Yamaha SY99 Owner's Manual page 26

Two output groups
These two effect units can be arranged in either serial or parallel mode. (The
diagram at the beginning of this section shows the effect units arranged in
parallel mode.)
Depending on the effect type (single, cascade, or dual), each effect unit is driven
by either one or two effect sends. Depending on the effect mode (serial or
parallel), the two effect units will be driven by a total of 1-4 effect sends. Each of
the one, two, or four elements in a normal voice (and each of the 76 AWM
sounds in a drum voice) has its own effect send level adjustment for each of the
four effect sends. Velocity and keyboard scaling can also be made to affect the
effect send level, for example allowing you to apply a greater amount of reverb
(or other effect) for loudly (or softly) played notes, or for high or low notes. This
allows you to apply effects to a voice with great precision.
In addition, you can specify any two parameters of the two effect units to be
controlled from two assigned control sources (control change messages, after-
touch, velocity, key scaling, or the effect LFO).
All effect settings are stored as part of each voice.
The stereo output from the dynamic pan table of each element can be sent to
either or both of the two output groups, where it is mixed with the stereo output
from each effect unit. The output level of each effect and the "wet:dry" balance
for each output group can be set independently.
The final mix is sent from the two stereo pairs of rear panel jacks
OUTPUT 1/1+2 and OUTPUT 2.
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