Hi, and welcome back.
I sometimes wonder what it's like to try to learn audio engineering these days.
I mean, I did it the hard way, right? I got stuck in and learnt on the job from my mistakes:
there was no YouTube in those days, no tutorials to guide me in the right direction.
Sometimes, as a house engineer, I would work with a touring engineer that really knew their
stuff and was happy to share some of that knowledge. But mostly I just had to figure
it out for myself.
But then sometimes I wonder if nowadays we have the opposite problem: the internet is
awash with information and advice, some of which is questionable at best, and some of
which is just plain wrong. How do you tell the difference?
A video recently popped up on my timeline which illustrates this quite well. It talks
about the difference between linear and nonlinear plugins, and almost everything it says on
the subject is wrong.
I'm not going to name the creator, as it's a smaller channel and it would feel like bullying.
But I feel like the world clearly needs a "what is a non linearity video", so here it
is.
Because the term has a very specific meaning, when it comes to plugin design. Let's see,
what does Wikipedia say?
"In mathematics and science, a nonlinear system is a system in which the change of the output
is not proportional to the change of the input."
Ok, maybe that's not so helpful on its own. So let me explain.
Here's Pro-Q3. It's a linear EQ of course. But that's not because the frequency response
is perfectly flat when no bands are active. Nor is it because this bell filter is nicely
symmetrical, or because it's at precisely the frequency I've dialed in. None of that
is relevant at all.
And incidentally, its also nothing to do with linear phase mode at the bottom, confusing
as this may be. Pro-Q3 remains linear, in the mathematical sense, whichever processing
mode you choose.
It's linear because the input level doesn't matter. If I drop the input level by 12dB,
the output level drops by 12dB, but remains otherwise identical. Likewise, if I boost
the input level by 20dB, the output level increases by 20dB… and whatever signal level
I feed it, this EQ cut always remains exactly the same.
Analogue gear is usually designed to be as linear as possible, when fed with signals
at its intended operating level. But of course no analogue gear has infinite headroom: if
you boost up the input level sufficiently, sooner or later the signal will start to distort.
At this point the equipment becomes profoundly non linear: adding another 12dB of gain at
the input won't give you a 12dB increase in output level any more, because the signal
peaks will get clipped and distorted.
Incidentally, a side note for my fellow guitar players: gain just means volume. Increasing
the gain in a preamp means multiplying the voltages to make a bigger signal: that's it.
Gain only means distortion when it's pushing the signal into a non linearity of some kind.
And while we're at it: vibrato means pitch modulation, and tremolo means volume modulation.
Just because Leo Fender got confused 70 years ago I don't see why we all need to still be
getting it wrong today.
Sorry, I needed to get that off my chest
Anyway, the reason Pro-Q3 is a linear EQ is because there's no distortion at all, whatever
the input signal level. You'd have to boost the levels by hundreds of decibels before
you ran into any issues, which means practically speaking we can consider it totally clean
and distortion free.
In fact, the only way to stop Pro-Q3 from being totally linear is to enable the dynamic
features. Because dynamics processing such as compression and expansion is also inherently
non linear. You can think of compression and limiting as an attempt to create a non linearity
that's free of distortion, but of course fast attack and release times still result in distortion
and added harmonics, and that's half the fun.
Because here's the thing: distortion sounds good! Now, if you're a rock guitarist, that's
not going to be controversial at all. Guitarists have thoroughly embraced distortion, to the
point that they think that's what the word "gain" means, and it probably wouldn't be
hard to persuade them that a tiny drop of the same poison will also work magic on the
drums, or the vocals.
Other types of musicians are more cautious: don't tell a classical cellist that you're
distorting their sound, whatever you do. But if you call it something with fewer negative
connotations like "saturation" or "analogue magic" they'll hear it as extra richness,
more resonance, more detail, more clarity, and those added harmonics will tickle their
ears just like yours or mine.
Harmonics are what make instruments sound interesting after all: we don't just listen
to arrangements of pure sine waves, as a general rule, and it shouldn't suprise us that adding
more harmonics can make things sound more interesting.
In analogue land, distortion is basically free. The expensive stuff often does it subtly
and gently when run at normal operating levels, which gives the kind of gentle enhancement
that we might perceive as more "hi fi" rather than less.
But you always have the option to drive it harder if you want to, and when you do the
extra added harmonics that result stick to the same musical harmonic series that our
instruments create in the first place.
In digital land, distortion is not free. It has to be explicitly coded, in all its potential
dynamic complexity. And furthermore it really needs to be oversampled, otherwise you risk
some of those harmonics you're adding folding back down as aliasing, which doesn't stick
to the harmonic series any more.
