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Sound Synthesis Tutorial

:: Introduction to analogue synthesis - Filters

This chapter will focus on several types of filters. Only the simpler of these filters are usually used in subtractive synthesis, but knowledge of what the rest of them do is always useful. A filter allows to remove unwanted frequencies or boost certain frequencies. Which frequencies are removed or left depends on the type of filter used. Before starting to learn about some common types of filters, it is recomendable to understand some concepts about filtering. And always remember that filters add nothing to the sound, but on the contrary they always subtract something, and that is why they are called filters. When filters are applied to the sound, one has to be sure that what they contribute to the sound is more important that what they remove from it. However, filters, because of their variety, are a great shaping tool for any type of sound and their applications are innumerable.

The first concept to learn is cut-off frequency, which is the point (frequency) at which the filter begins to filter. The filter will block or attenuate the amplitude (volume) of the frequencies that are below or above the cut-off frequency, depending on the type of filter used. In modern synthesizers that work with digital technology and algorithms, this process of attenuation can be instant, but in analogue synthesizers, whose filtering circuits are based on resistors and capacitors, it takes some time for the filters to attenuate frequencies, in proportion to the distance that those frequencies are from the cut-off point. This kind of gradual, sloped cut-off produces a sound that is warmer than the one produced with more modern synthesizers. The speed at which a filter attenuates is called slope or gradient, and the process itself is called attenuation. The slope, which is measured in deciBels per octave, creates a transition band that will determine if the sound produced tends to be warmer or punchier.

On the filter knobs found on analogue synthesizers, one can often see values of 12 dB or 24 dB of attenuation per octave. This means that each time the frequency doubles, the filter attenuates by 12 dB or 24 dB any signal in that frequency. These filters are known as two-pole or four-pole filters, each pole representing 6 dB of attenuation. Well, at this moment this concept is not essential to know, but just a detail that adds something, maybe unexpectedly useful, to the infinite ladder of knowledge. So let's see now some of the most simple and known types of filters:

- Low pass filter (LPF): this filter attenuates the frequencies that are higher than the cut-off frequency, leaving intact the remaining frequencies. Because of the attenuation of the higher frequencies, this kind of filter is useful for creating a bass effect on the sound or for attenuating any noise or hiss (artifacts) that are often found on higher frequencies. But in return it will reduce brightness on the sound, since the attenuation of higher frecuencies affects the higher frequency harmonics.

Low Pass Filter

- High pass filter (HPF): this filter is the opposite to the low pass filter; it attenuates the frequencies that are lower than the cut-off frequency, leaving intact the remaining frequencies. Because of the attenuation of the lower frequencies, this kind of filter can be useful for creating a sense of brightness in a muddy sound; however, this is done at expenses of the lower frequencies. When working in audio processing, for doing this kind of task without affecting any frequency present on the sound, it would be advisable to use a harmonics generator. But that is another tale...

High Pass Filter

- Band pass filter (BPF): this filter attenuates any frecuencies below and above a certain band of frequencies around the cut-off point. So in these filters, apart from the cut-off frequency, it should be possible to modify the range of allowed frequencies (pass band) around the cut-off point.

Band Pass Filter

- Band reject filter (BRF): this filter, also known as notch filter, is the opposite to the band pass filter; it attenuates a certain range of frequencies around the cut-off point. So in these filters, apart from the cut-off frequency, it should be possible to modify the range of attenuated frequencies (stop band) around the cut-off point.

Band Reject Filter

These filters that I have listed are certainly the most basic ones and those that are commonly found in analogue synthesizers. The following images show the filter controls that are present in the MiniMogue VA (left) and the Arppe2600 VA (right) analogue synthesizers; those two are great free VST synths that are available for download in this website and I recommend you to take them if you do not have them yet. On the MiniMogue VA panel you can notice the cut-off controls that are independent for each channel, along with other additional controls; labelled as Filter Contour you can see the four knobs for the filter ADSR envelope. On the Arppe2600 VA panel it can be seen additional parameters such as resonance or filter color - which just means type of filter to be used (LP or HP) -. Note how the filter bank is labelled as VCF (voltage controlled filter); in more modern synthesizers this is often replaced by DCF (digitally controlled filter). In this chapter I have explained the most basic concepts about filters and a glance at those small images shows that there are still much more facts to learn about filters. But that will be in a further chapter. In the next chapter I will continue exposing about the fundaments of subtractive synthesis and analogue synthesizers.

MiniMogue Luxus Filters Arppe2600 Filters



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