Overdubbing (also known as layering) is a technique used in audio recording in which audio tracks that have been pre-recorded are then played back and monitored, while simultaneously recording new, doubled, or augmented tracks onto one or more available tracks of a digital audio workstation (DAW) or tape recorder. The overdub process can be repeated multiple times. This technique is often used with singers, as well as with instruments, or ensembles/orchestras. Overdubbing is typically done for the purpose of adding richness and complexity to the original recording. For example, if there are only one or two artists involved in the recording process, overdubbing can give the effect of sounding like many performers.
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In vocal performances, the performer usually listens to an existing recorded performance (usually through headphones in a recording studio) and simultaneously plays a new performance along with it, which is also recorded. The intention is that the final mix will contain a combination of these "dubs".
Another kind of overdubbing is the so called "tracking" (or "laying the basic tracks"), where tracks containing the rhythm section (usually including drums) are recorded first, then following up with overdubs (solo instruments, such as keyboards or guitar, then finally vocals). This method has been the standard technique for recording popular music since the early 1960s. Today, overdubbing can be accomplished even on basic recording equipment, or a typical PC equipped with a sound card, using digital audio workstation software.
Because the process of overdubbing involves working with pre-recorded material, the performers involved do not have to ever have physically met each other, nor even still be alive. In 1991, decades after her father Nat King Cole had died, Natalie Cole released a "virtual duet" recording of "Unforgettable" where she overdubbed her vocals onto her father's original recording from the 1960s. As there is no limit in timespan with overdubbing, there is likewise no limit in distance, nor in the number of overdubbed layers. Perhaps the most wide-reaching collaborative overdub recording was accomplished by Eric Whitacre in 2013, where he edited together a "Virtual Choir" of 8,409 audio tracks from 5,905 people from 101 countries.