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Page 112 CD3000XL OperatorÕs Manual
CREATING 24db/8ve 4-POLE LOWPASS FILTER
There are two types of filters commonly used in analogue synthesisers. They are sometimes
referred to as ‘2-pole’ or ‘4-pole’. These offer two different filter response slopes:
HARMONICS
CUTOFF FREQUENCYROLL-OFF SLOPE
FREQUENCY
HARMONICS
CUTOFF FREQUENCYROLL-OFF SLOPE
FREQUENCY
12dB/Octave 2-pole lowpass filter 24db/Octave 4-pole lowpass filter
L
E
V
E
L
L
E
V
E
L
A 2-pole filter offers 12dB/Octave cutoff slope and a 4-pole has a 24dB/Octave cutoff slope.
Because the 12dB/Octave slope is slightly less severe, you can see that some upper
harmonics are still in the signal. The 4-pole filter’s 24dB/Octave slope, however, removes
these. Some people claim that 24dB/Octave filters are ‘punchier’ than the 12dB/Octave variety
which some people claim to be ‘fizzy’.
In the ‘good old days’ of analogue synths, some manufacturers used 2-pole filters, others used
4-pole filters. Moog synthesisers had 4-pole filters and many people attribute the classic
MiniMoog’s punchy bass end to this (other factors are actually responsible as well, as it
happens). Early ARP and Oberheim synths (the SEM Synth Expansion Module and the
Oberheim 4-Voice) had 2-pole filters and yet no-one accuses them of lacking any ‘punch’!
Some synths offered a switch to choose the cutoff frequency’s response slope.
However, it must be said that for certain sounds, particularly bass synth sounds, a
24dB/Octave, 4-pole filter is better. On the CD3000XL, it is possible to create a 4-pole filter
with a 24dB/Octave cutoff slope when the IB304F is installed. To do this, select LP as the filter
type in FILTER 2 and set FILTER 2’s parameters identically to FILTER 1. I.e.:
Because the two 2-pole filters are in series, this creates a 4-pole filter with a 24dB/Octave cutoff
slope. You can make ‘A/B’ comparisons between 2-pole and 4-pole using the
switch to hear the difference between the two filter types.