Akai S6000 Operator's Manual page 120

EDIT PROGRAM
Filters, therefore, can be used to remove harmonics from the sound (this is referred to as 'subtractive
synthesis') but other filters can be used to boost or emphasise certain frequencies.
The most basic filter is a LOWPASS filter, so called because it lets low frequencies pass though
unaffected but filters out higher frequencies.
For example, a lowpass filter whose filter cutoff frequency is set to maximum would have no effect
on the sound because all the harmonics would pass through unaffected:
This is the equivalent of selecting one of the S6000's lowpass options with cutoff set to 100.
However, if you lower the cutoff frequency, some of the higher harmonics will be filtered out:
With the loss of upper harmonics, the sound will be more muted and duller in tone.
The lowpass filter is the most commonly used type because it mimics nature quite accurately.
In acoustic sounds, high frequencies usually have less energy and therefore die away faster than
low frequencies. As such, you can use an envelope generator applied to a lowpass filter to gradually
remove high frequencies throughout the duration of a note.
Also, the harder you play an acoustic instrument, the more you will 'excite' the higher harmonics
so the sound will not only be louder but also brighter in tone. Thus you can use velocity to control
timbre with a low velocity (i.e. playing 'quietly' so that cutoff is reduced) giving a muted timbre and
high velocity (i.e. playing 'hard' so that the cutoff frequency is raised) yielding a bright tone.
The lowpass filter is also the most commonly available filter type on most synthesisers and so can
be used to re-create the majority of synth sounds.
However, there are many different types of filter. Typically in the past, most synths were equipped
with nothing more than a simple lowpass filter but some advanced synthesisers (typically large
'wall-to-wall' modular synths) offered other filtering options. The S6000 is no exception and offers
not only a variety of lowpass options but also highpass, bandpass and many other filter types.
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Version 1.21