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Synthesis Tutorial
The Filter
The K-Station is an Analogue subtractive type of music synthesizer.
Subtractive implies that part of the sound is subtracted somewhere in
the synthesis process.
The Oscillators provide the raw waveforms with plenty of harmonic
content and it is the Filter that subtracts unwanted harmonics in a con-
trollable manner.
The Filter in the K-Station is a Low Pass type. A cut-off point is chosen
and any frequencies below that point are passed and any above are fil-
tered out. The setting of the FREQUENCY knob on the K-Station panel
dictates the point below which frequencies are removed. This process
of removing harmonics from the waveforms has the effect of changing
the sounds character or timbre. When the FREQUENCY knob is set
fully clockwise, the filter is set completely open and no frequencies are
removed from the raw Oscillator waveforms.
In practice, there is a gradual reduction in the volume of the harmonics
above the cut-off point. How quickly these harmonics are reduced in
volume above the cut-off frequency is determined by the Filter’s slope.
This slope is measured in ‘volume units per octave’. Since Volume is
measured in decibels, this slope is quoted in number of decibels per
octave (dB). Typical values are 12dB or 24dB per Octave. The higher
the number, the faster the harmonics are cut and the more pronounced
the filtering effect.
The button on the K-Station’s front panel marked SLOPE allows either
the 12dB or 24dB type filter slope to be selected.
A further important feature of the Filter is the RESONANCE control.
Frequencies at the cut-off point may be increased in volume by this
control. This is useful for emphasizing certain harmonics of the sound.
As the RESONANCE is increased, a whistling like quality will be intro-
duced to the sound passing through the filter. When set to very high
levels, RESONANCE actually causes the filter to self - oscillate when-
ever a signal is being passed through it. The resulting whistling tone
being produced is actually a pure sine wave, the pitch of which
depends on the setting of the FREQUENCY knob (the filter’s cut-off
point). This resonance-produced sine wave can actually be used for
some sounds as an additional sound source if desired.
This diagram shows the response of a typical low pass filter.
Frequencies above the cut off point are reduced in volume.
When resonance is added, frequencies at the cut off point are boosted
in volume.
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