Hi, and welcome back.
This has been a long time coming: I've been promising a parallel filter tutorial for some
time now, apologies for keeping you all waiting.
The reason for the delay is a bit silly really: I had an idea for an example, but it's a little
out of my comfort zone and it took me a few goes before I was happy.
You'll see what I mean when we get to it.
Anyway I’m guessing most of you have heard of parallel compression, where the output
of the compressor is blended with the original signal to create more subtle and complex compression
effects.
But I’m guessing you're less comfortable with the concept of parallel filtering; perhaps
you've never heard of it, or maybe you've just been warned not to do it because of mysterious
and unexplained “phase issues”.
There is some wisdom in this advice, as it is possible to get into a pickle with phase
cancellation if you’re not careful.
But if you know how to avoid the pitfalls a whole world of mixing possibilities opens
up.
So lets start by clarifying exactly what these phase issues are, and how to stop them being
a problem.
Here we have an analyser showing a 12 dB per octave high pass filter in Pro-Q3, and also
the result of adding that to the original dry signal, both at unity gain.
Actually, it doesn’t look too bad.
We basically have a 6dB high shelf boost, but with a little dip before the rise: this
is where the phase shift from the filter causes cancellation.
But its not too drastic: well under 2dB of cut, compared to the 6 dB of boost above the
cutoff, and this kind of frequency response, with a little pre rise dip, often works better
than just a straightforward high shelf boost.
This isn’t scary at all, its a perfectly usable setup.
Now lets try a steeper filter, say 24dB per octave instead of 12… things aren’t looking
so rosy anymore.
That dip is now a sharp cut of almost 12dB: way more than the 6dB of boost we have at
the top.
I would argue that this response is useless, in practical terms.
Not because there would never be a source that called for this response: there’s no
such thing as an inherently wrong EQ shape, as it always depends where you start and where
you want to end up.
But I would argue that no-one is ever going to try to tame a harsh resonance by sweeping
a parallel filter around on another channel.
If I’ve got a source that’s burning my eyebrows off at 2KHz, I’m going to fix that
with a conventional EQ cut on the channel before I move on to the fun creative stuff,
and I’m pretty sure I’m not alone in that.
So lets agree that this is an example of the dreaded phase issues you may have been warned
about: running a 24 dB per octave filter like this in parallel is indeed likely to get you
in trouble, and if you try going even steeper… really weird stuff starts to happen.
So I’m going to present you with three different ways to make sure you don’t run into this
kind of phase weirdness, starting with the one I actually use the least, although its
arguably the most powerful:
Switch to linear phase mode.
Now everything works exactly as you would intuitively expect: with no phase shift from
the filter there are no weird cancellation issues, and the filtered signal is just added
perfectly to the original signal to create a high shelf boost.
This remains true regardless of the filter steepness… right up to the crazy steep brickwall
type, which simply gives us a crazy steep high shelf boost.
The price you pay for this freedom is latency and increased cpu use.
Both very minor issues really, assuming the plugin delay compensation is working properly
in your host DAW.
But even with proper working PDC, latency can result in annoying delays between pressing
play and hearing any audio; between tweaking parameters and hearing the change; it might
mean plugin meters move out of sync with the audio; and as latency accumulates I find it
helpful to avoid it when it's not essential.
I rarely want steep and surgical shelves like this when I'm running parallel processing.
I'm much more likely to want a gentle, sweetening shelf response; and there are more efficient
ways to achieve this.
Let's go back to zero latency mode… and switch the high pass filter to the most gentle
6dB slope… and all those phase problems immediately disappear.
Now we just have a perfectly smooth high shelf boost.
Well ok, the phase shift is still there, but its no longer a problem.
Lets compare this to the same filter but in linear phase mode… and we can see that the
phase shift in zero latency mode is making a difference: its like the frequency has shifted
slightly higher, and the slope has become slightly steeper.
So for reference, lets compare the zero latency parallel filter response to a simple conventional
high shelf boost: here’s Pro-Q3 set to the same frequency, with a 6dB slope, and 6dB
of boost…
this time the parallel filter setup is lower in frequency… but if I shift the high shelf
down a bit to compensate…
I can match them pretty much perfectly.
