Stereo Imaging Plugins [Analyzing Mixing Effects]


Hey, guys. This is Eric Tarr for theproaudiofiles.com.
I’ve been working through a series of videos
where I’m demonstrating techniques that you
can use to analyze your audio effects plug-ins.
I’ve already worked through several different
categories of audio effects, and that brings
me to a group of effects that I’ll put together
under the type of stereo image effects. These
could be things related to mid/side processing,
and also get into things that have the perceptual
effect of stereo image widening, or narrowing.
In my first video, what I’m going to do is
take you through my basic setup for my demonstration,
and also get into some mid/side processing.
In my second video then, I’m going to build
off of these ideas and show you some of the
cool things – mysterious things about stereo
image effects. I’ll talk about some of the
similarities and differences between a couple
of different ones.
Next, let me take you through the basic setup
of what I’m working with. It might seem a
little bit confusing or unconventional at
first, but I hope that it’ll make sense after
I take you through it.
I’m going to be using a bunch of different
test tones that I will send through these
stereo image effects to analyze how they work.
So, I started out by creating five different
auxiliary tracks here in Pro Tools, and then
inserted the signal generator plug-in.
I’m generating five different test tones,
starting with 100Hz, 250Hz, 500Hz, 1,000Hz,
and 2,000. Also, they all have the same exact
relative amplitude of -12dB. The reason for
this is I really just want some tones that
are spread out across the frequency spectrum
that I can use to analyze.
So after I’ve generated those tones, I bussed
them from these auxiliary tracks, and printed
them onto mono audio tracks. That way, each
time that I send this signal through my stereo
image effects, I know that the exact same
signal, with the exact same amplitude and
phase is going through the audio effect.
So, I’m going to go ahead and get rid of these,
so they’re not confusing to you. So now, I’ve
got five different audio tracks, each with
a different test tone going through it.
Next, what I’ve done, is I’ve panned these
at specific places across the stereo image.
So for my lowest tone at 100Hz, I’ve panned
it all the way over to 100% to the left. Then
at 250Hz, I’ve got it panned to 50% to the
left. 500 is panned to the center, 1,000 here
over to 50% to the right, then 2,000 all the
way over to 100% to the right.
The reason for this is that I’m going to take
advantage of a frequency spectrum analyzer,
and that’s – I’m going to use here the iZotope
Insight plug-in. What I can do here is use
the frequency decomposition from this plug-in
to look at the relative amplitude of these
tones on a left channel, and a right channel.
So, I’ve got these panned over here, then
I’ve bussed them over to this stereo auxiliary
track, where I’m going to be using a bunch
of different stereo image effects. Then, after
this, I buss this stereo effect out to two
mono auxiliary tracks.
That way, I can insert the Insight plug-in
here on the left channel and look at what
information is contained in the left channel,
after the effect, what frequencies show up,
and what are their relative amplitudes, indicating
the relative amplitude for something that’s
panned all the way over here to 100 to the
left, 50 to the left, something in the center,
something over here 50% to the right, and
then actually something that’s panned over
here to 100% to the right, doesn’t show up
on my left channel, but it does show up over
here on my right channel.
So I’m just using the frequency decomposition
in these relative frequency of these tones
so that I can look at amplitude whenever I
adjust them by using stereo image effects.
So, that’s the basic setup. I can even – you
can hear these things. They’re being played
back right now.
[sine waves play]
But hearing them is actually not very important.
Main thing is just to visualize what’s taking
place.
So, right now this is the basic setup where
I’ve got these panned to different locations.
This analyzer on the top is for the left channel,
the amplitude of these tones at different
locations when they’re panned for the left
channel, and this is the one for the right
channel.
Okay. So, next up, let’s get into some mid/side
processing. All of this should make sense
at this point. I’ve got a left channel and
a right channel.
Let’s talk about mid/side processing, though,
because there are some things to understand
that might seem a little bit confusing. I’m
going to go ahead and pull up this plug-in
from Waves.
It’s a very simple plug-in. It’s going to
perform mid/side encoding when I turn it on.
What that’s going to do is do a summed channel,
where you do Left plus Right, and that’s going
to get sent out actually on the left channel.
So, it does a mid/side encoding where the
left channel is the summed one, and then the
right channel that’s – exit this – is
the difference, or the sides, where you do
left minus right.
So I can do this. Let’s see what we have leftover
now.
So, again, the signals from this one are bussed
over here to the left and the right. Let’s
look at what the result is. So this plug-in,
very simple, all it does is just mid/side
encoding. You can see that this one that used
to be just for the left, now is taking the
mid/side encoding for just the mid. This is
essentially the mid-channel, or I’ll call
it the summed channel.
You can see that even though this one is called
the mid-channel, we actually still have each
one of these test tones in there. Things that
are panned all the way to the left, and things
that are panned all the way to the right,
still show up in our mid-channel, or our summed
channel.
I think that many times, people, engineers,
when they’re thinking about doing mid/side
processing – mid/side equalization, mid/side
compression – they just assume, “oh, that’s
the mid-channel that I’m applying this to,
so I’m only doing it to signals that show
up in my mid.”What you’re really doing is
applying mid/side, or mid/side equalization
or compression, when you just affect the mid-channel,
you’re still affecting everything across the
whole stereo field, because it’s summed together.
There’s nothing that gets cancelled out. So
it’s things that are in the middle, and also
things that are on the side.
What has happened though, is that the relative
