Hi, and welcome back.
Today I'm going to introduce you to the concept of filter inverting. And we're going to start
with the same setup as last time, a bandpass filter running in parallel with the original
signal, both mixed together at unity gain… and the result is a bell shaped EQ boost of
almost exactly 6dB.
Why 6dB you might ask? Because at the cutoff frequency we have unity gain and zero phase
shift, so this specific frequency is still exactly the same as the source. When they're
added together the amplitude of the waveform doubles, in the same way that 1 plus 1 equals
2, and a doubling of the amplitude means 6dB of gain.
Ok, so what do you think will happen if I pop open the output section for Pro-Q3 and
flip the polarity for just the filtered signal? I'll give you a moment to think about it…
ok? If you said "it will turn into a 6dB cut instead of a 6dB boost" then you're clever,
but you're still wrong: in fact we get a hard notch, with infinite attenuation at the cutoff.
Infinite attenuation? Really? Well again, the filter doesn't change anything at that
one specific frequency its tuned to, but we're now subtracting it instead, and of course
1 minus 1 always equals zero.
Now, while we've got the output section open, notice what happens if I reduce the gain of
the filtered signal… that hard, infinite notch smoothly morphs into a gentle EQ cut.
Of course we can also turn the gain the other way and boost the filtered signal above unity…
perhaps unexpectedly, this starts to flatten out the frequency response again… at plus
6dB we're more or less back to flat again. Be aware however, that the frequency we're
filtering is now flipped in polarity: if I switch to show the phase response, we see
a 180 degree phase shift at the target frequency. So we've created an all pass filter. This
is likely to be harmless, but also probably pointless: I can't think of any reason you'd
want to do this, so I'm going to suggest you do as I do and keep the filtered signal at
or below unity gain when inverting like this.
So before we move on let's have a practical example of this in action: here's some classic
DMX drum samples, running from NI Battery. These samples could definitely use a cut in
the midrange, around the 5 or 600 Hz mark, but instead of EQing the drum bus as I might
normally, I'm going to send all my drum channels to a parallel bandpass filter with the polarity
flipped… and the result is a hard notch, as predicted by Plugin Doctor.
It's a bit too extreme to be honest. But that's not a problem, I can just turn it down a bit…
and it becomes a gentle cut instead.
Let's get a bit more adventurous however: I'll put the fader back to unity to restore
the hard notch… but then add a compressor instead. I'm a bit fussy about the type of
compressor I add however, not so much regarding the flavour of compression, but more to do
with the control scheme. I'll avoid the T-Racks Comprexxor for example, not because it's not
a great compressor… in fact I've got one on my mix bus… but because it has a fixed
threshold topology with input and output gain controls: this makes it really awkward to
figure out where unity gain is, and I really want to get back to exactly unity gain when
the compressor fully releases.
I'll avoid all my 1176 emulations for the same reason. The Waves H Comp is also ruled
out because you can't turn off the automatic gain compensation, although that’s not going
to stop me putting an affiliate link down below. But any compressor with a threshold
control and the option to manually control the makeup gain will work fine.
I'm going with something that can eat the transients effortlessly, because we're not
just inverting the filter now, we're also inverting the dynamics processing: whenever
the compressor turns the gain down, that hard notch will turn into a gentle cut, so the
dynamics of this 500Hz region are moving in the opposite direction and we've created a
downward expander.
Or more than an expander really: with the makeup gain at unity we return to a hard notch
whenever the compressor releases, so this is more like a hard gate for that frequency.
This is a great way to retain some of the vintage midrange thump in drum sounds like
these, while still tightening up and de-cluttering that boxy region, and creating more space
for other parts.
Of course we can create a subtler version of this by simply turning down the filtered
signal again… now the gain never returns to unity so we never get the hard notch. Instead
the response varies from a gentler cut to a harder cut, and we get gentle downward expansion
of that region.
This technique isn't only useful on drums: it can work very well on stuff like acoustic
guitar, which can sound thin without some lower midrange, but can quickly clutter up
that region if you're not careful.
We can also do the opposite in terms of dynamics however: let's think about that part of the
song where everything is in together… and the midrange starts to build up and become
overpowering. In this case its the lower mids around 300Hz that are getting carried away,
so let's tune a nice broad gentle bandpass filter to that frequency, running on a parallel
channel, and with the polarity flipped, and I'll send it anything that's got any content
at all in that region. In this case I'm sending from subgroups, so it's going to get everything
except drums.
We don't want anywhere near a hard notch in this case, so I'll turn the filter down a
good deal to start with, and create a gentle low mid scoop instead… and then I'm going
to add a downward expander, with a gentle ratio of around 2:1 or perhaps a touch higher.
Again, we're inverting the dynamics processing as well as the filtering, so this becomes
gentle downward compression. The resulting cut will be gentler when there's less energy
in that region, and the parts you send to this channel will all start to interact with
one another in that frequency band.
