Fun With Filters

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And so starts the stupendous summer of synths! After establishing the basis of synthesis, the operations of oscillators, and the world of waveforms, we’ll kick off the summer with a look at filters. Like in weeks past, we’ll be using the Minimoog Voyager as our example of how a specific filter works, but first, let’s discuss filters on a broader scope.

A filter in a synthesizer is just like a filter in any other facet of reality: we’re sending a medium through a filter to remove an unwanted part of that medium. In this specific case, we’re sending sound through a filter to remove certain frequencies of said sound. Generally speaking, we’re either trying to remove low sounds, high sounds, or occasionally both.

When you’re trying to remove high sounds, you call it a “low-pass” filter as the low sounds are what are passing through. When the low-pass filter is off, you’ll have a rich, full, bright sound. As you filter more of the high sounds out, the tone will become softer, darker, and more mellow.

By that logic, a “high-pass” filter will allow the high sounds through and will filter out the low ones. Once again, not having a filter will result in the full sound. This time, though, as you filter out the low sounds, you lose a great deal of body and depth, so the sound becomes very nasally and tinny. It can almost have a kind of “mosquito” quality to it.

You can also have a low-pass and a high-pass filter work together to only allow sounds in the middle to come through. This resulting “band” of sound that makes it through the filter is why a combination of these filters is called a “band-pass” filter. Not only does this allow you to control where the highs and lows begin and end, but it also allows you to choose how large of a “band” makes it through.

With the basis of filters sorted, we can put it to practical use with the Minimoog Voyager’s filters. And yes, that’s supposed to be plural! The Voyager actually has two filters that can be set in two different ways: dual low-pass or high-pass/low-pass.

In dual low-pass mode, two different low-pass filters are working together. If you were listening with headphones, one filter would be assigned to the left ear and the other to the right ear. This can allow for some very unusual and unique sounds in that you can essentially have two different sounds going on at once! Since most of the recordings and videos will be in mono, as I am just using one cable to send signal out of the Voyager, you won’t really be able to hear this effect, so we’ll save it for something more in depth further on down the road.

In high-pass/low-pass mode, you have one of each filter. It’s essentially how you create a band-pass filter. You’ll notice that there isn’t a standalone high-pass filter for the Moog. You can get that effect though by letting all the high sounds through the high-pass filter while letting very few low sounds through the low-pass filter. Basically, you’ve made a super small “band” of sound where only the high sounds can get through.

To set these filters on the Voyager (and many synths), you have what’s called a Cutoff Knob. This cutoff point is the frequency at which sounds (high or low) can no longer pass through. The Voyager Cutoff Knob can go from 20hZ to 12KhZ (which is close to the entire audio span of human hearing!).

Right below the Cutoff Knob is the Spacing Knob. For the Voyager, the Spacing Knob acts in two ways. For the Dual Low-pass mode, it essentially acts as the Cutoff Knob for one ear while the main Cutoff Knob functions for the other ear. In High-pass/Low-pass mode, it acts as the Cutoff Knob for the High-Pass filter (which establishes the size of that “band”).

Below that is a particularly interesting knob: the Resonance Knob. When you set the Cutoff Knob, you’re setting it as specific frequency. That frequency can be emphasized by using the Resonance Knob. It often gives the sound a vocal-like quality but can lead to many other interesting effects. On top of that, when you turn the Resonance Knob up to its highest settings, it will emphasize the frequency so strongly that the filter begins to self-oscillate and winds up creating a pure sine wave! This is how I made a sine wave in the last post regarding waveforms, as the Moog oscillators don’t naturally create sine waves on their own.

Then we come to a potentially confusing knob: Keyboard Control Amount. It boils down to this: if you have this knob turned all the way up, it means that as you play higher, the Cutoff Frequency gets higher, too. It’s as though you had an invisible hand turning the Cutoff Knob as you play higher and lower on the keyboard. If you have this knob turned all the way down, the invisible hand goes away and the Cutoff Knob stays the same regardless of the pitch you play. This can be very helpful if you want to make your higher sounds stick out (turn up the knob) or make them more muted and mellow (turn down the knob). Additionally, the use of this knob can really help emulate acoustic instruments in different ways.

Lastly, you come to the one switch amongst all these knobs. It’s just a red switch to go back and forth between Dual Low-pass and High-pass/Low-pass.

With all the knobs and switches explained, let’s end with a video showcasing all of these items back to back using the Dual Low-pass mode:

Next time on Synth You Asked: the power of envelopes and A.S.D.R.!