Filters are another important part of a Eurorack system. Like its name suggests, a filter removes part of the signal, and lets the other part of the signal pass through. The type of filter determines what part of the signal gets filtered, and what passes through.
The most common types of filters are low pass, high pass and band pass. Low pass filters let low frequencies pass through and filter out high frequencies, which can give things a sort of underwater quality. These are sometimes referred to as high cut filters because they are cutting the high frequencies.
High pass filters do the opposite, allowing high frequencies to pass through while filtering out low frequencies. High pass filters are often used to create a rising sensation during the breakdown of a song to emphasize the bass drum once it kicks back in.
Band pass filters are a combination of high and low pass filters. They cut the signal above and below the cutoff frequency, letting only a narrow frequency band through. Notch filters, or Band reject filters, do the opposite and remove a band from the signal, which can create a swishy effect similar to a flanger.
The filter cutoff frequency is the point where the filtering begins to occur. By sweeping or modulating this cutoff frequency with various sources a huge range of timbres can be achieved.
The number of poles, or slope of a filter determines how gradually or abruptly the signal is filtered beyond the filter’s cutoff point.
A filter’s resonance, or Q refers to the amount of feedback applied at the filter’s cutoff point. This is sometimes referred to as emphasis because it emphasizes and exaggerates the filter’s cutoff point. Some filters have the ability to self resonate with high resonance or Q. This means that without an input signal connected, the filter is able to generate enough feedback at the cutoff frequency to generate a tone. This tone takes the form of a pure sine wave, which enables self resonating filters to also function as oscillators.