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.