The Violin Phase reference article from the English Wikipedia on 24-Apr-2004
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Violin Phase

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Violin Phase, written by minimalist composer Steve Reich in 1967, is an example of his phasing technique previously used in Piano Phase in which the music itself is created not by the instruments but by interactions of temporal variations on an original melody. Music of this kind is generally referred to as process music.

In Violin Phase, two violins are recorded and played back, together at first. They are then made to go slowly out of sync by slowing one of the tapes. A new melody is formed by the interaction of the two out of sync instruments and is then accented by a third violin. This process is repeated with variations throughout the rest of the piece.

See also: Piano Phase