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Mute a side channel

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Mute a side channel

Postby GRAHAM99 » Fri Nov 27, 2020 5:27 pm

I have a wav file of a mix of a song with two guitars panned hard left and right.
I would like to replace one of the guitars using my daw.
How can I isolate and mute one side?
I have tried using Voxengo Msed but this just isolates the mid and both sides.
I use Reaper.
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Re: Mute a side channel

Postby The Elf » Fri Nov 27, 2020 5:59 pm

Both left and right sides will also carry audio that appears in stereo centre. In other words, simply muting one side isn't going to only mute the guitar panned over there, and when you do mute it you're going to be left with a mono signal. You could pan the mono signal stereo centre, but then you're going to end up with one guitar stereo centre and your own to one side.

Of course you could then record both a left *and* right guitar of your own making, but the rest of the mix will be 'very mono' - and will still contain one of the original guitars.

So using mid/side and muting the sides may be as good as you're going to get.
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Re: Mute a side channel

Postby Hugh Robjohns » Fri Nov 27, 2020 6:56 pm

GRAHAM99 wrote:How can I isolate and mute one side?

You can't. At least, not with simple MS processing.

A hard-panned guitar exists in both the Mid and Sides channels. It is present only in the Left channel, obviously, but not exclusively -- it's buried in with everything else placed anywhere from the centre out to the hard left. So you can't simply delete one part of the stereo signal without disrupting everything else.

You could try spectral editing, or possibly something like R-Mix or Spectralayers.... But it would be tricky and time consuming.
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Re: Mute a side channel

Postby blinddrew » Tue Dec 01, 2020 5:26 pm

I was thinking about this the other night, because I was lying awake and hoping it would send me to sleep, and it occurred to me that it is possible to remove one side of the sides signal.
Obviously it'll only remove content that exists solely hard-panned that side, but if anyone's curious:
Track 1 - source track, sending to track 3.
Track 2 - duplicate of source track, with a plugin set to play mid-only, polarity reversed, sending to track 3.
Track 3 - panned hard left or right depending on which you want to remove, polarity reversed, sending to master.
Track 4 - duplicate of source track, sending to master.
Play with the level of track 3 to dial in or out the reversed side signal.
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Re: Mute a side channel

Postby GRAHAM99 » Tue Dec 01, 2020 6:13 pm

I am sorry this has kept you awake at night!!
I sort of gave up and used the Voxengo plug in to block the sides.
I overdubbed two guitars, left and right.
Thanks anyway, I will try your idea out!
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Re: Mute a side channel

Postby blinddrew » Tue Dec 01, 2020 6:20 pm

As Hugh and The Elf mention, it won't do a complete removal because of the content that exists in the mid as well, but depending on what you're trying to do it might add a bit of value.
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Re: Mute a side channel

Postby merlyn » Wed Dec 02, 2020 5:37 pm

blinddrew wrote: ... it occurred to me that it is possible to remove one side of the sides signal.

What do you mean by one side of the sides signal? I don't think your routing is doing what you think it is doing. :)
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Re: Mute a side channel

Postby Hugh Robjohns » Wed Dec 02, 2020 5:55 pm

blinddrew wrote:... it occurred to me that it is possible to remove one side of the sides signal.

I fear that's wishful thinking! The side signal doesn't have 'sides'. It's just a single signal coming from a single microphone or being matrixed from a stereo signal. It is is simply : L-R

It recreates the left and right 'sides' only when matrixed with the Mid signal.

And if you try to do the inversion and add thing as you describe, you end up introducing the right channel into the left... So the whole thing just becomes the right channel....
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Re: Mute a side channel

Postby blinddrew » Wed Dec 02, 2020 7:34 pm

Interesting. I tested it with a hard LCR mix of just three sources and it worked for me.
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Re: Mute a side channel

Postby blinddrew » Wed Dec 02, 2020 7:51 pm

Possibly some poor terminology on my part, track three isn't a sides signal per se, it's the stereo signal with the mid removed.
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Re: Mute a side channel

Postby Hugh Robjohns » Wed Dec 02, 2020 8:04 pm

Okay, I've probably misunderstood your patching description... But I remain sceptical that you're removing only the fully panned element.
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Re: Mute a side channel

Postby blinddrew » Wed Dec 02, 2020 8:09 pm

Oh yes, it's not pristine, but it could be useful. I shall try and make a video to demonstrate.
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Re: Mute a side channel

Postby merlyn » Wed Dec 02, 2020 8:30 pm

blinddrew wrote:Track 1 - source track, sending to track 3.
Track 2 - duplicate of source track, with a plugin set to play mid-only, polarity reversed, sending to track 3.
Track 3 - panned hard left or right depending on which you want to remove, polarity reversed, sending to master.
Track 4 - duplicate of source track, sending to master.

Track 1 is easy enough : 1L = L 1R = R

Track 2 is : 2L = -L-R 2R = -L-R

Track 3 adds 1 & 2 : 3L = L + (-L-R) = -R 3R = R + (-L-R) = -L

With track 3 panned hard left : MasterL = L-R MasterR = R

Are you confusing 'mid' with centre? The centre signal doesn't exist -- the centre is the part of the signal that's exactly equal in L and R.

If you could do what you're suggesting it would be possible to isolate the centre and that isn't possible with simple routing -- it needs a special plugin.
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Re: Mute a side channel

Postby blinddrew » Wed Dec 02, 2020 8:43 pm

The only plugin I'm using is bx-solo from brainworx on track 2, set to mid. i.e. mono sum.

I should add that track 3 needs to be boosted by 6dB to null out.
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Re: Mute a side channel

Postby blinddrew » Wed Dec 02, 2020 10:27 pm

Nope, I've checked the maths, and the frequency response, and can confirm that I'm talking bollox again. :headbang: :headbang: :headbang:
Sorry about that. You're both completely correct that panning the track three just goes from mid+R to mid+L.
But...
If you raise track 3 by 6dB that difference in the level is 12dB, so it is quite significant.

But there is still something I don't quite understand. If I solo track 3, and stick a frequency analyser on it (i.e. an EQ with nothing active), or on the master, it shows no signal at all when the panning is central, so I'm assuming everything is nulling, but I can hear it through the speakers. As soon as the panning goes slightly off to one side the scope shows the left and right but no centre signal.
What really, really obvious thing am I missing hear?
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