So, we used to judge analogue gear by how clean and free of distortion it was: now we
judge digital software by how well it can do analogue style distortion. And again, that
shouldn't be surprising: a little bit of distortion is usually a good thing: in the past the struggle
was not to end up with too much: these days the risk is that, unless you explicitly add
a little bit, you might end up with too little.
Anyway, let's talk about measuring distortion using Plugin Doctor: here's the first graph
you're presented with when you launch the software, and it shows the frequency response
of the plugin in question. Or the phase response if you switch it at the top.
But notice what this first tab is labelled at the top: linear analysis. This type of
analysis is designed for a linear system, with no distortion or compression. If you
try to measure a non linearity with this method you're just going to break the test.
Now, in this case of a plugin that's almost linear, like this Lindell channel strip with
the dynamics bypassed and the preamp turned down, we can safely disregard the added harmonics,
as they're low enough in level not to mess up the reading significantly, and this graph
is likely to be a pretty accurate representation of the frequency response through the EQ and
filter modules.
But if I add more distortion… these graph shapes we’re getting just become nonsense:
we’re not getting any useful information about the behaviour of the plugin any more,
we’ve just broken the test.
But the Doctor does provide some useful ways to measure non linearities. Lets start with
the Dynamics tab: this is intended to show the transfer curve of dynamics processors
such as compressors and limiters, but it can also show the way that analogue style distortion
affects your dynamic range.
Or the Harmonic Analysis tab, which is more intended for measuring distortion. Here we
have a 100 Hz sine wave test signal, at a conservative level of -20dBFS, running through
Pro-Q3 to begin with. Notice the output is just a sine wave still, with no extra harmonics
on top.
Now here’s the Lindell channel strip, with the input gain all the way down: even with
its cleanest setting we’re getting a significant harmonic series added. Now look what happens
when I boost up the gain…
Incidentally, don’t pay any attention to the noise down here at -150dBFS: this is way
too low in level to ever be audible at all. But that third harmonic though: thats not
insignificant, even with the gain all the way down.
There's a limitation with this kind of test however, in that we're only presenting it
with a single frequency. If I shift the test signal to a different frequency… we might
see a very different pattern of harmonics… and measuring the harmonics at just a single
frequency doesn't tell us much about the way it might sound.
So let's try switching to the Hammerstein tab. Hammerstein? Hammerstein? I don't know
to be honest, and continuing to be honest I hadn't heard of this type of test until
Plugin Doctor introduced it to me.
Here's what seems to be going on, as far as I can tell: the key at the top right tells
us which colours correspond to which harmonic, and we can see which harmonics are added to
which frequency ranges, and by how much.
Probably the best way to illustrate this is using Spectre from Wavesfactory, a very useful
plugin that allows you to add non linearities much as you'd dial in EQ.
Starting flat we see just the first harmonic tooling along at unity, no change whatsoever.
Now let's add a non linear band at 200Hz… and now we see a prominent third harmonic,
centred at 600Hz, and a fifth harmonic at 1KHz.
If I switch the band to the asymmetric Tube mode… now we've got lots of even harmonics
as well, at two, four and six times the fundamental frequency.
Contrast this with a linear 200Hz EQ boost from Pro-Q3: we get a bump in the fundamental,
but no extra harmonics at all.
Here's Fabfilter Saturn running its Warm Transformer style: notice the third and fifth harmonics
are fairly significant down in the low frequencies, but reduce in level as the frequencies increase.
This stays very much the same as I increase the drive… until all of a sudden the higher
harmonics kick in with a vengeance: this is presumably the transformer core saturating
fully and starting to clip the signal brutally instead of just thickening it subtly.
Contrast that with its Warm Tube style, with the drive set low… and now with the drive
cranked… asymmetric tube style saturation gives us lots of the sweet sounding even harmonics
at 2, 4 and 6 times the fundamental frequency.
Here's Warm Tape used subtly… and now abused disgracefully.
This one's good for a giggle: Broken Tube.
Disclaimer: Plugin Doctor is quite slow and sluggish running this analysis, at least on
my aging processor, and dare I say it, also a little buggy at the time of writing. It
often stops responding to changes in the plugin, and I have to reload. So that's why I've probably
pieced this segment together from static screenshots rather than attempt to capture video as usual.
And it's also why I'm not as confident as usual that all my readings are accurate.
Nevertheless, if you want to analyse a non linearity, this is a better method than a
test called Linear Analysis, which is kind of like trying to weigh something using a
tape measure.
Ok that's all for now, thanks for watching.