﻿Hi and welcome back. I got a little push back in the comments
for my last video, from a few people who thought I had been unnecessarily hard on Paul Third.
Paul made a video in which he repeatedly used the terms "linear" and "non linear" incorrectly,
which inspired me to make a video attempting to explain the terms better.
I wasn't going to identify him, but he outed himself in the comments.
Too be clear, I've no beef with Paul beyond the meaning of that one technical term.
Except perhaps I might be slightly jealous of that lovely looking Gretch guitar in the background.
This was only the second of his videos I've watched: YouTube popped his Harrison 32c
channel strip review into my timeline while I was editing my own review of the same plugin,
because Google knows my every thought and deed apparently, and he came to broadly the same
conclusions as I did, although he did express them rather more colourfully and entertainingly.
So I thought I would give it the best of three and try another of his videos: one called
Understanding Harmonics and Aliasing. And I'm happy to report that it's solid
information and I didn't find anything to disagree with.
I'd like to offer a constructive practical tip however, for anyone who wants to
test a plugin for aliasing or check the oversampling is working correctly.
Paul uses a sine wave sweep at one point in the video,
and shows the added harmonics as a spectral graph in iZotope RX.
I endorse this method; its the best way I've found so far to visualise aliasing. But there is
a small tweak that will make the resulting graphs both easier to read, and easier to understand if
you're still trying to grasp the concept of what aliasing is and how it manifests.
I don't actually have RX: I've got a light version I think, but I'm afraid I haven't dug into it all,
so I usually use the Acoustica audio editor from Acon Digital, not to be confused with Acustica
spelt differently who make the Aqua plugins. Let's start by generating our sine sweep:
this will go from 20Hz to 20KHz over four seconds… I'm keeping it fairly short to make
it less offensive to my viewers, but the duration doesn't matter much… and it's going to be -20dBFS
so I don't blow my ears out if I play it at normal monitor gain settings.
And, importantly, its an exponential sweep. That means it increases at a constant rate
in terms of musical pitch. But in terms of frequency the rate of increase starts slow,
then speeds up for the higher frequencies. We can see that clearly on this spectral view:
the sweep curves upwards; its not linear.
So let's open the settings, and change the frequency scale to logarithmic,
which is the opposite of exponential… and now the sweep looks like a straight line.
Ok, great. But we do have this big fat carrot shape at the bottom, which looks a bit odd.
And we also find that, if we generate some naive harmonics with no oversampling,
the reflected aliasing looks odd: the exponential curve of the original test signal
has been reflected and turned upside down, so the logarithmic frequency scale
no longer corrects that curve. The result is kind of confusing:
you can see that the harmonics are reflecting, but if you're not familiar with the maths behind the
exponential sweep and the logarithmic scale you might be left wondering why the straight line of
the harmonic reflects down as a curve. So here's my suggested tweak:
first of all, generate a linear sine sweep, instead of exponential.
This sounds kind of odd to human ears: the sweep rushes through the lowest frequencies,
then lingers on the very highest frequencies… but the sine wave is now increasing in frequency at
a constant rate in Hz, rather than a constant rate in semitones.
In the spectral view we now see the sweep curving in the opposite direction,
but this is just the logarithmic frequency scale: if I switch back to a linear scale
it shows as a straight line. Bonus: we've eaten the fat carrot
at the bottom. That looks much better. But better still, when I generate some aliasing…
its now really obvious that those added harmonics are just simply reflected back down from Nyquist…
and we can now also see that, when they get to zero they still don't disappear,
they reflect back up again.
And it's much easier to understand that, if a harmonic is trying to be 5KHz above Nyquist,
its aliased reflection will end up 5KHz below Nyquist instead.
To be clear: this is really exactly the same test Paul was running, I'm just displaying the
results slightly differently to make them easier to interpret and understand.
And just in case you wondered, if I redo the processing with oversampling enabled…
those reflected harmonics now get filtered out before they can get
back down below 20KHz and become audible.
A nice thing about this test is that you don't have to load the plugin in question
into an editor or analyser, so you can also test native plugins that only run in your DAW:
just load in that linear sine sweep, bounce it through the plugin in question,
and then check the results back in whatever spectral editing software you have available.
Or if you're a Reaper user you can switch to the spectral display and view it right there.
The only other thing I might add to Paul's video is a touch of perspective.
Now, these tests are perfectly valid: they help us to understand how our tools work,
and how we can get better results from them, help us to evaluate purchases, or sometimes
to see through misleading marketing claims. I would not dispute the claim that a plugin that
includes the right kind of saturation and non linearities is a better, more accurate copy of
the analogue gear than another that doesn't. Nor would I deny that, all else being equal, a plugin
that properly oversamples it's non linearities is objectively better than one that doesn't.
But I would say that it's not necessary for every plugin in a mix to include saturation,
and indeed it may not be desirable. A totally linear EQ plugin can still be really useful:
you may not need any more harmonics, or if you do you can follow the EQ plugin
with some kind of saturation instead. And I would also add that, just because a plugin
isn't oversampled when it technically should be, that doesn't make it useless. The classic example
is Sound Toys Decapitator: yes it's a distortion plugin; no it's not oversampled; yes it aliases
as much as you'd expect. But nothing sounds quite like Decapitator, and I use it regardless.
Because in real world use we don't normally apply hard guitar pedal style distortion to
every channel. The saturation we add tends to be much more subtle than that.
In fact Paul notes this himself in his video, when he runs a test tone through a real Pultec
and observes that the added harmonics are actually quite subtle…
Also, remember that real world audio signals have much less energy at high frequencies.
Our sine wave sweep stayed resolutely at the same level all the way up the spectrum,
but guitars and pianos and vocals don't behave like that at all, and will usually only have
a tiny bit of energy up in the highest octaves that are most likely to alias.
And final point: sometimes aliasing sounds good. I'll often deliberately
use samplerate reduction effects, like the Destroy style in Fabfilter Saturn…
and sometimes I'll turn off oversampling even when using more analogue distortion styles…
because added partials don't necessarily have to be harmonically related to the original signal
to sound good. So what I'm trying to say is, don't let
these analyser tests mislead you as to the scale of the problem. Adding harmonics plus aliasing is,
in most cases, still going to sound better than leaving your mix too clean and sterile.
And if your ears tell you something is good, trust them and ignore the graphs.
Because ultimately that's the only thing that matters.
Ok that's all for now, thanks for watching.