Phaser

Electric Guitar Effect

Phasing is a popular effect for electric guitar. The term often referred to the original tape flanging effect heard on many psychedelic records of the late 1960s. In 1968, Shin-ei's Uni-Vibe effects pedal, designed by audio engineer Fumio Mieda, incorporated phase shift and chorus effects. This became a favorite effect of guitarists like Jimi Hendrix and Robin Trower. By the early 1970s, phasing was available as a portable guitar effect—one of the most notable early examples is the MXR Phase 90.

Modulation Family

A phaser is a modulation effect. This broad term describes a family of effect types, including phasers, flangers, and chorus. Overall, these effects sound like they are moving or constantly changing. For an exact definition, modulation describes any instance in which the audible signal (known as the carrier) is being modified (modulated) by another signal (known as the modulator). Under the hood, all three modulation effects use slightly different means to manipulate the signal phase. Therefore, they use the EQ and a time delay. The sweeping of these parameters earmarks the effects of 'modulation effects.' While you could move these parameters by hand and achieve the effect, an LFO is often employed to control the speed and depth of the sweeping. In this instance, the LFO is the modulator, while the audible signal is the carrier. Flanger and chorus usually rely on delay to create this effect. On the other hand, the phaser is unique because it employs a chain of 'all-pass filters to generate frequency notches and peaks. Sweeping (moving) these notches/peaks creates the signature sound of a phaser.

Electronic Phasers

Traditional electronic phasers use a series of variable all-pass phase-shift networks, which alter the phases of the different frequency components in the signal. These networks pass all frequencies at equal volume, introducing only phase change to the signal. Human ears are not very responsive to phase differences, which creates audible interferences when mixed back with the dry (unprocessed) signal, creating notches.

Phaser Structure

The number of all-pass filters (usually stages) varies with different models; some analog phasers offer 4, 6, 8, or 12 stages. Digital phasers may provide up to 32 or even more. This determines the sound's number of notches/peaks, affecting the general sound character. A phaser with n stages generally has n/2 notches in the spectrum. So, a 4-stage phaser will have two notches. Additionally, the output can be fed back to the input for a more intense effect, creating a resonant effect by emphasizing frequencies between notches. This involves returning the output of the all-pass filter chain to the input.  

Computer Voice

In motion picture or television production, the effect created by a phaser is often used to imply that the sound is synthetically generated. Simply put, it turns a natural human voice into a computer or robot voice. The technique works because the frequency filtering produces sound commonly associated with mechanical sources, which only generate specific frequencies. In contrast, natural sources have a range of frequencies. A vocoder is a different effect that producers use for similar purposes.

Additional Resources & Source Texts

• Phaser Effect