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S3000XL Operator’s Manual Page 107

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

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, a 24dB/Octave, 4-pole filter is better. On

the S3000XL, it is possible to create a 4-pole filter with a 24dB/Octave cutoff slope when

the IB304F is installed. To do this, select LOWPASS 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.

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Akai CD3000XL Betreibershandbuch Seite 113