Listen to the difference when I take this channel out of the mix… the low mids get
all congested and boxy sounding… now let's put it back in… lovely!
Alright then, so the opposite of a bandpass filter is a notch. And perhaps not surprisingly,
the opposite of a notch is a bandpass filter. This works fine, even in zero latency mode.
So what's the opposite of a lowpass filter? A high pass filter of course. And the opposite
of a high pass filter is a lowpass filter. And it doesn't matter how steep you set the
filter, or what Q setting you use, as long as, and this part is important, you're using
linear phase mode.
Linear phase filters will always invert perfectly and give you the exact opposite: the pass
band becomes the stop band and the stop band becomes the pass band, and there's no phase
shift to complicate the issue.
Awesome. So again, before moving on let's think about some practical applications. First
of all this can be useful when constructing crossovers: no need to calculate a separate
high pass filter when you can just subtract the lowpass filter from the full range signal
instead. As an extra bonus, if you use this method your separated low and high bands are
guaranteed to mix back together to create exactly the original signal, in the same way
that X - Y + Y = X.
I've shown this method in a previous video, which is a bit Reaper specific, so I won't
repeat myself here.
But it can also be useful as an interesting and different way to introduce drastic filtering
for certain parts of the mix or certain parts of the song.
Here for example is a notch filter, set as steep as Pro-Q3 can make it, in linear phase
of course, and with the polarity flipped. And now any channel I send to that filter
will be hard bandpass filtered with exactly the opposite response. This gives us an easy
way to filter specific parts of the mix, without them all necessarily having to be routed to
the same sub group. In this case I’m sending everything except that deep bass part… and
as a bonus that means I’m safe to break the ‘no reverb on bass’ rule, which I
like to do at every opportunity.
Anyway, I can now bring this filtering in and out simply by automating the level of
the filter channel… and the transition between hard filtering and flat is just gradually
decreasing shelving EQ… very natural and smooth sounding.
Ok, let's grab the bull by the horns and talk about inverting minimum phase filters. Here's
a 12 dB low pass filter, in zero latency mode, and here's what we get when we turn it upside
down… well, yes, it's kind of a high pass filter. But it's not a good mirror image of
the original lowpass filter: what's with this weird resonant bump at the cutoff?
You could build a crossover like this, and it would basically work. The low and high
bands would still recombine to recreate the original signal perfectly. But the lumpiness
of the high pass filter will start to re-emerge whenever there are gain differences between
the bands, and that resonant bump might cause unwanted behaviour in dynamics processors
like compressors.
It gets worse with steeper filters: 24dB filters, which are pretty standard for crossover duties,
produce a vicious 6dB spike in the inverted response… and if you go any higher you can
expect weird notches to appear as well.
There are ways to design minimum phase crossovers that avoid all these issues, so filter inverting
tricks like this are rarely used. With one important exception: if I go the other way
and set the filter to a 6dB slope… now we get a perfect ideal inversion: a 1 pole lowpass
filter will invert to create a perfect 1 pole highpass filter.
And if I reduce the gain of the filtered signal… it turns into a low shelf cut. Don’t underestimate
how useful this can be!
Ok, final example: here’s a big deep bass part… and notice how its competing with
the kick drum. Every time the kick drum hits the low end of the mix gets muddy and confused,
plus we’re kind of losing the low end of the kick, which sounds a bit flappy and weak.
Ok then, parallel channel with a 1 pole 6dB lowpass filter, and the polarity flipped.
And note: zero latency mode, no linear phase required. And I’ll send that bass part to
this filter… and now the bass part is high pass filtered.
We’ve achieved a couple of important things: we can hear the low end of the kick clearly,
and that part is thumping again. We’ve tightened up the whole mix, by avoiding that excessive
boom on every kick drum. And the gentle 1 pole high pass filter that we’ve created
for the bass part is rolling off very smoothly without changing the basic character of the
bass part.
The big price we paid however: the bass isn’t deep enough any more! I miss that powerful
deep fundamental that we had without the filter! No problem: I’ll add a gate to the filter
channel, which I’ll sidechain from the kick drum. Now we’re only subtracting that lowpass
filtered signal when the kick drum hits, so the bass part ends up filtered briefly for
every kick drum, then goes back to flat as soon as the gate closes.
Incidentally, this is about the only situation where ReaGate's infinite attenuation is actually
optimal. Usually I reach for something else because this one lacks a range control, but
actually infinite attenuation means we return to perfectly fiat whenever the gate is closed.
Of course, the maximum 100dB range available in Pro-G will be plenty and you'll never hear
that difference, but we might as well use the optimal option if it's available.
Lets run through that again quickly so you can listen to the difference: here’s the
original balance with no filtering… here’s the filtering but without the gate... and
here with the gate in as well…
Ok I think I'll leave it there for now. Will there be another part? Perhaps… I do have
more to say, but I fear it will only benefit fellow Reaper users. I'll have to think about
it…
Thanks for watching.