Rock music is a genre of popular music that developed in the 1960s from 1950s rock and roll, rockabilly, blues, and country music. The sound of rock often revolves around the electric guitar or acoustic guitar, and it uses a strong back beat laid down by a rhythm section. Along with the guitar or keyboards, saxophone and blues-style harmonica are used as soloing instruments. In its "purest form", it "has three chords, a strong, insistent back beat, and a catchy melody". The traditional rhythm section for popular music is rhythm guitar, electric bass guitar, drums. Some bands also have keyboard instruments such as organ, piano, or, since the 1970s, analog synthesizers. In the 1980s, pop musicians began using digital synthesizers, such as the DX-7 synthesizer, electronic drum machines such as the TR-808 and synth bass devices (such as the TB-303) or synth bass keyboards. In the 1990s, an increasingly large range of computerized hardware musical devices and instruments and software (e.g., digital audio workstations) were used. In the 2020s, soft synths and computer music apps make it possible for bedroom producers to create and record some types of music, such as electronic dance music in their own home, adding sampled and digital instruments and editing the recording digitally. In the 1990s, some bands in genres such as nu metal began including DJs in their bands. DJs create music by manipulating recorded music on record players or CD players, using a DJ mixer.