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SOUND SYNTHESIS TUTORIAL (IV)

Tutorial about synthesis of sound: introduction to analogue synthesis (filters).

Introduction to analogue synthesis - Filters

This chapter will focus on the characteristics of several basic types of filters that are commonly found in analogue synthesizers. A filter allows to remove or boost certain frequencies; which of them are removed or left depends on the type of filter used. Before starting to learn about a few common types of filters it is recommendable to understand some concepts about filtering. Filters are called like this because they add nothing to the sound but, on the contrary, they always subtract something. When applying filters to the sound we have to make sure that the effect achieved is more important than the removed signals. However, because of their variety, filters 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 subtract signals. 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 which 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 warmer sound than that generated by 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 determines whether the sound tends to be warmer or punchier.

On the filter knobs found on analogue synthesizers we 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 are known as two-pole or four-pole filters and each pole represents 6 dB of attenuation. On this moment this is not an essential concept to know, but just a detail which adds something maybe unexpectedly useful to the infinite ladder of knowledge. Let us see now some of the most common and simplest types of filters:

- Low pass filter (LPF): this filter attenuates frequencies 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) which often dwells on the higher frequencies. But in return, this filter will reduce the brightness of the sound, for the attenuation of the higher frecuencies affects the higher frequency harmonics.

Low pass filter (LPF)

- High pass filter (HPF): this filter is the opposite to the low pass filter; it attenuates the frequencies which are lower than the cut-off frequency, leaving intact the rest of frequencies. Because of the attenuation of the lower frequencies, this type of filter can be useful for creating a sense of brightness in a muddy sound; but obviously, 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 an harmonics generator instead.

High pass filter (HPF)

- Band pass filter (BPF): this filter attenuates any frecuencies below and above a certain band of frequencies around the cut-off point. 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 (BPF)

- Band reject filter (BRF): this filter, also known as notch filter, is the opposite to the band pass filter, as it attenuates a certain range of frequencies around the cut-off point. 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 (BRF)

The following pictures show the controls that are present in the filter banks of the MiniMogue VA (left) and the Arppe2600 VA (right) analogue synthesizers. in the panel of the MiniMogue VA we can see cut-off controls which are independent for each channel, along with other additional controls (labelled as Filter Contour we can see the four knobs for the filter's ADSR envelope). In the panel of the Arppe2600 VA we can see additional parameters such as resonance or filter color (which just means the type of filter to be used - LP or HP, in this case). 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).

MiniMogue Luxus filter bank Arppe2600 filter bank

In this chapter I have showcased only the most basic concepts about filters; in the next chapter I will continue exposing the fundaments of subtractive synthesis and analogue synthesizers.

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