Phaser Shifters (or "phasers") were the most common 'effects' used throughout the 1970s, and have remained popular since. Their slow, evolving sweeps were a mainstay of rock keyboards,and were used extensively on sounds such as electric 12-stringguitars, where a faster modulation speed produced a sound akin to playing the instrument through a Leslie rotary speaker cabinet.

Phasers work by combining the input signal with a delayed(and therefore phase-shifted) version of itself. Where the shifted frequencies are in phase with the original signal, they reinforce any harmonics that lie at those frequencies. Where the shifted frequencies are out of phase with the original, they interfere destructively, and attenuate any harmonics that lie at those frequencies. If the amplitudes of the original and shifted signals are equal, there are notches in the output spectrum.

The number of notches is determined by the number of delay stages in a unit's architecture. One design employs four stages that shift the phase of the input signal from 0º at low frequencies to 720º at high frequencies. This means that the original and shifted signals reinforce at low and high frequencies, and also at 360º, with two regions of cancellation at 180º and 540º. We can conclude, therefore, that the number of notches in the output is equal to half the number of phase-shifting stages in the device. The figure overleaf may make this clearer.

Some vintage phasers use more stages to create higher numbers of troughs and peaks. However, increasing the number of stages does not necessarily create a more pleasing effect, and in some circumstances the results can be too subtle to be of use.

Many phasers have a positive feedback loop that sharpens and emphasises the peaks between the notches. Increasing the feedback signal makes these peaks progressively sharper until - in extreme cases - the phaser begins to oscillate. The Electro-Harmonix Small Stone offered a switch that offered two levels of feedback, but some owners modified their units to allow them to determine the amount of emphasis using a knob. These modified units became the sonic benchmark for the design of the RS-400.

The classic phaser effect is generated by sweeping the frequencies at which the notches occur. This creates a complex spectral response that is not easily duplicated using conventional filters. To this end, all stand-alone phasers incorporate an LFO with a frequency range of a few percent of a Hertz to 10Hz or there abouts. For a given number of stages it is the speed and depth of this LFO that determine the nature of the output.

More sophisticated phasers also offer CV inputs that allow you to affect or even replace the LFO modulation. This makes it possible to produce a further range of 'phasey' sounds that do not display the more common cyclic characteristic.