Title: The Synthesizer – Filters Description: The filter unit is one of the most important components of a synthesizer. This article introduces the main filter types: low-pass, high-pass, band-pass, and notch filters. Author: Raphael Petri
The Synthesizer Filter
So far, we have only looked at the oscillator, the component of a synthesizer that is at the very beginning of sound and noise generation. Before the signal generated by the oscillator is amplified and sent to the speaker, it can be modified by additional components. One of the most important of these components is the filter.
The filter processes this signal in such a way that only a part of the original signal remains, or certain parts of the signal are emphasized and brought to the forefront.
The Different Types of Filters
There are four main types of filters:
Low-pass Filter
This filter allows low frequencies to pass through while attenuating high frequencies.
High-pass Filter
This filter allows high frequencies to pass through while attenuating low frequencies.
Band-pass Filter
This filter allows a specific range of frequencies to pass through while attenuating frequencies below and above that range.
Notch Filter
This filter attenuates a specific range of frequencies while allowing frequencies below and above that range to pass through largely unaffected.
Cutoff and Resonance
The exact position of the filter can be adjusted using a parameter called cutoff. The cutoff frequency is measured in Hertz (Hz) and determines the point at which the filter begins to affect the signal.
For example, if the cutoff frequency of a low-pass filter is set very low, most of the higher frequencies will be filtered out, leaving only the lowest frequencies of the signal audible.
With a high-pass filter, however, a low cutoff frequency would mean that most low frequencies are still allowed to pass through without being attenuated. To hear mainly the very highest frequencies, the cutoff frequency would need to be set much higher.
The cutoff therefore determines the position of the selected filter's frequency range.
Increasing the resonance causes the frequencies around the cutoff frequency – the exact point where the filter starts to take effect – to be amplified.
As a result, changes to the cutoff become much more noticeable when moving it while playing the synthesizer. The filter becomes more prominent, and the overall 'filter effect' is considerably stronger.
The Sine Wave
As we have already learned, the sine wave is the only waveform that consists of just a single frequency. When we apply a filter to it, we can only make this one frequency quieter, since there are no other frequencies available to be filtered.
At the same time, other waveforms can be filtered so extensively using a low-pass filter (by setting the cutoff frequency as low as possible) that only their fundamental frequency remains audible.
At this point, the waveform is effectively 'transformed' into a sine wave.
Here is an opportunity to try this out for yourself: