Alesis S4 Plus Reference Manual page 62

Editing Programs: Chapter 6
Sound Overlap (00 to 99)
This determines how many voices can overlap on the same pitch. Example: If you
hold the sustain pedal down and play the same note over and over, Sound Overlap
determines how many voices are available for that note, and thus how many voices
will overlap (play simultaneously).
In the old days, different brands of synthesizers offered different voice allocation
schemes. One brand used a method called "rotate mode" in which each time a note
was struck, a new voice was used. Another brand used a different method called
"reassign mode" whereby if a note is played and then played again, the same voice is
used both times. In other words, a new voice is used each time a new note of a
different pitch is played.
The Sound Overlap value lets you choose a comfortable setting between rotate mode
and reassign mode. When the value equals 99, you are always in rotate mode, using
up polyphony; if the value is 00, you are always in reassign mode, conserving voices.
Set the value anywhere between 00 and 99 and you will get a combination of both,
with partiality toward whichever mode the value is closest to.
A piano sound requires some Sound Overlap, but not a lot; it isn't natural to hear too
many voices on the same note. On the other hand, having only one voice per pitch
isn't natural either; let's say you played a loud note with the sustain pedal held,
followed by a soft note--the second note would abruptly cut-off the first. On a real
piano, the string would still be resonating from the first (loud) note when the second
(soft) note was played; thus the two notes would overlap.
Sound Overlap is especially useful in Drum Mode. In Drum Mode, each drum sound
is designated to one note (or sometimes a group of notes). When playing one note for
a drum sound, such as a snare, and you attempt to play a roll, the Sound Overlap
controls how much of an overlap will exist between each pair of notes. If the Sound
Overlap value is set low, you are more likely to hear old notes getting cut-off.
This function has added value when the Program's Keyboard Mode parameter is set
to Mono, instead of Poly. When playing monophonically, Sound Overlap controls the
amount of overlap between notes. Normally, even if the sound has a long release
time, playing a new note in Mono Mode will cut-off the previous note. By adjusting the
Sound Overlap value, you can still hear the previous note fading away after the new
note has been played.
It is important to note that Sound Overlap can have a negative effect on polyphony.
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If you have Sound Overlap set to 99, hold the sustain pedal and play a series of
notes, you will run through all 64 voices in no time. By adjusting the Sound Overlap to
a lower value, you decrease the number of voices used by each new note, and
thereby ensure there are voices available to play other sounds, if necessary.
S4 Plus Reference Manual 61