![Making acid sounds with a low-pass filter [Fusion Core VCF #1]](/articles/filter-1-intro/feature_hu_b6ec02e56a335fae.png)
Hi!
The next synthesizer module on my to-do list is a voltage controlled filter. I love it for its capability of shaping sounds! To understand what a filter is (in this case a so-called low-pass filter), here a vibe-coded widget:
Introduction: What is a filter? #
I hope, the vibe-coded widget above provides some clarity – I encourage you to play with the buttons and sliders! What do you hear?
The widget shows a sound signal chain: The topmost block outputs a sound signal of a beat or melody I put together some other time. It is plugged into a so-called low pass filter, which modifies the sound somehow. Finally, the output of the filter goes into the audio output block.
The main objective here is the filter in the middle – my goal is to build it using analog electronic components in real life!
Please, in the sound source, select “beat.wav” and press start.
You initially hear a pretty dampened beat. When sliding the cutoff frequency-knob to the right, then the beat becomes crisper. It is like a wall going away between you and the beat.
Please, in the sound source, select “melody.wav” and press start.
The melody contains notes with increasing pitch (or frequency). The low-pass-filter makes the notes quieter at some point (you call that cutoff frequency).
The filter diagram #
The window inside the low pass filter block shows a green line and if the sound is activated, a wobbling, blue-shaped area. It actually is a diagram with frequencies (tone pitch) on the x-axis and the volume / sound level on the y-axis.
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The blue area shows the so-called “sound spectrum” of the melody or beat at the current time frame. The height of the area at a specific horizontal x-position indicates, how loud the frequency is in that specific moment. Take a look again, try the “melody.wav” in the source block and watch the melody notes with increasing pitch are shown in the filter’s blue area as peaks wandering to the right!
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The green line is the big thing, the filter is about - it indicates, how much which frequencies in the sound spectrum get attenuated. It is called the filter frequency response or filter transfer function. The lower the green line, the quieter. If we reduce the cutoff frequency (slide it to the left), less high frequencies pass through the filter – so that at some point, we only hear the bass. In our case, the green line starts flat and then decreases: It passes the low frequencies and attenuates the high frequencies. That’s why our filter is called, a “low-pass” filter!
Please, in the sound source, select “melody.wav” and press start. Then in the filter, slide the cutoff-frequency to about 33%
Do you observe, that with increasing tone pitch, the melody gets more quiet?
And now, try the same with the “beat.wav” source.
Even though, we don’t hear a proper melody with notes of individual pitches, the filter also makes a huge difference in the sound!
Now, what was the point? #
In audio synthesizers and digital audio workstations, filters in general are great for timbre-shaping and sound and also used in equalizers to manage the volume of bass, mids and treble. Of course, the filter frequency response can also look different – in our case, it started flat and then went down (low-pass behavior). But it can, for example, start rising and then stay flat! That’s called a high pass filter.
Acid synth performance #
A low pass filter with a high resonance is needed to make acid techno sounds! In VCV rack, I put together some more synthesizer modules to make a small acid techno jam.
Btw., VCV rack is free to download for linux, mac and windows!
Here is a proper track with acid sound as main content (starting at second 31):
Acid Zdoch
Provided to YouTube by Tripalium CorpAcid Zdoch · MaelstromAcid Avengers 021℗ Tripalium CorpReleased on: 2022-03-18Composer: Joan-Mael …
The challenge #
For electrical engineers, filters are a core concept used for many applications different to sound design: In telecommunications, one uses a filter to listen to only one selected radio frequency span, and in a robot car, it’s used to control the torque of electrical motors to move exactly to a given distance. Engineers put a lot of thought into it!
As said, I want to develop a filter module for my synthesizer myself! In hardware, using low-level electrical components.
In the following chapters, we’ll combine a resistor and a capacitor to build such that. Then, we’ll measure that filter response graph and how to make the cutoff-frequency adjustable!