Yamaha Electone EL-57 Owner's Manual page 93

Envelope Parameters
By using the modulators and setting them to various output levels, you can
create all kinds of sounds. These sounds will be static, however, unless
you have the output levels change in time. That is precisely what an
envelope does. It varies the output of an Operator in time so that you can
vary the tone or the level. If the output of the modulator changes, the tone
will change. If the output level of the carrier changes, the volume will
change.
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Feedback
The output of a modulator can also be circled back to its input, allowing it
to modulate itself as well as the Operator below it. This is called feedback,
and it adds harmonic complexity to the tone. Operators using feedback
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are indicated by a line that encircles the right side of the Operator.
Feedback cannot be directly controlled; however, you can adjust the
parameters of Operators that have feedback to change the characteristics of
the feedback sound.
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