Yamaha DX21 - Manual (Page 28)

Still more variations can be achieved by changing the relative output levels between
operators; the greater the level of the modulating operator, the more harmonics
are present.
FEEDBACK
Note that every algorithm has one operator with a “feedback loop”—represented
by a line from the output of the operator which feeds back to the input of the same
operator. In effect, a feedback loop means that the operator is modulating itself.
While every algorithm has one feedback loop, feedback is not necessarily used in
every voice. One of the DX21 editing functions permits the feedback level to be
set between 0 (no feedback) and 7 (maximum feedback).
ENVELOPE GENERATORS
Consider what happens when you play a note on, say, an acoustic instrument. The
level of the sound initially goes up to some value, then eventually falls to nothing,
following some sort of pattern that is characteristic of the particular instrument
played. For example, a low note on a pipe organ starts slowly when you press a
key, because it takes a while for the large column of air within the pipe to build
up to maximum oscillation level, and takes a while to die down once the key is
released a note played on a wood block, on the other hand, starts quickly as the
mallet strikes the block, and stops quickly as the block stops resonating. The
characteristic volume pattern of any note played on any instrument is known as
its “volume envelope”. Most acoustic instruments also have a “timbre envelope”,
in which the harmonic content of the note changes (the timbre changes) from the
time the note is initiated to the time it decays.
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