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A disc jockey, also known as DJ, is a person who selects and plays recorded music for an audience. Originally, "disc" (sometimes spelled "disk", although this is now uncommon) referred to phonograph records, not the later Compact Discs. Today, the term includes all forms of music playback, no matter the medium.

There are several types of disc jockeys. Radio DJs or radio personalities introduce and play music that is broadcast on AM, FM, shortwave, digital, or internet radio stations. Club DJs select and play music in bars, nightclubs, discothèques, at raves, or even in a stadium. Hip hop disc jockeys select and play music using multiple turntables, often to back up one or more MCs, and they may also do turntable scratching to create percussive sounds. In reggae, the DJ (deejay) is a vocalist who raps, "toasts", or chats over pre-recorded rhythm tracks while the individual choosing and playing them is referred to as a selector.[1] Mobile DJs travel with portable sound systems and play recorded music at a variety of events.

 

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.

Several techniques are used by DJs as a means to better mix and blend recorded music. These techniques primarily include the cueing, equalization, and audio mixing of two or more sound sources. The complexity and frequency of special techniques depends largely on the setting in which a DJ is working. Radio DJs are less likely to focus on music-mixing procedures than club DJs, who rely on a smooth transition between songs using a range of techniques.

Club DJ turntable techniques include beatmatching, phrasing, and slip-cueing to preserve energy on a dancefloor. Turntablism embodies the art of cutting, beat juggling, scratching, needle drops, phase shifting, back spinning, and more to perform the transitions and overdubs of samples in a more creative manner (although turntablism is often considered a use of the turntable as a musical instrument rather than a tool for blending recorded music). Professional DJs may use harmonic mixing to choose songs that are in compatible musical keys.