User Manual

SA241 Lunar Phaser User’s Guide 9
Basic Phaser Operation
A phaser, or phase shifter, is an effect that was originally designed to reproduce the sound of a
“Leslie” rotating speaker cabinet. The phaser effect can be produced by several methods, the most
common of which involves a network of all-pass filters, which introduce phase shifts into the input
signal. When the phase-shifted signal is mixed with the original input, audible interferences are
created, creating notches. If the phase shift amount is modulated over time, the notches sweep back
and forth across the frequency spectrum, creating the phasing effect.
In technical terms, a phaser sweeps non-uniformly spaced notch filters through the frequency
spectrum of an input signal.
The amount of phase shift in the phaser effect needs to be changed over in order to sweep the notch
filters across the spectrum. In order to do this, a low frequency oscillator (also known as an LFO) is
used to control the phase shift, as well as several other parameters. This is illustrated by the
following graph:
The blue line in the graph is known as the LFO. It changes the phase shift over time.
The frequency (a.k.a. “rate” or “speed”) of the LFO can be increased to get a more rapid change in
the phase shift, as shown in this graph:
PHASE SHIFT
TIME
PHASE SHIFT
TIME