Filters: Going Deeper

Filters: Going Deeper

10 December, 2007 | 2.09PM

• Section: Music News
• Written by: francispreve

Last week, we looked at the four most common filter modes and how each affects the oscillators’ sound.

This time out, we’re going to go a bit deeper into the specifics of the filter parameters.

As many readers have discovered, some synthesizers are extremely complex with elaborate functionality in each sound-processing module.

Other synths are more much straightforward.

To keep things consistent, we’ll be focusing on parameters that are common to all filter implementations.

Filter Parameters

Each filter includes an array of parameters that govern its overall sonic behavior.

The two main parameters are frequency (sometimes called the “cutoff” or “center” frequency) and resonance (also referred to as “emphasis” or “Q”).

Other common parameters include keyboard tracking, filter mode and modulation amounts for the LFO and envelope.

Frequency

The aptly named frequency knob (or slider) determines where the filter begins operating on the oscillators’ frequency content — subtracting highs, lows or mids, depending on the filter mode selected.

In the case of a low-pass filter, this knob serves to select the cutoff frequency, above which all higher frequencies are attenuated.

In a high-pass filter, the cutoff frequency has the opposite effect and all frequencies below the cutoff are lowered in volume.

In band-pass and notch modes, the frequency parameter sets the center frequency of the filter.

This is similar to the behavior of an EQ’s frequency knob.

In any of the modes, sweeping the cutoff frequency manually, or with a modulation source, will yield dramatic results.

Resonance

The resonance parameter is sometimes referred to by different names.

On vintage Moog synthesizers, like the Minimoog pictured on the right, it’s called “emphasis”.

On Ableton’s synthesizer devices it’s abbreviated it as “Res”, whereas Ableton’s Auto Filter device labels it “Q.”

In high- and low-pass filter modes, the resonance knob emphasizes the cutoff frequency by increasing the amplitude of the signal at that exact frequency.

At medium values, this creates a squelchy effect that gives a synth that “boingy” sound.

At higher values, a whistling sound is superimposed on the signal content.

This whistling is called “self-oscillation” and is often associated with the TB-303 sound of acid house, as well as funky 1970s and 1980s sounds.

In the case of band-pass and notch filters, things can get a little confusing.

On many synths, the band-pass resonance knob works in a consistent manner, emphasizing the frequencies at and/or near the center frequency.

On others, it serves a somewhat different function, setting the width of the range of frequencies that are passed or attenuated by the filter, much like the Q parameter of an equalizer.

The difference can be subtle, so when in doubt, use your ears!

Keyboard Tracking

Sometimes abbreviated simply “keyboard” or even “kbd”, this parameter allows the filter frequency to track the pitch of the keyboard with higher notes raising the frequency and lower notes reducing it.

Why is this useful?

Think of it this way.

Say you have a low, muted bass tone with a low-pass cutoff frequency tuned to around 260Hz (approximately middle C on a keyboard).

When you play your bassline, everything works as predicted, but if you jump above middle C, the sound becomes quieter.

If you play higher still, the sound fades away entirely.

This is because the filter cutoff frequency is set lower than the notes you are playing, so it cuts off those very notes!

By adjusting the keyboard tracking parameter, you “tune” the filter to the keyboard so that the higher you play, the greater the cutoff frequency.

This creates a more consistent behavior for certain types of sounds.

With high, self-oscillating resonance values, keyboard tracking can even be used to turn the filter into an additional oscillator.

Envelope Amount

The vast majority of subtractive synths allow the filter frequency to be modulated, creating dynamic motion in a sound.

The most common type of filter modulation is envelope modulation.

Ableton’s Sampler, shown above, has a graphically adjustable envelope for its filter.

Envelopes control how a sound changes over time.

For instance, to create that “wow” sound, you could use a filter envelope with a longer attack.

To create a percussive sound, you could create an envelope with a short attack and a rapid decay.

The overall intensity of the envelope’s effect on the filter is governed by the filter’s envelope amount control.

Note: Envelopes will be discussed in greater detail in a future lesson.

LFO Amount

LFOs are a terrific source of repeating often rhythmic effects like vibrato and tremolo.

When applied to the filter frequency, they can create undulations that give a sound a lot more character and motion.

One of the most dramatic examples of this effect is Timo Maas’s legendary remix of ‘Dooms Night’, which has several LFOs routed to various destinations, including low-pass filter frequency.

As with envelopes, the specifics of LFOs will be discussed in an upcoming tutorial, but in the meantime, I highly recommend exploring the effects of this parameter.

Start with a low-pass with a low cutoff frequency, say 25-35%.

Then increase the value of the LFO amount until you hear its effect on the filter.

From there, you can experiment with various LFO settings.

Coming up…

Take some time to experiment with these parameters.

Once you understand the filter modes and functions of each knob, we’ll dive into the practical uses of filtering.

• (3) Comments
• (740) Views