Synthesis Made Simple Part 5: Intro to Filters

Next to the oscillators, the FILTER is probably the most important facet in a synth’s sound. In fact, some might argue that the filters are actually more important than the oscillators. Most of the time when people are discussing the way a particular synth sounds, the sound of the filter is a large part of what they’re talking about.

So what is a filter? You may recall at the beginning of this series, I told you about the concept of subtractive synthesis. This technique involves taking a raw sound (your OSCILLATOR) and subtracting unwanted frequencies from that raw sound. The filter is what does that subtracting. So, at the most basic level, a filter is simply a tool for removing the unwanted frequencies in a sound.

Why would we want to do this, you may be wondering? Take a look at the way real world sounds work. When you pluck a string on a guitar, you get a very bright initial sound as the pick plucks the string. After this initial brightness, the sound settles into a more tonally subdued sustain portion. Of course on an simpler level, real world sounds also simply have different tonal qualities overall. A bass guitar may have mostly low frequencies with very little high frequency content. Conversely, a tinkly bell sound may be almost nothing BUT high frequencies. Filters allow us to replicate this behavior, but also allow us to do some wilder, more synthetic/artificial effects that have no real world equivalent, such as the famous filter sweep in Rush’s “Tom Sawyer”.

The two most important parameters when it comes to synthesizer filters are CUTOFF and RESONANCE (sometimes called ‘Q’ or ‘EMPHASIS’). The filter cutoff is the frequency at which the filter does its thing. Confused? Here’s another half-baked analogy for you. Have you ever been to an amusement park and seen the sign on certain rides that says “you must be at least this tall to go on this ride”? The height restriction on that ride is like the cutoff. No one below a certain height gets through. Depending on the type of filter (more on that in a bit), either frequencies above or below the cutoff don’t get through in your final sound.

One important thing to keep in mind is that a filter doesn’t keep out ALL the frequencies past its threshold. Instead, it rolls off those frequencies somewhat gradually for a more musical sound. This is what is known as the filter’s SLOPE. The two most common slopes are 12db and 24db with the latter having a much steeper rolloff. A steeper slope generally allows sharper, punchier sounds. This is why the Minimoog (which has a 24db filter) is so famous for its punchy bass sounds, whereas many Oberheim synths (which mostly have 12db filters) are perhaps less well-suited for that type of sound.

Resonance is the second important part of a filter. Simply put, the resonance value is an emphasis or boost in the frequencies near the cutoff frequency. When I was learning to program synths, I often thought of resonance as a ‘wetness’ effect, as higher resonance levels can indeed have an almost ‘splatty’, liquid quality to them. It can also sound nasal, ringing, or bassy depending on how the rest of your filter settings are programmed. Resonance is great for making spacey synth sweeps, cutting acid basslines, and at high enough values it’ll allow you to add a discernable pitch to noise when combined with lower cutoff settings. It’s a bit hard to explain the concept of filter resonance in an easy to understand way, so to give you a general idea of what it does, here’s an audio example. The first sound is a filter sweep (which means a sound where the filter cutoff is open and closed slowly) with no resonance and the second is the same thing, but with the resonance level boosted.

Some filters have the ability to self-oscillate. At extremely high resonance levels, the filter itself will produce a sound (a sine wave) even if there is no oscillator being fed to it. This isn’t all that common on modern synths, but it can be a great tool for making synth kick drums and special effects when it's available.

Most filters have additional controls that allow the filter cutoff to be modulated (changed) automatically via the keyboard position, an envelope, or an LFO. Don’t worry if that doesn’t mean anything to you yet, we’re going to get around to those things soon. Just know that you can expect to find these additional controls on most synths.

You may be wondering exactly how you determine just WHICH frequencies are being filtered out. We’ll cover that next time when we discuss different types of filters.