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That's Not an Oscillator!

Unconventional Sound Sources for Your Eurorack System

Robin Vincent · 12/05/24

The oscillator is traditionally the primary source of sound in your modular system. They can conjure up waveforms through analog circuitry or reconstruct shapes using digital wizardry. They can be fat, fun, fruity and fabulous, but they are not the only source of audio signals in your rack. With a little bit of bending and a fistful of abuse, we can subvert other innocent modules into having a voice. Sometimes it's by design, and other times, it's through the threat of violence.

Let's learn how.

Filter Self-Oscillation

The most common and probably the sweetest source of sound is the pure and lovely sine waves that grow on the edges of waveforms that are pouring through a fully resonant filter. It's a fascinating thing to watch on an oscilloscope. Before you pile on the resonance, you can witness a complex waveform get pared back to simpler forms as harmonics are removed by a drifting cut-off. But as you lean into the resonance, a higher sound emerges on the waveform, as if it is starting to boil and overflow with pure tones.

Filter resonance is a feedback circuit that is used to emphasise the frequency around the position of the cut-off. The cut-off is the point at which the filter begins to reduce frequencies to zero. As you use resonance to feed more of the output back into the input there becomes a point where the energy is large enough to sustain the feedback and we are treated to the ringing of self-oscillation.

[Above: an excelletion video about filter-self oscillation prepared by Marc Doty on behalf of the Bob Moog Foundation, using the Minimoog for demonstration.]

This is very much like pointing a microphone at a speaker, but where that tends to be uncontrolled and undesired, feedback in a filter is met with the warm embrace of nonlinear saturation in the amplifier designed to reign in the instability. The results can be very, very nice. With the right amount of resonance, the filter will self-oscillate beautifully, and the cut-off becomes the tuning knob. Many filters have a 1v/Oct input to the cut-off in order to use the self-oscillation in a musical context. There's an added bonus that as the sine wave is generated naturally, it's actually much purer than ones made from a triangle core oscillator circuit in your VCO.

Of course, you may not want a nice sine wave, in which case there are a few ways you can subvert the beautiful self-oscillation into something gnarlier. The filter itself may have the ability to overdrive and flatten the edges, so that's a good place to start. From there, a wavefolder would be a really good way of giving sine waves some flavour. One neat trick is to use a clock divider. A clock divider will happily receive an audio rate sine wave as the input clock. If you patch the half clock output to a mixer, you will find it to be a pulse wave running at an octave lower. It’s produced a very useful sub-oscillator. You could take other outputs too, giving further notes depending on the ratio.

Filter Pinging

There's another way you can use resonance in a filter to generate sound, and that's by giving it a nice big ping. You can "ping" any resonant system to produce a high, resonant tone. One example is that of a wine glass. The glass is a system that can resonate and all it needs is for you to excite it. You do that by flicking it with your finger and you get a ringing tone. There are all sorts of things in modular you can use as a resonant system but the easiest and most common is the filter. For the role of the ping we need something that will inject a short burst of excitement. This would normally be a trigger signal.

[Above: filter pinging diagram from a vintage manual for the Serge Modular Music System, regarding the Variable Q Filter (VCFQ).]

So, patch your trigger into the input on the filter and turn down the cut-off so you're not really hearing the trigger pulse. Turn up the resonance, and before you reach self-oscillation, you will hear a beautiful ringing sound from each trigger. It could be seen as controlled self-oscillation with a quick decay, but the use of a trigger introduces a percussive and potentially rhythmic element that regular self-oscillation doesn't possess. The shape of the trigger can have an effect on the tone of the sound. Introducing a bit of an envelope to it can have interesting results. You could also add a short envelope to the CV input of the filter cut-off and once you tune down, you'll find yourself with an 808-style analog kick drum—groovy.

Be sure to check out the dedicated Signal article all about filter pinging.

Filtered Noise

Another potentially melodic use of a filter is to run a noise generator through it. Filtering noise starts off as the sound of wind or the sea but as you increase the resonance it starts to take on a pitch and tone of its own. If you narrow the filtering down to a band-pass filter you can find a range of sizzling percussive tones before it falls into its own self-oscillation. Add a couple more resonant band-pass filters and you could find yourself generating chords.

