This very easy-to-read article is from Koert and is taken from www.infodrom.north.de/~matthi/jv-1080/boosterinfo.txt. I just copied it because I realized that several XP-related webpages went offline after some time and I don't want to have this really helpful information to get lost.
Originally, it was written for the JV-1080, but is of course almost the same for the other JV and XP keyboards.

Date: Tue, 05 Sep 95 17:44:56 CET
From: "kve"
Hi there,


The Booster & Ring Modulator features are very powerful in the JV,
since it is the only way to create entire new waveforms from the
existing ones.

They're not much used, probably because it is not easy to use these
functions properly (it is very easy to make some dirty sounds coming
from the gates of hell with it, but a good sound without too much
dirty harmonics - that's not easy!).

Very careful use of the filters do a great job here (remember - you
then have 2 filters in series!). Small modifications in keyfollow,
resonance etc. here give drastic differences in sound.


Let me summarise some of my previous mails:



RING MODULATOR:

Basically multiplies both waves. Those people knowing about Fourrier
analysis and trigonmetry know that this operation results in added and
subtracted frequencies, including all harmonics. Hence, you obtain a
sound with a lot of non-harmonic overtones.

FIRST LESSON: Try to combine two sines, one detuned Coarse +7.

I also had some good results with the subsonic waves (Low sine, Low
saw etc) combined with a "normal" wave. Obviously, the pitch settings
of both waves also play a major role.



FXM:

FXM is mainly for noisy sounds. In fact, it's a kind of ring
modulation, but using twice the same wave. I had good experiences with
simple waves (sine etc).

There is though one very interesting use (execpt for all the strange
sound effects): adding some growl-like effects. You can here this
clearly in some of the sax parts: when hit they key hard, you get a
typical growl. When analysing how they did this, I found out that it's
by using the FXM (check it!).

FIRST LESSON: Try to add some FXM growl to a sharp trumpet sound. Make
it happen only when you hit they keys very hard.



BOOSTER:

When I just bought my JV, I obviously was very courious about this
booster, and hence took some preset and changed the structure to 3 or
4. I was rather disappointed: it merely gave some distortion.

I now discovered that this is NOT how to use the booster. Instead, if
you carefully think about what the booster does, it is really a great
thing (just as the ring modulator).

Boost pushes the waves until they clip, even if boost is set to 0.
Wave gain increases the effect.

The TVA envelope on the other hand, is used -after- this, and hence
has no effect on the clipping.

Using for instance a simple triangle wave, with a high gain and boost,
and combined with the DC wave, creates a kind of pulse wave. If you
take the structure with at least one TVF after the boost, you have
created a new wave and can treat it with the TVF.

Since TVA1 or TVA3 adjusts the balance of the wave and the DC wave,
you can make this one velocity sensitive and hence having a sound that
changes from triangle to pulse depending on the velocity (of course,
you can also use the LFO, so your sound sweeps between triangle to
pulse).

Now, replace the DC with a LOW SINE wave. You now have some additional
sweep, which lets you create (together with the LFOs) some very
complex sweeping.

If you use instead of the triangle some other wave, you can get very
wild effects with the booster. But my experience is that you can
better 'control' the booster function if you use at least one very
simple wave (DC, LOW SINE, LOW TRIANGLE,,,,).

So, if you spend some time learning using the booster, you really get
great analog sounding sounds.

FIRST LESSON: Boost a saw or triangle with a DC wave.



Try it! Get some of my patches on pacificrim, and create some
yourself.

AND LET ME HEAR YOUR CREATIONS! DUMP PATCHES ON PACIFICRIM AND MAKE
THE WORLD HAPPY!


In some next contribution, I will explain how tuning inluences ring
modulation. Note already that our mean tempered 12-tone system is just
a lousy simplified system of what it should be.



Koert

Lord of the Ring Modulators


_________________________________________________________________

a  little adding by Matthias Kopfermann (---------------------

if you want to combine e.g. a low sine wave and another wave
by the booster
you should know that to avoid the effect of increasing the pulse rate
of a boosted wave by key you can set the keyfollow to 0. then
the pulse will be the same whereever you play on the keyboard.
and if you change the pitch envelope of the second wave
the pulse will change with time.