Club DJ equipment may consist of: • Sound recordings in a DJ's preferred medium (for example, vinyl records, Compact Discs, computer media files, etc.); • A combination of two devices (or only one, if playback is digital) to play sound recordings, for alternating back and forth to create a continuous playback of music (for example, record players, Compact Disc players, computer media players such as an MP3 player, etc.); • A multiple Sequencer which can mix MIDI tracks with Digital Audio; • A sound system for amplification or broadcasting of the recordings (for example, portable audio system, PA system) or a radio broadcasting system; • A DJ mixer, which is an electronic (usually 2- or 4-channel) audio mixer usually equipped with a crossfader used to smoothly go from one song to another, using two or more playback devices; • Headphones, used to listen to one recording while the other recording is being played to the audience, or to listen to both recordings simultaneously; and • Optionally, a microphone, so that the DJ can introduce songs and speak to the audiences. Other equipment could or can be added to the basic DJ setup (above), providing unique sound manipulations. Such devices include, but are not limited to: • Electronic effects units (delay, reverb, octave, equalizer, chorus, etc.). Some club DJs use a subharmonic synthesizer effect which either doubles low frequencies with energy added an octave lower or synthesizes harmonics such that the impression of a very low bass sound is added to the mix. • A computerised performance system, which can be used with vinyl emulation software to manipulate digital files on the computer in real time. • Multi-stylus headshells, which allow a DJ to play different grooves of the same record at the same time. • Special DJ digital controller hardware can manipulate digital files on a PC or laptop; • Samplers, sequencers, electronic musical keyboards (synthesizers), or drum machines.