Karplus-Strong

Moving in a completely different direction, we have a very simple method of synthesis called Karplus-Strong that has a very remarkable sound. Karplus-Strong is an early form of physical modelling and is an algorithm designed for digital simulations to generate the sound of a struck string. But the marvellous thing is that it can be replicated in an entirely analog modular system.

All you need is a short burst of noise and a delay module with a really short delay time. It's a similar process to pinging a filter. You fire the noise in and dial down the delay until you begin to get these percussive string textures. The delay feedback controls the length of the decay, while the delay time, provided that it remains short, can control the pitch. You get plenty of interesting tones around that of a recognisable string pluck which makes it a lot of fun to play with.

Really, Really Fast Control Voltage Generators

Anything that generates a voltage change can ultimately be used as an oscillator if you spin it fast enough. Many LFOs have the ability to spin up to audio rates but they are designed to oscillate; what about something that isn't? An interesting module to wind up to audio rate is the envelope, or even better, a slope generator.

The idea is that the rise and fall of a slope generator, or attack/decay of an envelope, is essentially the shape of a waveform. So, if you can trigger it with a pulse running at audio rate (an audible frequency) like from a fast-moving LFO, Clock or even a square wave oscillator, you will get sound from the output. You can then use the Rise/Attack and Fall/Decay knobs to shape the waveform into ramps, sawtooths, triangles and other things. Slope generators can work better because they tend to complete their cycle regardless of the gate length of the input.

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Oscillation experiments using Make Noise XPO oscillator to drive Make Noise 0-Ctrl sequencer. First, gradually changing sequence stage values; then, sequencing the XPO while gradually changing sequence values; then a more complex patch using XPO, 0-Ctrl, and Bastl Ikarie with complex cross-modulation.

Try clocking a sequencer at audio rate and—provided there's a difference between some of the steps—it can generate an audible waveform. Altering the voltage on each step will create different colors, as will reducing or increasing the step count. Interesting things happen when you clock the Music Thing Turing Machine at audio speeds. As you turn the central knob, you can move from noise, through uncertainty and into some pretty pure tones. The Scale knob controls the level as it dictates the size of the movement, and so the amplitude of the waveform.

Feedback

Finally, we’ll return to feedback loops, but this time, we will be taking in one or more modules rather than just part of the internal workings of a filter. This is perfect if you want to get a bit experimental. The basic idea is that you take a patch cable and plug the last output into the first input. That may require a bit of multing or mixing of signals but it's usually possible to do. Be mindful that feedback can overwhelm the audio outputs and potentially do nasty things to your speakers, so do take care.

A good place to start is with a stereo reverb. Being stereo it has a built-in feedback option where you can plug either output into the mono input and find that sound will bellow menacingly out of the other output. The size of the reverb will control the pitch, the feedback will sculpt the ferocity, and you might also have some tone controls. The trick to finding the feedback is in the Dry/Wet knob. You can do something similar with stereo delays or really any audio processing module. (In the video above, Perfect Circuit's own Jacob Johnson demonstrates ideas about using reverb feedback as a sound source—albeit with pedals, not Eurorack modules.)

To set up an experimental chain of feedback patch together a series of modules such as a filter, a wavefolder, delay, frequency shifter and so on. Mult the output and take one through an attenuator or mixer, back to the input of the first module. Turn everything down to remove any squealing and then slowly start to turn things up. Add small amounts of CV or slow LFOs to some of the controls and you will become a feedback master.

Rethinking Signal Flow

In this article, we've talked about generating sound without oscillators, but when it comes to playing with feedback loops they are also fabulous things to use with regular oscillators. They can be patched back into themselves for dynamic FM or waveform shaping. Or you can start the feedback loop at the filter and have your regular oscillator driving the signal. Many filters have more than one input which makes it easy to patch the output of the chain back in again. There's always another way of doing something in modular.