Mid-Side manipulation of stereo signal...

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brad

Guest
1] In your mixer, pan the first piece (1&2) as a normal stereo track of music, hard left and right.
2] Next, pan the second track (3&4) dead center for both channels. This will make the track mono.

3] Now phase reverse both channels of 3&4.
If you kept the tracks sample tight, when you play, you will only hear the stereo information - anything common to both channels (mono) will go away. If you bounce this resulting output to a stereo file, you have the side information isolated for further processing (we'll call this 5&6) . Now, if you mute 1&2 (leaving only 3&4 in mono and phase-reversed) you can bounce this resulting output for your mono information (we'll call this 7&8). If you line up the 5&6 and 7&8, they'll recombine, giving you your normal stereo output.
 
M

Michael Kiaer

Guest
Brad Blackwood eloquently explained:

1] In your mixer, pan the first piece (1&2) as a normal stereo track of music, hard left and right.
2] Next, pan the second track (3&4) dead center for both channels. This will make the track mono.

3] Now phase reverse both channels of 3&4.
If you kept the tracks sample tight, when you play, you will only hear the stereo information - anything common to both channels (mono) will go away. If you bounce this resulting output to a stereo file, you have the side information isolated for further processing (we'll call this 5&6) . Now, if you mute 1&2 (leaving only 3&4 in mono and phase-reversed) you can bounce this resulting output for your mono information (we'll call this 7&8). If you line up the 5&6 and 7&8, they'll recombine, giving you your normal stereo output.
Brad, your explanation makes total sense, except, wouldn't you have to phase-reverse the bounced mono track (now 7&8) again (bringing it back in-phase) before adding it in?

Also, do you need to reduce your levels before summing (panning) L and R? In Protools, I hit over 0dBFS on the Master fader (using an extracted song from a CD so each track by itself has peaks at 0dBFS).

Does this depend on the pan behavior of the individual DAW or desk, and how do you determine what to do? I guess it isn't trivial whether the pan just sums or whether it also scales? Am I making this more complicated than it is?

When you mix M and S back together, it makes sense they should be mixed back together at equal levels, right? So, if I need to reduce L and R by -6 dB before panning to M, I should also bring down the S tracks by 6dB, correct?

Sorry for not being nearly as coherent in my questions as you were in your explanation. I am just trying to wrap my poor brain around the concept. :eek:

Thanks!

Michael Kiaer
New York City
 

Dave McNair

Well-Known Member
Joined
Mar 6, 2001
Michael, I think you keep the mono tracks phase reversed, because you might want to eq or compress center information only and to do this corectly you need to effect the phase reversed mono so that it will sum properly with the side information.
 

Kev

Well-Known Member
Joined
Nov 13, 2001
OK that's Middle and Side ... or Sum and Difference.

But I have been trying to find a way of making Left - Centre - Right... easy you say. I don't think so.

The centre is not mono. and if I could get centre I could get Left only and Right only. I have tried various I-r and r-l configs but I just cannot get centre only info.

How did Dolby do it? Am I missing something simple here? I have been avoiding hooking up my old ProLogic decoder as it would be a cop-out.
:confused:
 

Kev

Well-Known Member
Joined
Nov 13, 2001
I think you are wrong but now have to set up a test mix to prove it???

I'm sure a signal hard in one speaker doesn't appear in the centre channel. Also the left and right are independant signals so you could change the left right balance whereas the side of m/s doesn't allow for this. Am I making sense?
 
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Kev

Well-Known Member
Joined
Nov 13, 2001
I'm still working on this??

I had a few guys at work today scribbling on paper ... l-r ... r-l L-(l-r) no L-(r-l)
no phase change the whole thing ?? Thats mono again!.. then we moved into a control room and tried?? Didn't get much work done for about an hour.

Later in the after noon one guy .. not from the original group, collected a few of us together and said "I've got it"

No he didn't have it! :D

I'm not giving up .... now where did I put that ProLogic decoder?
 
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Kev

Well-Known Member
Joined
Nov 13, 2001
We are still working on it.
:D
... and I still can't find my old ProLogic decoder.
 
R

Ronny Morris

Guest
Originally posted by Brad Blackwood:
Originally posted by mikkel:

Originally posted by mikkel:
Also, do you need to reduce your levels before summing (panning) L and R? In Protools, I hit over 0dBFS on the Master fader (using an extracted song from a CD so each track by itself has peaks at 0dBFS).
Yes, you probably do. I didn't dig into it far enough to see if the relationship is linear (eg: lowering both sets by 3db or one by 3 and one by 6) as I don't have to do this in my setup. It'll take some trial and error on your part to figure it out, but if you keep the orignal, un-touched file in your EDL as well as the bounced information, you can solo the original vs. the bounced stuff to get the relationship/levels right.
Yes, he needs to account for +3dB gain increase that you get when you double a signal bus. Load-in tracks 1 & 2, reduce gain by 3dB and copy to 3 & 4, otherwise if the material is close to -0dBFS, the final stereo output bus will produce overs.
 

Kev

Well-Known Member
Joined
Nov 13, 2001
Originally posted by Brad Blackwood:
Originally posted by Kev:
I think you are wrong but now have to set up a test mix to prove it???
Yes, you will.
I give up... :(

stereo doesn't not give L-C-R no matter how many phase changes and l-r and r-l and take away the number you first thought of etc....

I did find the ProLogic decoder and it does seem to do more than just Sum and Diff but I haven't worked out what they have done.

However it is not as defined in the L-C-R as I had remembered.
;)
 
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