So a really easy way to avoid any issues is to stick to one pole, 6dB per octave filter
slopes: these will give you nice gentle shelf EQs when run in parallel, just the same as
a gentle EQ inserted on the channel, and you don’t even have a Q control to worry about,
which keeps things nice and simple.
So the question you might be asking now is "why"?
What's the point of running a parallel high pass filter instead of a high shelf?
Well, not much really.
Unless perhaps you have many channels that all need a similar high frequency boost, like
say, a bunch of stacked harmony vocal parts..?
In that case this one parallel high pass filter
might replace half a dozen or a dozen individual EQ bands, and starts to make a lot more sense.
With the sends and return all at unity we get a 6dB high shelf boost… but we can mix
in less of the filtered signal for a more subtle boost… or turn it up above unity
to boost more than 6B.
This is the example I mentioned at the top of the video: I don't normally sing: All Ii
can say is thank god for the Waves Tune plugin, affiliate link in the description.
But perhaps you're still not convinced.
Why not just stick them all in a subgroup and EQ the group instead?
Well, one advantage to this method is that we can still adjust the amount of high frequency
boost for each individual channel: just turn up the send if that part needs more brightness,
or down if it's already too much.
And as a bonus, you can do this from the mixer window without having to open and close individual
EQs.
Another bonus: you can add more plugins after the filter, like saturation for example.
This is only saturating the high frequencies that we're adding, so the result is more like
an enhancer or exciter, generating extra high frequency harmonics as well as boosting those
already present.
Or we could add compression… and now we've got a dynamic high frequency shelf, compressing
just the high frequencies.
Yes, you could achieve something similar by running a dynamic EQ or multiband compressor
on a subgroup, but there's an important difference: this is parallel compression.
We're compressing an isolated high frequency band which we're then adding back to the full
range signal, so all the changes to the compression curves that happen when you run a compressor
in parallel will be happening here, except only affecting the high frequencies.
Ok, final method.
If you’re feeling a bit more adventurous, try switching to the 12dB slope.
Now we do have a Q control, and its actually quite critical: above 1 there be dragons!
This starts to introduce a resonant peak in the filter response, which we see as a positive
bump in the resulting high shelf, and we also get a corresponding sharp dip below the cutoff.
Again, this shape is not necessarily useless, but in this sweetening, parallel processing
context I usually go the other way instead: now we have a gentle shelf boost, with a gentle
dip a little higher.
When using a lowpass filter like this to create a low shelf, the result is kind of remeniscent
of the Pultec low end trick, except we're achieving it with just one parallel band instead
of two.
And, instead of running it on the master, or a subgroup, I can use aux sends from any
track that needs extra fat bottom end; in this case drums, and two different bass parts.
Of course we can saturate this again, if we want thicker, richer low end as well as more
low end.
And again we can add dynamics processing such as compression, and make this a dynamic low
shelf instead.
One interesting consequence of this is that, while the low sub bass frequencies that are
being boosted by the shelf will be compressed, that higher bass region that's being dipped
slightly will be moving in the opposite direction, or in other words will be expanded instead
of compressed.
This might help to preserve some punchy thump in the kick drum, while still clearing out
clutter in the manner of the Pultec trick.
An important observation however: Pro-Q3 doesn't use standard definitions for the Q value of
it's filters.
A value of 1 in Pro-Q3 is equivalent to 0.72 in most other filter or EQ plugins, so if
you're not using Q3 remember to stay below about 0.7 to get this kind of behaviour.
Ok, great.
But it gets better.
You may have noticed that the pass band of the filter trends back to zero phase.
This is why we get nice straightforward shelf responses from our parallel low or high pass
filters, with the most phase weirdness tending to happen where the filter starts to roll
off.
So it would be reasonable to assume that a bandpass filter, which rolls off both sides
of the cutoff and has a phase response more like this, will be a hot mess when run in
parallel.
You would be quite wrong.
And in fact, parallel bandpass filters is one of the most powerful and useful drum mixing
tricks I've yet to discover.
But that will have to wait for the next installment.
Thanks for watching.