The focus of art music in the 20th century was characterized by exploration of new rhythms, styles, and sounds. The horrors of World War I influenced many of the arts, including music, and some composers began exploring darker, harsher sounds. Traditional music styles such as jazz and folk music were used by composers as a source of ideas for classical music. Igor Stravinsky, Arnold Schoenberg, and John Cage were all influential composers in 20th-century art music. The invention of sound recording and the ability to edit music gave rise to new subgenre of classical music, including the acousmatic and Musique concrète schools of electronic composition. Sound recording was also a major influence on the development of popular music genres, because it enabled recordings of songs and bands to be widely distributed. The introduction of the multitrack recording system had a major influence on rock music, because it could do much more than record a band's performance. Using a multitrack system, a band and their music producer could overdub many layers of instrument tracks and vocals, creating new sounds that would not be possible in a live performance.