Hi.

I've been reading about AC3 lately and I'm not clear on something: does an AC3 file contain six discrete channels of highly compressed information that are bundled together in one file and managed by metadata?

Or is it a single stream of compressed audio data that gets decoded and properly placed in the correct channels via use of frequency bands, etc?

Thank you.

AC-3 allows for a lot of things. It can have up to 5 full bandwidth channels plus 1 limited bandwidth (afaik up to 120 Hz) channel for extra bass effects. That’s 5.1 discrete channels.

Yes, AC-3 (and most other surround codecs) use discrete channels.

If you are interested in a surround codec that has "psychoacoustically-discrete" channels, take a look at MPEG Surround

But of course, AC-3 is not always 5.1 discrete channels. I've made a lot of DVDs from broadcast TV (analog TV), and from video tapes. I use Dolby AC3 encoding and these contain 2.0 AC-3, with the optional "Dolby Surround" (Matrixed surround).

Most commercial DVDs will contain both a 2.0 AC-3 track and a 5.1 AC-3 track.

Ok, thanks!!


Now, another question: is there a way to extract these discrete channels from an AC3 file such that one could have six mono channels? Example:

LEFT.wav
CENTER.wav
RIGHT.wav
LEFT_SURROUND.wav
RIGHT_SURROUND.wav
LFE.wav


I see a number of programs that will encode six .WAV's to an AC3 or will convert an AC3 to a stereo .WAV, etc....but none that will, in effect, "unravel" the AC3 into it's respective components.





Ah, I understand. That explains the audio options on a DVD menu (stereo, 5.1, etc).

Just out of curiosity, DVDdoug, what encoder do you use? Is it part of a DVD authoring package? I'm trying to find a standalone PC application that will do AC3 encoding for some professional products my company is producing. There's countless shareware ones, but I'd like to find something with the Dolby stamp on it to ensure it will play in most DVD players, etc...

Yeah, I can only do it with with my DVD authoring software. I don't have any way to make a stand-alone AC-3 file. I use Ulead/Corel Video Studio and DVD Workshop. Both of these generate the complete audio/video VOB files for a DVD.

But... You know.... Now that I think about it, I've never actually tried to make a stand-alone AC-3 file. Video Studio does allow you to split the audio onto a WAV or MP3 file. I'll take another look at the audio format options this weekend.

Surcode makes a licensed stand-alone Dolby AC-3 encoder ($600 to $1000 USD). I don't know why it's so dang expensive! You can get DVD authoring programs with licensed AC-3 encoders for around $100.

I think the Surcode software can do that. And, I think here are unlicensed freeware/hacker tools that can do it too. (People seem to do a lot with BeSeweet, but I've never used it.)

"Most" might be an overstatement. I haven’t seen a DVD with a 2.0 track in addition to an identical 5.1 track in a long time.


BeSweet works great.

This procudes six 16bit mono waves. Use -6chfloat instead of -6ch for 32bit floating point waves. BeSweet automatically attaches the channel name and .wav extension to the given output filename.
Full reference for BeSweet can be found here: http://brother-john.net/besweet-reference.html

To obtain the full original channels don't use -azid() or -ota() parameters, just:


The use of -azid( -n 1 ) means apply the Dialog Normalization (attenuate by a fix amount indicated in the ac3 header), and is useless if after you request amplify the signal until 95% (the ota -norm 0.95)

One AC3 can have more than one DNR level. Think of commercials in DVB captures. It’s unlikely to happen for DVDs but the switch doesn’t hurt either. Btw: Azid is always active for AC3 decoding. If -azid() is missing default values apply.

One more reason to don't apply DNR. You can't trust the commercials have the correct DNR, always have high volume values and after the max peak to normalize is always in commercials and the movie part remain attenuate.

I think is better preserve the original volume.


Yes, this is my recommendation, use the default values: don't apply DRC and DNR.
These parameters are useful in play time but not for edit or transcode purpose.

Hi! ...just wanted to mention that AC-3 also uses a technique that's sort of an extension of intensity stereo for multiple channels where some upper frequency band of some selected channels are jointly coded (as one coupling channel + coupling coordinates. coupling coordinates = intensities and polarities). If done well this should increase the perceived-quality-per-bit ratio. So... it's not exactly discrete in this case... but can be very close to in terms of psychoacoustice I guess, assuming a sane encoder.

Cheers!
SG