Electric guitar effect
Phasing is a popular effect for electric guitar. The term was often used to refer 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 soon became a favourite effect of guitarists such as Jimi Hendrix and Robin Trower. By the early 1970s, phasing was available as a portable guitar effect. One of the most notable early examples being the MXR Phase 90.
Modulation family
A phaser is a modulation effect. This is a broad term that describes a family of effect-types that includes 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 of these modulation effects use slightly different means to manipulate the signal phase. Therefore, they use the EQ and/or a time delay.
It’s the sweeping of these parameters that earmarks the effects of ‘modulation effects’. While you could move these parameters by hand and achieve the effect, most commonly an LFO is employed to control the speed and depth of the sweeping. In this instance then, 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 create 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, but this creates audible interferences when mixed back with the dry (unprocessed) signal, creating notches.
Phaser structure
The number of all-pass filters (usually called stages) varies with different models, some analog phasers offer 4, 6, 8 or 12 stages. Digital phasers may offer up to 32 or even more. This determines the number of notches/peaks in the sound, 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 feeding the output of the all-pass filter chain back 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 is turning 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 produce a range of frequencies. A vocoder is a different effect that producers use for similar purposes.
Additional resources & source texts
https://en.wikipedia.org/wiki/Phaser_(effect)
