The voltage-controlled filter is a major part of subtractive synthesis.  There are many different ways to design a filter, and although they basically do the same thing, they all sound a little bit different.  The filter is the part of the synthesizer that is most responsible for shaping the tones you get out of it, and it gives each synth its own character and unique sound.

In general, filters block some things while letting other things through.  A filter in a synthesizer is no different; it blocks some frequencies while letting others through.  There are several different types of filter you can choose from depending on which frequencies you need to block/pass to achieve your desired sound.

Filter Types (Modes)

A low pass filter passes (allows) frequencies below a certain point known as the cutoff frequency.  Frequencies above the cutoff frequency are blocked.  This filter lets you cut the high-end out of your signal or get rid of harmonics.

Low pass filter sweep

A band pass filter passes frequencies around the cutoff frequency, but blocks the ones above and below it.  It only allows a narrow band of frequencies through.  It is useful if you are trying to highlight a certain frequency.

Band pass filter sweep

A high pass filter passes frequencies above the cutoff frequencies and blocks the ones below it.  It lets you cut the low end out of your signal or only allow harmonics through.

High pass filter sweep

A notch filter blocks the frequencies around the cutoff frequency and lets all others pass.  This can come in handy if you have one particular frequency you want to block (maybe it’s causing feedback or just sounds bad).

Different synths or filter modules may have one or more than one type of filters in them.  Sometimes they’re all available at the same time from different jacks, and sometimes you can switch between them using a switch or a knob.

Cutoff Frequency

The above descriptions are just generalizations; signals aren’t usually completely passed or blocked on either side of the cutoff frequency.  The gain actually starts curving down then dropping steadily.  Let’s take a look at a low pass filter as an example:

Note that the cutoff frequency isn’t the point where the filter starts affecting the gain.  It’s the point where the gain is -3dB (half).

The cutoff frequency can be adjusted on almost all VCFs with a knob or a control voltage.  This is where you can start using your filter to create more dynamic sounds.  Maybe you patch a low pass filter so that the cutoff is fairly high when you first play a note, then it drops down as the note is held.  This gives you a note that is bright when it is first hit and gets more dull over time, just like many acoustic instruments do when they are played.

Slope

The rate at which the gain drops after the knee at the cutoff frequency is known as the slope of the filter.  As you’re shopping for a filter, you will see slope specs like 12dB/octave or 24dB/octave.  This means that every time the frequency doubles (goes up an octave), the gain drops 12dB or 24dB respectively.  Sometimes the slope is described in “poles”, with each pole equaling 6dB.  A 2-pole filter would have a slope of 12dB/octave, a 4-pole filter would have a slope of 24dB/octave, etc.

6dB isn’t a very steep slope for a filter.  You can use it to brighten or muddy up your sound, but it doesn’t really cut any frequencies to the point where you can’t hear them.  The tone knobs on your old stereo probably have a 6dB slope.

12dB will filter out most of the sound, but you’ll still be able to hear some of it.

24dB is pretty steep.  You can use a 4 pole filter to cut unwanted frequencies to the point where you won’t notice them.

Resonance

Turning up the resonance knob (or increasing the control voltage) on your filter boosts the frequencies right around the cutoff frequency.  It also tends to attenuate the lower frequencies (some filters compensate for this so you don’t lose your low end as you crank up the resonance).  The width of the peak where the frequencies are boosted is called “Q”.

As you continue to turn up the resonance, the sound will start to ring a bit right around the resonant frequency.  If you turn up the resonance even more (if your filter will let you; some won’t) then the filter will start to self-oscillate and you’ll find a sine wave at the output.  The peak is so tall and narrow that very little of your input signal will be coming through.  If your filter is calibrated so that it tracks with your keyboard then you can use it as a sine wave VCO.

Note: While they can be used in this way, VCFs don’t make very good replacements for VCOs.  Oscillators are very sensitive to changes in temperature, and a true VCO has extra circuitry to compensate for this and make it more stable.  VCFs don’t, so they’re not very good at staying in tune.  On some of them you can drastically change their pitch just by blowing on their circuit boards.

Let’s take a look at an actual VCF.  This is the Filtare SEIII by Division 6.  It’s a Eurorack filter that is designed to sound like the SSM2040 filter chip, which is no longer in production.  It is a 4 pole filter, so its slope is 24dB/octave.

It has 4 different filter types, and they each have their own separate output jack.

The notch output is a little unique in that it has a “balance” control that lets you shift between all low pass and all high pass.  When set in the middle, it passes both low and high frequencies with a dip (notch) in the middle.

All the controls on this filter can either be controlled by a knob or by an external control voltage.

There are two CV inputs for the cutoff frequency.  One has a level knob so you can adjust how much the incoming voltage affects the frequency.  The other is a fixed 1 volt per octave, making it easy to use this filter as a sine wave oscillator if you want (there’s also a trim pot on the back so you can tune it).