Apple Soundtrack - Manual (Page 131)

Chapter 8 Working with Audio Effects 131
Modulation Effects
Modulation effects begin with a delayed signal, like time-based effects, but vary (or
modulate) the delay time, typically using an LFO. This can be used to double a sound,
making it seem stronger and “fatter,” to simulate a group of voices or instruments
playing together, or to add a distinctive character to the sound. Modulation effects
include chorus, phase shifters, and flangers.
Chorus
Chorus effects play back multiple repetitions of the delayed signal (like reverbs), but
vary the delay time for each one, using an LFO. As the name implies, this effect can
strengthen the sound, and create the impression that the sound is being played by
many instruments or voices in unison. The slight variations in delay time created by the
LFO simulate the subtle differences in timing and pitch heard when several people play
together. Using chorus also adds fullness or richness to the signal, and can add
movement to low or sustained sounds.
Phase Shifters
Phase shifters produce a characteristic “whooshing” sound by combining the original
signal with a copy of the signal that is slightly out of phase with the original. This means
that the amplitudes of the two signals’ sound waves reach their highest and lowest
points at slightly different times. The time between the two signals is modulated, typically
using an LFO. As the two signals go in and out of phase, certain frequencies, called notch
frequencies, are created, which give phase shifters their distinctive sound.
The main difference between chorus and phase shifting is the amount of delay time.
Chorus effects typically use delay times between 20 and 30 milliseconds (ms), while
phase shifters (and flangers, discussed next) typically use shorter delay times, between
1 and 10 ms.
Flangers
Flangers work in much the same way as do phase shifters, but additionally change
the pitch of the delayed signal slightly. Flanging is typically used to create a more
extreme change than phase shifting, sometimes described as adding a “spacey” or
“underwater” effect.
All of the modulation effects include parameters for the delay rate (also called speed or
frequency), which let you set the minimum delay time; depth (also called width or
intensity), which you use to set how much the LFO modulates the delay time; and mix,
which you use to control the ratio of the effected (wet) signal to the original (dry)
signal. They can also include parameters for feedback (or regeneration), which add part
of the output back into the input signal.