Apple Soundtrack Pro 3 - Manual (Page 276)

Modulation Effects
Modulation effects begin with a delayed signal, like time-based effects, but vary (or
modulate) the delay time, typically using a low-frequency oscillator (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.
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.
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.
276 Chapter 10 Working with Audio Effects