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7.1 to 5.1 fold down
Old 25th September 2014
  #1
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ggegan's Avatar
7.1 to 5.1 fold down

If I fold down 7.1 to 5.1 by summing Lss with Lsr and Rss with Rsr there is a 3dB bump in the surround level whenever audio is bussed 50/50 between a side surround and a rear surround. If I drop the surround level 3dB to compensate then anything bussed discretely to a side surround or rear surround is 3dB too low. Has anyone figured out a way to compensate so the level remains constant regardless of whether the audio is bussed discretely or to a phantom position between the two side surrounds?
Old 25th September 2014
  #2
Hi Gary -Its been awhile but from memory there are two different approaches.

One that sends some of the 7.1 side (Lss, Rss) material to the 5.1 front speakers (L, R) and also to the 5.1 surrounds (Ls, Rs), The 7.1 rears (Lsr, Rsr) are fed only to the 5.1 surrounds (Ls, Rs) -this is the standard Film type 7.1 to 5.1 down mix that you'll likely find in the Euphonic S5.

And another which is the Dolby plus methord where the front speakers must remain the same between the 7.1 and 5.1 and the surrounds are combined the maths for the Ls/Rs I think is

Ls =[(Lss 0dB) + (Lsr -1.2dB) + (Rsr -6dB)]
Rs =[(Rss 0dB) + (Rsr -1.2dB) + (Lsr -6dB)]

Cheers

Bruno
Old 25th September 2014
  #3
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Quote:
Originally Posted by bru95bg View Post
Hi Gary -Its been awhile but from memory there are two different approaches.

One that sends some of the 7.1 side (Lss, Rss) material to the 5.1 front speakers (L, R) and also to the 5.1 surrounds (Ls, Rs), The 7.1 rears (Lsr, Rsr) are fed only to the 5.1 surrounds (Ls, Rs) -this is the standard Film type 7.1 to 5.1 down mix that you'll likely find in the Euphonic S5.

And another which is the Dolby plus methord where the front speakers must remain the same between the 7.1 and 5.1 and the surrounds are combined the maths for the Ls/Rs I think is

Ls =[(Lss 0dB) + (Lsr -1.2dB) + (Rsr -6dB)]
Rs =[(Rss 0dB) + (Rsr -1.2dB) + (Lsr -6dB)]

Cheers

Bruno
Thanks Bruno, I'll play around with those formulas and see what feels best. It seems like I should be able to use Spanner to accomplish either scenario.
Old 25th September 2014
  #4
I tend to sum by side. The reasoning being that when in 7.1, each of the 4 channels is calibrated for 82dBC.
When going back to 5.1, the two channels are each at 82dBC.

So if one sums the two channels, and drops the level by 3dB (like doing a L+R downmix to mono), some information will be played back too low, just like you stated.

I prefer to have the levels a bit high on the SL and SR tracks in 5.1, rather than have some audio information low in the mix. And as 7.1 does make for less muddled surround info, one can easily be brought to putting more stuff in the surrounds.
But that means that the subsequent Stereo LtRt downmix, from 5.1, might be a bit weird in the phase department as it carries more surround info than the average mix.

For my last 7.1 mix, I did the 5.1 version with all the surround channels of my stems on faders on the desk. I adjusted accordingly, but started with just a pure summation.
Old 25th February 2015
  #5
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Quote:
Originally Posted by bru95bg View Post
Hi Gary -Its been awhile but from memory there are two different approaches.

One that sends some of the 7.1 side (Lss, Rss) material to the 5.1 front speakers (L, R) and also to the 5.1 surrounds (Ls, Rs), The 7.1 rears (Lsr, Rsr) are fed only to the 5.1 surrounds (Ls, Rs) -this is the standard Film type 7.1 to 5.1 down mix that you'll likely find in the Euphonic S5.

And another which is the Dolby plus methord where the front speakers must remain the same between the 7.1 and 5.1 and the surrounds are combined the maths for the Ls/Rs I think is

Ls =[(Lss 0dB) + (Lsr -1.2dB) + (Rsr -6dB)]
Rs =[(Rss 0dB) + (Rsr -1.2dB) + (Lsr -6dB)]

Cheers

Bruno
FYI, just got the formula directly from Dolby:

Ls = Ls + (Lrs-1.2dB) + (Rrs-6.2dB)
Rs = Rs + (Lrs-6.2dB) + (Rrs-1.2dB)

Anyone got the exact formula of the standard Film type downmix parameters?

Last edited by kosmokrator; 25th February 2015 at 10:58 AM..
Old 4th March 2015
  #6
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Quote:
Originally Posted by kosmokrator View Post
FYI, just got the formula directly from Dolby:
Ls = Ls + (Lrs-1.2dB) + (Rrs-6.2dB)
Rs = Rs + (Lrs-6.2dB) + (Rrs-1.2dB)
Really? Right rear in the Left surround?
Someone has been reading geometry textbooks instead of watching movies.

Also not sure why Lss and Lsr should have different gain with respect to each other. They're lined up to the same level right?

Quote:
Originally Posted by ggegan View Post
...there is a 3dB bump in the surround level whenever audio is bussed 50/50 between a side surround and a rear surround.
There really is no good solution to this. There may be a theoretical way to solve the bump for a mono object between a single pair of channels, but with multiple signals mixed into a complex 7.1 field there's really no hope.

You just have to keep the downmix in mind and try not to park signals right between 2 channels - or suffer a 3dB boost in the 5.1.

On the question of the side/rear position:
For static signals like AMB I think it makes sense for side and rear to be summed into the surround channels of a 5.1 - so they still play out of the side-wall arrays in a theatre.
For moving signals spinning or flying front/back it seems to make sense to set the Side channels 50/50 or a bit further back - thus maintaining something like the geometry of the original move.

But when you have a combination of signals in a mix it's always going to be a compromise.

And yeah, Spanner can do any or all of the above, even automated to deal with specific issues thru the reel.

justin
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