Downmix Output --> volume drop - why?

Skaven252 wrote on 10/25/2010, 1:38 AM
Our team works on video material with 5.1 audio, but our art director's and editor's workstations don't have a 5.1 speaker setup, so they keep the Downmix Output button pressed to hear all the audio in stereo.

The problem is, that when the button is pressed, the volume drops. This leads to those people constantly tweaking the volumes up, after which I hear the Vegas project in 5.1 and it's been set way too loud... over and over again.

Why does the Vegas Downmix Output button introduce a volume drop? I have a similar button on my 5.1 amplifier: it switches off the 5.1 audio and downmixes it to stereo. It introduces no volume change like the Vegas Downmix button does.

Why? What am I doing wrong?

Comments

Geoff_Wood wrote on 10/26/2010, 1:42 PM
Because if not attenuated, downmixing 6 channels to 2 would likely cause digital clipping.

Either that or all the tracks are being scanned and the output level calculated (and set) prior to downmixing, to ensure 'overs' do not occur.

geoff
Skaven252 wrote on 10/27/2010, 5:49 AM
Thank you.

But then I wonder, why doesn't my amplifier's similar "downmix to stereo" button introduce a volume drop?

P.S. We just found out that if our producers set their Vegas Preferences: Audio Device to DirectX Surround Mapper (whilst keeping their Windows audio set to Stereo), they can listen to the stereo mixdown on their stereo monitors without having to press the Mixdown button. Problem solved. :)
Geoff_Wood wrote on 10/27/2010, 2:47 PM
Glad the 'problem' is easiy solved by that method.

FWIW your amplifier presumably isn't a digital device where there is an absolute 0dBFS clipping limit, and they rely on commonsense or an internal limiter/indicator to avoid amp clipping.

geoff
ChristoC wrote on 10/27/2010, 4:44 PM
"> P.S. We just found out that if our producers set their Vegas Preferences: Audio Device to DirectX Surround Mapper (whilst keeping their Windows audio set to Stereo), they can listen to the stereo mixdown on their stereo monitors without having to press the Mixdown button. Problem solved. :)"

Well, not really; they are getting a 1:1 mix to Stereo of all channels sent equally to stereo output, but NOT a proper 5.1 Downmix as would be realised in a true 5.1>>Stereo professional downmix, or the downmix occurring in a DVD player; there is every likelyhood if the 5.1 mix is loud that they will experience overload and distortion.

It's anyone's guess what your amplifier is doing!

Vegas does do Downmix correctly, albeit with a seemingly drastic but inevitable change in volume.

If, for example, all 5.1 channels are mixed so they all peaked on some event at 0dB full scale, unless a correct downmix is done, the cumulative addition of all tracks into just 2 will result in gross distortion and overload.

So there are 4 things done to the levels in Vegas downmix to Stereo:
- The Front L/R channels are reduced by approx 7.7dB and added to Stereo Output.
- The Centre Channel is reduced by 11dB and added to Stereo Output.
- The LFE channel is not added to Stereo Output.
- The Rear channels are reduced by 11dB and added to Stereo Output.

You will find if ALL channel are driven to 0dBfs at same time, the Stereo downmix does not exceed 0dBfs.

The reason the Centre channel is reduced by 3.3db more than Front is that it is sent equally to Stereo Output L/R and added acoustically by 2 speakers, and therefore is equal level to a single speaker 3.3dB louder (that is the basis of Equal Power Panning).
The additional 3.3dB reduction to Rear channels is because often too much rear info in stereo mix can sound weird.

Enthusiastic and knowledgeable user can control the downmix to some extent in the Vegas (secret) Internal Preferences:



musicvid10 wrote on 10/28/2010, 7:25 AM
In addition to Christo's answer, which is 100% correct, the nobrainer answer is, Don't set 5.1 levels using a stereo mixdown.