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There is one further aspect to filters one has to consider, however, and that is ROLL-OFF.
You will see filters referred to as 1-POLE, 2-POLE or 4-POLE - maybe even 6-POLE. A ‘pole’ is
equal to 6dB of attenuation per octave. Thus, a 2-POLE filter offers 12dB/Octave roll-off whilst a 4-
POLE offers 24dB/Octave rolloff. A 6-POLE filter offers a theoretical 36dB/Octave roll-off but these
are unusual largely because the difference between 4- and 6-POLE is so subtle that it is largely
inaudible in a properly designed filter - most of the ‘classic’ analogue synthesisers only offered 2-
or 4-POLE filters (i.e. 12dB/Octave and 24dB/Octave roll-offs). You can see the difference between
a 2-POLE and a 4-POLE lowpass filter in the following diagram:
FREQUENCY
L
E
V
E
L
Cutoff
A 2-POLE or 12dB/Octave lowpass filter
FREQUENCY
L
E
V
E
L
Cutoff
A 4-POLE or 24dB/Octave lowpass filter
Although the cutoff frequencies in both diagrams are the same, the roll-off on the 4-POLE is slightly
steeper. The effect is that the 2-POLE filter is a little ‘fizzier’ sounding than the 4-POLE as extra
harmonics are passing through because its roll-off is less severe.
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). Sequential
Circuit’s Prophet 5 and Pro-1 also used 4-pole filters.
However, Oberheim synths typically 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.
The S6000 offers both options and a wealth of other filter types besides.
So, with the theory out of the way, let’s now look at the filters’ parameters.
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