Synth Programming – Filters

Sieves, AD 1568 (wikimedia commons, public domain)

Sieves, AD 1568 (wikimedia commons, public domain)

I’ve posted about base synthesizer waveforms yesterday. The signal from the oszillator(s) most often gets used to feed a filter. Almost all synths at least provide a lowpass filter. Some synths provide a multimode filter, which may contain lowpass filters in different characteristics, a highpass filter, a bandpass filter, a notch filter and maybe a comb filter. Those filters are not static. Instead one can change the cutoff frequency of the filter, which effectively means that the range of filtered partials can be influenced.

The lowpass filter is the most commonly used filter. It cuts all partials above a certain freqency, resulting in a less brilliant sound. Applied to a sawtooth wave, which contains almost all partials, the result is a dramatically changed sound. In the following example, the filter first is completely open. Then it slowly closes until no frequencies are left at all. Then it opens again:

A Sawtooth wave filtered by a lowpass filter

Additionally to the cutoff frequency, most filters provide a resonance parameter. It raises the level around the cutoff frequency, resulting in a narrower sound:

Sawtooth filtered by a lowpass filter with resonance applied

Most synths allow the musician to automate the filter cutoff by some envelope. In the filter section, there’s a parameter called »Envelope Amount« or similar. In this example, first the filter gets set up. Then the envelope gets adjusted to achieve a pad like sound. Here we go:

  • Most synths provide a init feature for sounds, which sets the instrument to a sawwave and a basic filter setup. It is recommended to use it.
  • Go to the filter section.
  • Set the cutoff frequency of your filter to a value so that the filter is almost closed, but still some frequencies are audible.
  • Raise the Resonance a little bit, but not too much, as this may result in some filter self oscillation.
  • Set the envelope amount to a value of about one quarter of its range.
  • Go to the filter envelope section.
  • Set both the attack as well as the decay times to a value around a second.
  • Set the sustain level to a value of zero.
  • Apply some reverb to the sound if you like beautiful sounds.

The result could sound like this recording:

Lowpassfilter on Sawtooth – Pad Sequence

That’s still boring and theoretical stuff, as a lot of spices are still missing. But anyway filters driven by envelopes are key features of synthesizers.