By Jerry Kovarsky
|Fig. 1. The Filter Mod page in a sound program on the Korg Kronos. AMS means “alternate modulation source.” I’ve assigned the up direction of the joystick (JS+Y) to open up the filter cutoff and the down direction (JS-Y) to close it down.
SINCE WE LAUNCHED THS COLUMN IN DECEMBER 2011, WE’VE COME A LONG WAY
in exploring pitch-related performance techniques. This month, we’re changing gears
to learn some programming concepts to enhance the presets you use, and to help you
create your own presets as well. We’ll start with something nearly every synth has in
common regardless of what kind of technology it uses to generate sound: the filter.
Back when all synthesizers had knob-laden front
panels (thankfully, this design trend has been
coming back), it was second nature to reach out
and change settings during performance. Since
synths had limited preset memory or none at all,
you had to constantly interact with the synth,
unless you had a rock-star budget and could afford
a spare keyboard for every sound you needed
to make. Today, if you have an instrument with
plenty of front panel controls, or some modular
gear, you know what I’m talking about. Varying
your sound while playing keeps things interesting
and avoids the common trap of simply playing a lot
of notes—not that there’s necessarily anything
wrong with that! Let’s start exploring some ways
you can get into this, and we’ll move deeper into
associated programming and playing techniques
in the next few months.
The most common move you can make is to sweep
a filter, starting from a closed sound and opening
up, or increasing the cutoff as you go. Most synths
have a controller assigned, or dedicated to filter cutoff
, and most soft synths will have that parameter
assigned to a MIDI continuous control (CC) number
for easy access. Hold a low note and sweep the filter—
and try not to move your lips while doing it!
To be sure everyone is on the same page, I’ll
start with some basics. A filter is a component of
a synthesizer that blocks some of the harmonic
content that’s coming from the oscillator(s). It’s
subtractive, meaning it takes away from the harmonic
content of the sound. Some filters can be
considered to add to the sound due to their emphasizing
certain frequencies, but we’ll save that
for a future column.
The most common type is a lowpass filter,
which passes frequencies below the cutoff
point and blocks frequencies above it. Starting
with this type of filter “closed” will produce
a very dark or muffled sound, and as you
increase the cutoff, the sound gets brighter.
How much change you hear depends on what
you’re feeding the filter from the oscillator. If
it’s harmonically rich (like a sawtooth wave or
a sample of an organ with lots of stops pulled
out), you’ll hear a lot. If it’s is very simple (like a
sine wave), not so much.
It’s common to assign cutoff frequency to a knob,
slider, or sometimes the modulation wheel. Control
sources like an envelope can vary the cutoff over
time. So can velocity, based on how hard you hit a key.
Another common filter type is highpass,
which lets the frequencies above the cutoff
through and blocks those below it. Starting
from the lowest level, your sound will be unchanged;
as you sweep higher, you’ll hear a
brighter or thinner sound.
A bandpass filter passes frequencies within
a defined range, blocking everything but this
“slice” of harmonic content. A band-reject filter
does the opposite, letting everything through except
for the frequencies within the defined range.
|Fig. 2. Absynth’s filter cutoff and resonance are controlled by MIDI CCs 25 and 26, respectively. In turn, I’ve assigned these to the Y and X axes of an external Kaoss pad, letting me “play” both parameters with one finger.
I’ve been saying how a filter blocks frequencies,
but it’s not always a severe cut. It’s usually a slope or
rolloff , and the reduction in volume of the blocked
frequencies is measured in dB (decibels) per octave.
The steepest cut is usually a 24dB-per-octave design,
and it’s common to see 18, 12, or even 6dB designs.
Resonance emphasizes a narrow band of frequencies
right near the cutoff point, creating a sharp, nasal
quality. Increasing the resonance, holding a low
note, and sweeping the cutoff frequency, is a classic
technique for getting a wet, “squirty,” sharp sound.
With an initialized or very basic sound, try setting
the resonance of your synth around 75 percent of
the way up, then sweeping the cutoff low to high.
Adjust the resonance to various levels and get to
know what works best for your taste, with different
waveforms or samples, and for different ranges of
your keyboard. Then, “reverse engineer” the filter settings
of your favorite factory presets from your synth.
Not all synth filters are resonant (those that
aren’t don’t have a resonance control/parameter),
but these days, most are. A word of caution:
Sweeping the filter with the resonance set
too high can be hazardous to your speakers and
eardrums, so keep the volume down during your
Playing the Filter
Most modern ROMplers and workstations offer
many ways to modulate filter cutoff and resonance
beyond simply turning dedicated knobs, so you
can keep the hardwired settings but add another
controller to make “playing” the filter more easy
during your screamin’ keyboard solo. On Yamaha
Motifs, you set this up as part of a Controller Set,
which is a modulation matrix for assigning hardware
controls to various destinations. On Korg keyboards
like the Triton, Karma, M3, and Kronos,
you do it using the AMS1 and AMS2 sources on
the Filter Mod Page (see Figure 1).
In soft synths, the most common way to control
these parameters is with a MIDI CC assignment,
either factory-assigned or customized via
MIDI Learn as part of your own controller map.
Additional assignments can be added if the synth
offers a Modulation Matrix, which we discussed
in December 2011’s column.
What other controllers would you want to
sweep the filter cutoff ? If your synth/controller
has sliders, try one—it’s a different feel than a
knob. Using an X/Y pad (such as on the Novation
SL controllers, Minimoog Voyager, or Korg
Kaoss) or vector joystick (such as on the Korg
Kronos) is a very cool way to do it. In Figure 2
above, I’ve assigned the cutoff in Native Instruments
Absynth to the Y-axis (up and down) to
filter cutoff , and resonance to the X-axis (left
and right). Th is lets me interact with both parameters
using one controller, which is much
easier than trying to twirl two knobs while also
Jerry Kovarsky has had a more than three decade
career in product development,
brand management, and sound design.