How to avoid clipping when downmixing 5.1 to Stereo?

aka · Post by **aka** » Sat Jan 24, 2009 7:57 pm

Have extracted the 5.1 files from a DVD-A - (using DVDAExplorer - Stereo DownMix was not supported by this particular DVD-A).
I wish to build a Stereo only (as I don't listen in 5.1) DVD.
When I downmix (default settings) in Audacity the resulting files show rather extreme clipping (whereas the originals did not) making me think I need to reduce the gain on the 5.1 channels before exporting/downmixing.

Is there an option in audacity to do this (eg, avoid clipping, etc) automatically?

Thanks!

Post by **kozikowski** » Sat Jan 24, 2009 10:49 pm

I'm curious how you got the mix. How, for example, did you deal with the vocal track in the middle?

What was the forumla?

NewLeft = LeftFront + 1/2 Center
NewRight = RightFront + 1/2 Center

Delete LR, RR, LFE

That's how I would have mixed it down. YMMV. If you're compulsive about it you can mix some of the LFE back in, but it usually isn't necessary. Never use the surround tracks.

Anyway, based on that formula and starting with the original files, apply Effects > Amplify with fixed amount of reduction:
Center -12dB
LeftFront -6dB
RightFront -6dB

Given that none of the original files clipped, that should give you a correctly balanced stereo version of the show with no clipping. It satisfies the above forumula by simple mixing the center into the left and right with no other changes.

If you picked a different mix-down formula, tell me what it is and I'll tell you the reductions. They're always the same, so you don't have to derive them again unless you change the mix-down formula.

I would totally not push this decision off onto auto tools. There is a significant debate raging on the boards about the damage you can cause with some of the automatic effects.

Now. Having said all that. That's the mixdown if you have a straight performance in the front like a movie. If you have one of those listener sitting in the middle of the band deals, that's not going to work. I gotta mount my Buena Vista DVD-A again and see how they did it.

Koz

aka · Post by **aka** » Sun Jan 25, 2009 5:21 pm

Thanks Koz!
Really appreciate the detailed response.
The 'original' tracks/channels were extracted and encoded to WAV from a DVD-A using Merge Groups option (again, DVDAExplorer - xp/parallels) - channel separation is preserved (like layers in photoshop) and separates into individual channels when opened in Audacity.
I like the looks of the formula - this is precisely what I was hoping for - so thanks again!
Will give it a whirl asap!

Post by **kozikowski** » Sun Jan 25, 2009 5:35 pm

Managing the Center track is a little tricky. After reducing the level with the above formula, you need to make it into a stereo track so it will mix properly with the Front signals.

Koz

aka · Post by **aka** » Sun Jan 25, 2009 6:17 pm

So, Duplicate the center channel and push one R the other L? Or?
I am thinking that this 5.1 mix (Porcupine Tree - Stupid Dream) is most likely a 'you in the midddle of the band thing' - having no way to listen in 5.1 - dunno.
Given each will have its own nuances, any ballpark formula for downmixing such 'immersed' surrounds?

Thanks again, Koz!

aka

Post by **kozikowski** » Sun Jan 25, 2009 7:06 pm

<<<having no way to listen in 5.1 - dunno.>>>

You can listen to the two Rear tracks and listen for obvious echo effects like the microphones were in the back of the hall. Also listen for instruments that are very forward but suppressed in the Front tracks. That's the clue for immersion product.

In a Movie, it's pretty much obvious what they did because the Rear tracks are effects and surround work.

Yes, Audacity will mix all the Left tracks together when you export, for example. So the Center needs to be made into a Left and Right so it will appear in both left and right in the Export.

There is a rule of thumb about this. -6dB is half. -12dB is one quarter. So if you add two identical tracks up they will increase 6dB. Reduce both by half ahead of time (-6dB) and they will add up to about back where they started.

Koz

Post by **kozikowski** » Sun Jan 25, 2009 7:14 pm

By the way, your ears work funny. They don't hear "half volume" until about -18dB. This disconnect between your ears and the electrical signal is one of the reasons you need to double the size of your living room sound system to hear any difference at all. The reverse of that lets you hear somebody yelling at you a half-mile away (in the country). Your ears are still running for sounds that quiet.

Koz

aka · Post by **aka** » Tue Jan 27, 2009 3:29 am

OK - I'm getting it!
Thanks for taking the time to offer up the excellent primer, Koz!
Thankfully most of the DVD-As I've played with have a dedicated Stereo program or at least have the mixdown capability - so it will be rare to have to do this manually - that said, good to know how to attack when the issue arises
Most appreciated!

Cheers!
aka

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How to avoid clipping when downmixing 5.1 to Stereo?

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