Basic Synthesis: Part 2 – Filters

Filters are one of the most important parts of sound creation, and they are a foundation to the whole concept of ‘subtractive’ synthesis. The filters do exactly that – they filter out part of the sound, leaving you with a reduced portion of it, which sounds very different to the whole portion. The main control on any filter is the filter frequency, or ‘cutoff’, which is the key point at which all frequencies are cut off – be it all frequencies which are above, below, in between, or outside of the cutoff point. Common filter types include:

  • Low Pass – the most common type of filter, the low pass allows all frequencies below the cutoff point to pass through.
  • High Pass – the opposite of the low pass filter, the high pass filter allows all frequencies above the cutoff point to pass through.
  • Band Pass – allows a band on frequencies to pass through in the centre, but stops all frequencies outside of this band.
  • Band Notch/Reject – so called because it looks like a notch, this filter stops a band of frequencies in the centre from passing through.

Each of these filter types can have a different number of attenuation slope, usually 6dB, 12dB, 18dB, and 24dB per-octave – with the higher number being more effective. This is because of the increase in the steepness of each slope per-octave, so a 24dB filter is twice as effective as a 12dB filter, in relation to higher frequencies being higher pitched – e.g. 2000Hz is an octave higher than 1000Hz. Likewise, having more poles in the filter attenuates the signal more, so a 4-pole filter will create a duller, more muted sound than a 2-pole filter which is less effective in reducing frequencies.

Filters also have a feature called ‘resonance’, which basically boosts the frequency the cutoff point is currently set off. The more you boost it, the most the set frequency increases, in most cases to the point where the filter will ‘self-oscillate’, meaning that it creates it’s own sine wave – the pitch of which can be controlled by changing the filter frequency. On its own, resonance is useful for giving a sound a little more high-end, but it can really create spectacular sounds when used in conjunction with an envelope or an LFO, often creating what is known as a ‘filter sweep’.

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