amplitude of things in the mid have been increased
by this. So, because of the way that panning
laws work, you see that things that were panned
in the center are now relatively louder than
things on the side, by about 3dB.
We then move on to the sides channel – the
Left minus Right. It is the case that things
that were panned to the center – our 500Hz
tone – actually cancels out, because it’s
identical on the left channel, and the right
channel when they’re panned together, or summed
together – or subtracted more exactly. That
ends up being cancelled out.
So you have the sides are louder, and then
everything that was panned maybe 50% to the
left or 50% to the right, that’s relatively
quieter.
So this is the basic setup that’s used for
mid/side encoding. What you end up with – what
you have in the mid and the sides channel.
When you put them back together, you can – as
long as you don’t do any processing – recover
the original signals, too. The left and the
right.
So let me get into, then, analyzing something
that’s a little bit more complicated, where
it’s using mid/side processing, but also doing
some basic processing to the signal as well.
So I’ll bypass this one, and bring up a different
plug-in. This one is called Center from Waves.
It uses mid/side processing, but then what
you have control of is just the relative amplitude
of signals that are in the center channel,
and signals that are in the sides channel
– or the sums and the difference channels.
So just like before, even though I’m sending
these signals through this, and it’s performing
mid/side decomposition encoding and decoding
– this is my basic setup from before. Things
that are panned in the left channel – things
that are panned to the left are louder, things
that are panned to the right are quieter.
On the right side, the opposite is true.
Let me show you, then, what happens as soon
as I just start to change the relative amplitude
of these channels.
So, let’s say I go ahead and bring down this
center channel. Let’s see what happens.
So when I bring down the center channel, some
things start to happen – some strange things
maybe start to appear over here in the spectrum.
I can turn this center channel down. Let’s
look at what ends up being the result on the
left side and the right side.
So, what I started out with before was that
I just had things that were panned to the
left, and then this 2,000Hz tone that used
to just be in the right channel, didn’t exist
in the left channel, but as soon as I started
to turn down the amplitude of this center
channel, now I’ve got this tone that should
be just panned all the way to the right – it
starts to show up in my left channel.
This is because if you turn down the amplitude
of the center channel, when this plug-in goes
to do the mid/side decoding, it doesn’t perfectly
cancel out. So now it starts to show up in
the left side. Right?
It’s not just the case that this center channel
by itself turns down just this peak that’s
in the center. It also has a different result
where things that are on the sides are also
being affected. It has a lot to do with how
the mid/side decomposition works.
Similarly, if I turn down the sides – right?
If I turn down the sides, this tone on my
left channel is reduced in amplitude, but
what’s the result? 2,000 Hz starts to show
up on my left channel that wasn’t there before,
because it was just panned all the way over
to the right.
Now, this is what happens when you’re just
changing the relative amplitude here of the
center and the sides channel. Certainly, if
you start getting into mid/side equalization,
or mid/side compression, where you’re changing
the relative amplitude of certain frequencies,
or changing the relative amplitude of the
envelope of your signal, you can have even
more complex things going on.
The thing I want you to take away from it
is that mid/side processing – it can do
great things for your mix, but it can also
have some strange things start to happen,
where things that used to be panned to one
side, even 100% all the way to one side, the
start to show up on the left channel or the
right channel. The opposite one. Just by in
this case, changing the relative amplitude.
Any change that you make is going to affect
how perfectly that mid/side decomposition
– or decoding – works when it tries to
put things back together.
So just be wary of that, and know that it
exists, and be careful when you’re mixing.
For the most part, it’s not a big problem,
but you should just understand that these
things are true.
So that’s going to bring me to the end of
this video. In my next video on stereo image
effects, I’m going to take a closer look at
some stereo image widening effects. Compare
and contrast the differences between a couple
different ones, and show you some of the cool,
kind of mysterious things about them.
Until next time, take care.

6 Replies to “Stereo Imaging Plugins [Analyzing Mixing Effects]”

  1. Damn, I also fell for the trap that the mid isn't just the signals in the center but all of them summed. This is why sum and difference are actually the correct terms for that processing, mid/side is a microphone technique… Good video, not always easy to follow, but worth trying out for yourself…

  2. That´s pretty interesting, thanks. =) So if I take the middle out with the "centre" plugin and the side signals show up on the opposite speaker. Is that a flaw in the decoding/encoding of the plugin or is that generally true for mid side processing (part of the common encoding and decoding)?
    When taking down the sides channel it makes more sence for me that the side signals start to show up in the middle, making everything more mono/centered. Sounds like and effect we would be looking for.. Will the side signal in essence become lower in volume or just more towards the middle?
    But when taking out the middle, I´m basically making the sides more mono/centered aswell, because the left channel starts showing up in the right, which isnt really an effect we are looking for right? So the stuff on the sides gets louder in comparision to the middle, but also becomes more mono/centred.
    I´ve had that issue before when i would take out the mid signal to widen something and had the feeling everything became even more mono. I think it was with the brainworks saturator. Havent used its mid side processing for widening ever since lol.
    Thanks again!

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