track

Hi, I have Windows 8.1 in my 1TB HDD, 8GB of RAM, i7 4th gen. and Audacity 2.0.5. I’m trying to synchronize an AC3 audio track with 6 channels (Dolby 5.1).

First, the length of the audio track is 4h :mrgreen: . So it takes a HUGE lot to load in Audacity, and running my computer with a HDD doesn’t help much, xD.

Well, all I have to do is add 2 seconds more of silence in the middle of the track, and then it’s synced. AFTER doing that, I select “export…”, selected AC3 using FFMPEG and OK. After start exporting, Audacity asks me to put each track in the channels I want them to be. By default Audacity selected 1 channel for each track. I didn’t move anything cause everything seemed right, and clicked OK.

Took 20 min, lol. Then, I listen to it, and :astonished: I only hear voices in the left headphone. Seems like if the default selection of the channels were something like ALL the voices tracks to the left speakers, rest of surround and subwoofer, to the right. So here go the questions:

  1. When selecting in which of the 6 channels I wanna hear each track, how can I know if the channel I selected is the subwoofer center, left speaker, right speaker, centre speaker, back left speaker or righ back speaker? Because I was so unlucky that Audacity put all the 3 audio tracks in the left side.

  2. Is there any way I can leave the AC3 track in its original quality? Just add the 2 seconds and save it as it is, without compression. I’m saying this because my exported AC3 audio track is 300MiB and the original unsynced track is 700MiB, and I notice that the quality of my exported AC3 was worse.

Dolby AC3 is a compressed format. If you take it apart into “real” audio it gets huge (as you found), but more important, when you make a new AC3, you get double the compression damage because you’re compressing the music and the original compression damage.

If you had access to the music before the first AC3 compression, that wouldn’t happen. Exactly the same thing happens with MP3.

“How come my new MP3 sounds honky, tinkly and bubbling?”

That’s what double compression damage sounds like.

In the case of MP3, there are editors that can do simple editing without taking apart the original MP3. You may be able to find an editor for AC3 which works like that.


Thank you. What about question 1?

As I understand it there is no defined standard for which channels are which in AC3 (whereas it is defined in WAV, AAC and DTS).

AC3 5.1 is “often” FL , FC , FR , SL , SR , LFE but you cannot rely on it.

Is there any way you can listen carefully to some sections of the AC3 through your surround system and work out which channel is which? If you can mute some channels that would help you identify them.

Here are some possible AC3 lossless editors: .


Hi, thanks for the reply, I’ve solved IT!

I’ve tried to do it again, and this is how I did:

I have a 4 hours film in DUAL language: french and english (AC3 5.1 tracks, which means they have 6 channels). The french track is well, but the english track, in the middle of the film, gets out of sync.

So I first extracted them, and then added them to Audacity. It took a while cause is in total +8h of audio importation. Then, I synced with Audacity the english track, comparing it to the french one. It just needed to add 2 secs of silence, and then it was synced. Once this was done, I deleted the french track, and I had a sync fixed 6-channel audio track in Audacity. Then I had to export it. So I selected export…, AC3 file, AND THEN, OPTIONS…

This is what I did not did last time. Here you can select the bit rate, and by default, it’s too low: 112kbps. So I selected 384kbps, as the original NOT FIXED english track. Then, Audacity asked me to select in WHICH channel I’d like to put each track. I left it as it was (ATTENTION: if you move up/down a track working with more than 1 channel audio tracks, you may be change the assignation of each track, and, AS ME, you can get something like ALL the voice tracks sounding in the left speakers). So leave it as it is, do not move up/down tracks (if you do, after export, make sure you sort them to their original position). Well, after 25 min of exporting process, the new track is done. Its size is almost the SAME as the original english not fixed track (bytes of difference). And that’s it. May be I’ve REcompressed the audio, but I don’t notice any difference between the tracks.

Here goes a request:

When Audacity asks you to select in which of the 6 channel you want your tracks to be, it MUST show you to which speaker are you adding the track. Now you only read “Channel 1”, “Channel 2” and so. It should be “Channel 1: left speaker”, “Channel 2: center speaker”, “Channel 4: Back left surround speaker”…


When you exported, the Advanced Mixing Options window would have shown you if the tracks were not ordered 1 to 6.

Audacity doesn’t do what you ask now, but could only do so on the assumption there is a standard mapping for the format or the information is in the file somewhere. AC3 files (as I understand it) may have this information in the file but this assumes the application writing the file has provided it. Audacity doesn’t write or understand such information.

It’s unlikely this will be improved until Audacity supports multi-channel playback.


Oh, thanks. Well, I think there are some do more important than support Dolby 5.1 playback, such pitch changer in real time without processing the audio data, or tempo changer without change pitch in real time, no data modified. Nevertheless, support Dolby 5.1 would be an awesome feature.


Well that’s one you didn’t ask for before, so I added your vote.


Thanks Gale, God bless you! ^^