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Filtering without a filter board

By: Ethan Duni

 
HOW TO DO HIGH-PASS & BAND-PASS FILTERING ON AN AKAI WITHOUT THE FILTER BOARD

1) Create an inverted copy of the sample you want to filter.  On an s2000
this will require transferring the sample in question off to a computer editor.

2)  Place the original sample in one zone of a keygroup and the inverted one
in another. It is crucial that they be in the same keygroup as this
technique requires phase-coherent playback, which is only gauranteed inside
keygroups.

3)  Make sure that all of the settings in each of the zones are identical.
If all is well, you should not hear anything when you play a note.

4)  Set one of the zone's filter offset to +50 and the other's to -50.

5)  Now set the filter cutoff for this keygroup to 50.  From here, if you
modulate the filter downward, it will act like a regular lowpass (since one
of the samples is completely filtered out), but if you modulate upwards it
will act as a highpass, as one sample will have its filter all the way open
and the lower components of the inverse sample will begin to appear as you
open its filter, cancelling the lower components of the first sample.

A useful way to modulate the filter then, is with a modulation source that
can produce both positive and negative values (i.e. a pitchbender or LFO),
as opposed to something like the mod-wheel or velocity, which only produce
positive values.  Try setting a pitch-bender to control the filter.  Turn
the depth of the bender all the way to +50.  Now, when you bend all the way
down, you get nothing, and as you raise the bender towards its midpoint, you
get a regular lowpass sweep.  Raising the bender above its midpoint will
introduce the highpass effect, so you can get a morphing filter kind of
effect.  

Another interesting effect is to use LFO2 to control the filter, set to
random waveshape with zero rate, so that each note has a random filter
(could be highpass or lowpass).  

Of course, it is also fun to play with the resonance in this situation.  One
possible effect beyond the typical resonance type things is to crank up the
res and then turn the master filter cutoff up a bit.  This will cause *2*
resonant areas, one at the very top of the range and near the bottom; with a
cutoff of about 75 the bottom resonance brings out a lot of booming bass in
a loop, while leaving the rest of the loop intact. Of course, the high
resonant area will also exist, so be warned that this may bring out any
really high frequencies in the sample as well; to offset this, you may want
to turn down the zone filter offset of +50 to move this peak to somewhere
more agreeable.  

This brings us to the second point: Band-Pass filters.

Band-Pass filters 

To make a bandpass, set up the patch the same way, but don't turn the filter
offsets all the way up to +- 50.  This is convenient because you can create
bandpasses of arbitrary width; regardless of resonance, simply by tweaking
the difference between the filter offsets.  Getting this method to sound
just right for a given sample or effect may require tweaking the master
filter cutoff and then setting the offsets asymmetrically so that you don't
have a dea zone at the top/bottom of the frequency range where it stops
acting like a bandpass and acts like a highpass/lowpass.  also, keep in mind
that messing with the resonance will still cause dual peaks, so you can get
some wacky bandpass shapes this way.

More fun stuff 

Now that you are bored of playing with the new filters, try messing with the
vel>offset parameter in one of the zones.  This will throw the sample in
question off-phase depending on the velocity you play with, which can result
in some funky velocity-dependent bandpass/highpass effects.

Also, notice that phase-coherency seems to degrade during the release of the
notes (set the filter offsets of the zones back to 00 and the filter cutoff
back to 99 and try playing.  You should hear nothing until you release the
note, then you hear a quiet release).  Try using a long release and see what
you can get (I don't gaurantee it'll be cool, but you might find something
weird).

Also, bear in mind that this method will cut into polyphony as you are using
two samples for each voice.

And please let me know if you find any other cool applications for this.


Ethan Duni 

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