8 SYNthESIS BaSICS
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as you can see the curve is flat in the low register (which
means it doesn’t affect this part of the spectrum at all)
and then, at a certain point, gradually starts falling. When
applied to the wave above, this filter cuts away some of the
high frequency material in the wave, like this:
Filter types
there are many types of filters, all with their different pur-
poses. Let’s discuss the three most common ones, that are
found in the Nord Wave. as you may already have noticed,
the filters are explained in detail in the Panel reference
chapter
Lowpass filter
the lowpass filter dampens high frequencies and allows
low frequencies to pass through unaffected, as in the
example above. It is the most common synthesizer filter,
since it can be used to “round off” the sharp sound of saw-
tooth waves and pulse waves.
Highpass filter
this is the opposite of the lowpass filter. It allows the high
frequencies of the sound to pass through and cuts off the
low frequencies. this removes “bass” from a sound, without
affecting the high end.
Bandpass filter
this allows frequencies in a certain range of the spectrum
(the band) to pass through while dampening frequencies
both below and above this range. this accentuates the
mid-range of a sound.
Slope
Filters of one and the same type (lowpass, highpass etc)
can have different characteristics. one of the factors
determining the exact filter curve is the roll-off, which is
measured in dB/octave (“decibels per octave”) 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 low-
pass filters. the difference between the two can be studied
in the graph below. the 12dB filter let’s more of the high
frequency pass through which gives the sound a brighter
and “buzzier” character than the 24dB filter does.
Filter Frequency
Filter Frequency
Resonance = 10
Resonance = 0
Resonance = 3
Resonance = 7
Filter Frequency
Resonance = 10
Resonance = 0
Filter Frequency
Filter Frequency
Resonance = 10
Resonance = 0
Filter Frequency
Filter Frequency
Filter Frequency
Filter Frequency
Filter Frequency
Filter Frequency
Filter Frequency
Filter Frequency
Filter Resonance
Filter Frequency
Filter Frequency
Filter Resonance
Filter Frequency
Filter Vocal
Envelope range
Time
Filter Frequency
In the Nord Wave, the lowpass filter can be switched be-
tween 12 and 24dB modes.
Cutoff frequency
the most important parameter for a filter is its cutoff
frequency, which is the setting that determines where in
the frequency material it should start cutting. If the cutoff
frequency in a low pass filter is set to a very low value, only
the lowest harmonics (the bass) will pass through. If you
raise the cutoff all the way up, all frequencies will be let
through, as the figure below illustrates.
Changing the cutoff frequency is often referred to as
“sweeping the filter”. this is probably one of the most im-
portant ways of shaping the timbre of a synthesizer sound.
By using an envelope you can for example have a high
cutoff at the beginning of a sound which is then gradually
lowered (the filter “closes” as the sound decays). this would
emulate the way most plucked string sound (piano, guitar
etc) behave; the amplitude of the harmonics decreases as
the sound decays.
Keyboard tracking
When you play different pitches, the oscillators produce
different frequencies. this means that the overtones in
the waveform appear at different frequencies. the cutoff
frequency of the filter however, is fixed.
this means that different overtones will be cut off at differ-
ent pitches. to be more precise, the further up the key-
board you play, the muddier the sound will be.
to remedy this problem many synthesizers have a para-
meter called Filter Keyboard tracking. When this is acti-
vated, the filter Cutoff Frequency varies with which key you
play, just as the oscillator frequency does. this ensures a
constant harmonic spectrum for all keys.
amplitude
Frequency
