Phase music

Phase music is a form of music that uses phasing as a primary compositional process. It is an approach to musical composition that is often associated with minimal music, as it shares similar characteristics, but some commentators prefer to treat phase music as a separate category. Phasing is a compositional technique in which the same part (a repetitive phrase) is played on two musical instruments, in steady but not identical tempi. Thus, the two instruments gradually shift out of unison, creating first a slight echo as one instrument plays a little behind the other, then a doubling with each note heard twice, then a complex ringing effect, and eventually coming back through doubling and echo into unison. Phasing is the rhythmic equivalent of cycling through the phase of two waveforms as in phasing. Note that the tempi of the two instruments are almost identical, so that both parts are perceived as being in the same tempo: the changes only separate the parts gradually. In some cases, especially live performance where gradual separation is extremely difficult, phasing is accomplished by periodically inserting an extra note (or temporarily removing one) into the phrase of one of the two players playing the same repeated phrase, thus shifting the phase by a single beat at a time, rather than gradually.

It was popularized by composer Steve Reich. In Reich's tape music, where the composer sets off several copies of the same tape loop simultaneously on different machines. Over time, the slight differences in the speed of the different tape machines causes a flanging effect and then rhythmic separation to occur. Examples include Reich's Come Out and It's Gonna Rain. This technique was then extended to acoustic instruments in his Piano Phase, Reich's first attempt at applying the phasing technique to live performance, and later the change in phase was made immediate, rather than gradual, as in Reich's Clapping Music.

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