How to separate AC3 channels?

[JensG.]

On some DVDs the sound is dual mono with 192 kbps. As I understand this, it means that the sound consists of two identical mono streams of each 96 kbps. How do I separate these channels from each other, so that I get a 96 kbps mono file?

Background: I want to convert the movie to divx. To add the 96 kbps original AC3 makes more sense to me than to convert the sound to less quality mp3 with hardly lower bitrate.

[JensG.]

So I guess there is no way to extract single channels because of the ac3 file format?

:(

[killingspree]

there definitely is... i am just on the path of figuring it out... if you take the time and wait for another hour or two, or maybe a day, i'll post the results!

steVe

edit: sorry in the end i discovered that i wanted something else than you did. i just discovered that my 'suposedly' dual mono ac3 was a single mono ac3 and i wanted to convert it to mp3 anyway...

anyway, i guess the way to go for you is to demux the ac3 into two seperate wav streams and then reencode using ac3enc and mapping your single mono to your center speaker...

[harrya]

I use Sonic Foundry's Soft Encode... It is no longer supported and is not being released, but use your imagination of how to get it. :)

[JensG.]

Originally posted by killingspree
anyway, i guess the way to go for you is to demux the ac3 into two seperate wav streams and then reencode using ac3enc and mapping your single mono to your center speaker... Yes, but I guess there will be further quality degradation because of the compression. So it probably won't be better than mp3.
Anyway, thanks for your comments, steve and harrya.

[S_O]

Don´t reencode to ac3! If you have to reencode not to ac3. You´ll lose even more quality, because at low bitrates (96kbps) mp3 gives better results, also for mono files.

For the real seperation of the ac3, I don´t think that it is that simple, just take the frame, divide the frame and rewrite the headers.
Isn´t here anybody who has read ATSC A/52 (completly, not only the header specification) and could tell us how in dual channel a frame is divided for the two channels and if it is possible to separate both?? A tool doing that would be really useful.
I also need it for mp2. I have some stereo mp2, where one channel is only silence. I like to remove one channel and save the file at the same bitrate, just in mono.

[jaffree]

you can separate ac3 channels with besweet using besweet gui (see downloads section of doom9) select "six-waves" in output.

You will get six waves of same size, if your audio was 2 channels dolby digital, you will find XXXX-FL.WAV and XXXX-FR.WAV having sound from your original ac3 other files are useless. And if your source is 6 chennel ac3 then all six files will have sounds of each channel.

[S_O]

But the files are wav and decoded. What we need is an separator that simply splits the ac3-frames without decoding, add a new header to both new frames and write that into two ac3-files, so that you don´t lose any quality and could probably merge both files toghether again into one ac3 which is bit-identical to the original one.
I haven´t seen a program doing that yet.

[jaffree]

In that case you may need Sonic Foundry SoftEncode. Load your ac3 file (ac3=decode to pcm) select your required channel to be encoded as center channel set encoding mode to 1/0 C 192kpbs and encode, you'll get single channel ac3 @ 192k.

I don't think anyone can simply split a channel from an ac3 file, without involving decoding/encoding process.

[S_O]

"(ac3=decode to pcm)"
Exactly that is what we don´t want.
Theortically it can be possible, because thw two channels in dual channel file (1+1 ch) is not coupled and both channels have exactly the same space in the farme, so what you need is, to extract the part of the first channel in the frame, extract te second part of the other channel and then add to both parts a new frame header and then save each of these new frames to new files. That´s it.

[SeeMoreDigital]

Yes, it's a great shame there are no 'easy to use' applications out there that will generate 6 separate mono .wav streams from a multi channel .ac3 stream.

It would be great if an application like VOBedit could do it. Or better still, DVDdecrypter!

Here's hoping.

[JensG.]

Originally posted by SeeMoreDigital
Yes, it's a great shame there are no 'easy to use' applications out there that will generate 6 separate mono .wav streams from a multi channel .ac3 stream.
Well, at least it is possible. Whereas my desire to separate AC3 channels without reencoding seems impossible.
Pity.

[SeeMoreDigital]

JensG.

Have you given 'HeadAC3he' a go?

It's not the simplest application but may do what you want.

[S_O]

HeadAC3he can output multichannel wav and you can select the input/output channels, but lossless separation of two mono channels in a dual channel ac3 is not possible.

[SeeMoreDigital]

S_O

I have used HeadAC3he to extract the 6 separate channels of an .ac3 file to individual mono wavs (so I could create a 5.1 wma stream). But I had to encode one track at a time.

However, I have not tried to see if you can extract one .ac3 track and keep it as an .ac3 track.

But I do know that you can 'highlight' and extract single .ac3 tracks within a multi channel .ac3 file using SoftEncode.

[S_O]

I don´t want to create any wav files. Also no temporary.
It is not possible, neither with HeadAC3he, nor with SoftEncode (soft encode decodes the ac3 to temporary wav and encode it again then).

[SeeMoreDigital]

Actually, if you use SoftEncode there is a way you can separate and extract individual .ac3 tracks. So all your left with are the separate .ac3 tracks.

It's a long winded process.

First you have to load the entire file into SoftEncode. Then you have to highlight and delete the tracks you dont want and leave just 'one' of the tracks you do want.

Then you can extract that track to a single 'mono' .ac3 file.

I stopped using it in the end because it took me such a long time to delete, extract and convert each mono .ac3 track into a .wav file. But as it seems you only require mono .ac3 tracks..... it might be ok for you!

[SeeMoreDigital]

But, like you say it would be much better if there was a more direct way. I've been waiting a long time for an application to do this!

Cheers

[SeeMoreDigital]

Hello again JensG.

I had another look at SoftEncode today.

And it would appear that when you input an .ac3 file (or any othe audio file for that matter) it converts it to a temporary .wav file first!

What a pointless waste of time this is, even if the .wav files are high quality spec ones.

Even so, you are able to save the output as a single channel .ac3 file.

Good luck in your search and please let me know if you are successful

[JensG.]

Originally posted by SeeMoreDigital
Good luck in your search and please let me know if you are successful Thank you! Let's see if this thread still exists when a solution is found :-) Although I guess that with the compressed AC3 format it might be simply impossible to separate channels without decompressing.