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8. Synthesis basics: Subtractive synthesis
Bandpass filter
This lets frequencies in a certain range of the spectrum (the band) pass through while dampening fre-
quencies both below and above this range. This accentuates the mid-range of a sound.
Amplitude
Notch filter
This filter type (also known as Band Reject) can be seen as the opposite of a band pass filter. It cuts off
frequencies in a "mid-range" band, letting the frequencies below and above through.
Amplitude
Fc (Cutoff Frequency)
R R R R
- - - -
O O O O L L L L L L L L
O O O O F F F F F F F F
Filters of one and the same type (lowpass, highpass etc.) can have different characteristics. One of the fac-
tors determining the exact filter curve is the roll-off, which is measured in dB/Octave ("decibels per oc-
tave") or poles. The simplest possible filter has a roll-off of 6dB/octave, which is referred to as "1 pole".
The next step up is 12dB (2 poles), 18db (3 poles) etc.
The most common synth filters are the 12dB and 24dB lowpass filters. The difference between the two
can be studied in the graph below. The 12dB filter lets more of the high frequencies pass through which
gives the sound a brighter and "buzzier" character than the 24dB filter.
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Fc (Cutoff Frequency)
Frequency
Frequency
NORD MODULAR V3.0
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