An effect produced by running a signal through an allpass filter, and then combining with the original signal. The allpass filter introduces a frequency-dependent phase shift into the signal, which results in some reinforcement and cancellation when re-combined with the original signal. The effect is to produce a comb filter response, having one peak and one notch. If the cutoff frequency of the allpass filter is driven by an LFO, the peak and notch frequencies move back and forth through the spectrum, creating a swirling effect, somewhat like flanging although less intense. Most phase shifters use multiple stages of allpass filters for a stronger effect.
The first phase shifter devices for musicians were developed in the late 1960s. Phase shifters then were usually packaged in the form of simple guitar-effect stomp boxes, with few if any controls and a cloying sound. The effect was over-used to the point of becoming a cliche in '70s rock and jazz, and most musicians abandoned it when digital effects started to become more affordable around 1980. But phase shifting has made something of a comeback starting in the late '90s. Some phasers designed specifically for synth use, with voltage control over stages and filter sweeps, have become available since then.