Can we compress an AC3 5.1 soundtrack?

[Boathouse]

Please can you tell me if it's possible to compress an AC3 5.1 soundtrack?

With a normal stereo soundtrack I would convert the AC3 to MP3 and then mux the MP3 with Nandub. However I'd now like to include a 5.1 soundtrack with my DivX movie and preserve the surround sound.

I could just mux the AC3 file as it is using Nandub, but it's rather large, so is there a way of compressing it before muxing - does mp3 support 5.1 surround?

Thanks for your advice...

[Rooster6975]

None that I know of, you need to put it on 2 CDs

Your AC3 file contains 5 discreet channels of audio, plus a limited bandwidth Low-Frequency Effects (LFE) Subwoofer channel. All of that information takes more space than a 2-channel MP3 file. The MP3 format does not support the use of 5.1 audio, you have to downmix the audio to 2 channel. Theoretically, if you have the choice, you should choose the Dolby 2.0 stream on the DVD rather than 5.1 when you are creating a DivX with an MP3 audio track. The Dolby 2.0 on the DVD has a better mix for 2-channel MP3 files since it is one already. By taking the 5.1 and downmixing it to 2-channel, you are relying on the encoding software to do the best job it can, while you could be using the 2.0 stream which has already had it done professionally on the DVD.

What you should be doing is making 2-CD DivX when you want to have 5.1 audio. Otherwise it takes up too much space on a 700 MB CD and does not leave enough for top-quality video. I have found that on average, a normal 1h50 movie takes up roughly 70 MBs for 2-channel MP3, but over 300 for 5.1. The difference of 230 MBs can be put directly into the video quality which is why you should rip to 2 CDs. 300 MBs of 1.4 GB still leaves 1.1 GB for excellent quality video.

Cheers,
Rooster

[Vaughn]

"What you should be doing is making 2-CD DivX when you want to have 5.1 audio"

Just how does one go about keeping the 5.1 audio instead of compressing & making a mp3 using gknot?

[Enchanter]

Just how does one go about keeping the 5.1 audio instead of compressing & making a mp3 using gknot?

Use AC3 for your sound stream instead of MP3. DVD2AVI can extract DVD audio as AC3.

[UncasMS]

you can easily mux your ac3 audio with the avi file

take a look here:

http://www.atlandide.net/~uncasms/en/index.php?page=muxing

in your first posting you mentioned keeping SURROUND information.
this of course can be handled by mp3.
it cant, however, store dd5.1!

[Rooster6975]

Follow UncasMS' advice.

I followed his advice and his guide when I made my first AVI with AC3. The sound quality is fantastic, much better than anything I had made before with MP3 (or with OGG for that matter, although you will get a serious argument out of the OGG supporters).

The guide is easy to follow, and you will end up with a great quality DivX with AC3 sound. Pump that through your Dolby Digital decoder into your Home Theatre system, and you will be laughing.

Cheers,
Rooster