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Appendix B Synthesizer Basics 485
Filter slope
A lter will cut o the signal at the cuto frequency you set. This cuto doesn’t happen abruptly
but rather at a given slope, which is measured in decibels (dB) of gain reduction per octave. You
can dene how steep the “cli” is at the cuto point by choosing a severe or gentle slope.
Cutoff frequency
24 dB/Oct 6 dB/Oct12 dB/Oct
Envelopes in the amplier
Amplier envelope overview
The amplier module of a synthesizer is responsible for controlling the level, or loudness, of the
signal over time.
Consider the sound of a violin, for example. The sound slowly ramps up to a peak, or maximum,
level as the bow is dragged across a string, then it is sustained for a period until the bow is
moved away from the string, at which point it cuts o abruptly.
In contrast to the violin example, hitting a snare drum with a drumstick results in a very fast
peak level with no sustain portion, then the sound immediately dies out—although there will be
some decay, the time it takes to fall from the peak level.
These two sounds clearly have dierent characteristics over time.
Synthesizers emulate these sonic characteristics by providing control over dierent parts—the
beginning, middle, and end—of a sound’s level over time. This control is achieved using a
component called an envelope generator.
Note: Envelope generators are not limited to controlling signal amplitude. They can also control
the rise and fall of the lter cuto frequency or they can modulate other parameters. In short,
envelope generators can be used as a modulation source—or as a “remote control” for a given
parameter. See Modulation overview.