Released in 2007, the Halo 3: Original Soundtrack, composed and produced by Martin O'Donnell and Michael Salvatori, is a two-disc set much like the Halo 2: Original Soundtrack. The Halo 3: Original Soundtrack was released in one set rather than separate volumes like its predecessor. The Halo 3 OST deviated from the Halo 2: Original Soundtrack, and reverted to the structure used in the Halo: Original Soundtrack. Comprised of melodies, chants, and trumpets, the Halo 3: Original Soundtrack is broken up into chapters which correspond to in-game levels.