Multitrack recording (MTR)—also known as multitracking, double tracking, or tracking—is a method of sound recording developed in 1955 that allows for the separate recording of multiple sound sources or of sound sources recorded at different times to create a cohesive whole. Multitracking became possible in the mid-1950s when the idea of simultaneously recording different audio channels to separate discrete "tracks" on the same reel-to-reel tape was developed. A "track" was simply a different channel recorded to its own discrete area on the tape whereby their relative sequence of recorded events would be preserved, and playback would be simultaneous or synchronized.
Multitrack recording was a significant technical improvement in the sound recording process, because it allowed studio engineers to record all of the instruments and vocals for a piece of music separately. Prior to the development of multitrack recording, all of the singers, band instrumentalists, and/or orchestra accompanists had to sing and play together. Multitracking allowed the engineer to adjust the levels and tone of each individual track, and if necessary, redo certain tracks or overdub parts of the track to correct errors or get a better "take." As well, different electronic effects such as reverb could be applied to specific tracks, such as the lead vocals, while not being applied to other tracks where this effect would not be desirable (e.g., on the electric bass). Multitrack recording was much more than a technical innovation; it also enabled record producers and artists to create new sounds that would be impossible to create outside the studio, such as a lead singer adding many harmony vocals with their own voice to their own lead vocal part, an electric guitar player playing many harmony parts along with their own guitar solo, or even recording the drums and replaying the track backwards for an unusual effect.
In the 1980s and 1990s, computers provided means by which both sound recording and reproduction could be digitized, revolutionizing audio recording and distribution. In the 2000s, multitracking hardware and software for computers was of sufficient quality to be widely used for high-end audio recordings by both professional sound engineers and by bands recording without studios using widely available programs such as GarageBand, which can be used on a high-end laptop computer. Though magnetic tape has not been universally replaced as a recording medium, the advantages of non-linear editing (NLE) and recording have resulted in digital systems largely superseding tape. Even in the 2010s, with digital multitracking being the dominant technology, the original word "track" is still used by audio engineers.
Multi-tracking can be achieved with analogue recording, tape-based equipment (from simple, late-1970s cassette-based four track Portastudios, to eight track cassette machines, to 2" reel-to-reel 24-track machines), digital equipment that relies on tape storage of recorded digital data (such as ADAT eight-track machines) and hard disk-based systems often employing a computer and audio recording software. Multi-track recording devices vary in their specifications, such as the number of simultaneous tracks available for recording at any one time; in the case of tape-based systems this is limited by, among other factors, the physical size of the tape employed.
With the introduction of SMPTE timecode in the early 1970s, engineers began to use computers to perfectly synchronize separate audio and video playback, and or multiple audio tape machines. In this system, one track of each machine carried the timecode signal, while the remaining tracks were available for sound recording. Some large studios were able to link multiple 24-track machines together. An extreme example of this occurred in 1982, when the rock group Toto recorded on three synchronized 24-track machines. This setup theoretically provided for up to 69 audio tracks, which is far more than necessary for most recording projects.
For computer-based systems, the trend in the 2000s is towards unlimited numbers of record/playback tracks, although issues such as RAM memory and CPU available do limit this from machine to machine. Moreover, on computer-based systems, the number of simultaneously available recording tracks is limited by the number of sound card discrete analog or digital inputs.
When recording, audio engineers can select which track (or tracks) on the device will be used for each instrument, voice, or other input and can even blend one track with two instruments to vary the music and sound options available. At any given point on the tape, any of the tracks on the recording device can be recording or playing back using sel-sync or Selective Synchronous recording. This allows an artist to be able to record onto track 2 and, simultaneously, listen to track 1, 3 and 7, allowing them to sing or to play an accompaniment to the performance already recorded on these tracks. They might then record an alternate version on track 4 while listening to the other tracks. All the tracks can then be played back in perfect synchrony, as if they had originally been played and recorded together. This can be repeated until all of the available tracks have been used, or in some cases, reused. During mix down a separate set of playback heads with higher fidelity are used.
Before all tracks are filled, any number of existing tracks can be "bounced" into one or two tracks, and the original tracks erased, making more room for more tracks to be reused for fresh recording. In 1963, The Beatles were using twin track for . The Beatles' producer George Martin used this technique extensively to achieve multiple track results, while still being limited to using only multiple four-track machines, until an eight-track machine became available during the recording of the Beatles' White Album. The Beach Boys' also made innovative use of multitracking with 8-track machines of the day (circa 1965).Motown also began recording with 8-track machines in 1965 before moving to 16-track machines in mid-1969.
Multitrack recording also allows any recording artist to record multiple "takes" of any given section of their performance, allowing them to refine their performance to virtual perfection by making additional "takes" of songs or instrumental tracks. A recording engineer can record only the section being worked on, without erasing any other section of that track. This process of turning the recording mechanism on and off is called "punching in" and "punching out". (See "Punch in / out".)
When recording is completed, the many tracks are "mixed down" through a mixing console to a two-track stereo recorder in a format which can then be duplicated and distributed. (Movie and DVD soundtracks can be mixed down to four or more tracks, as needed, the most common being five tracks, with an additional Low Frequency Effects track, hence the "5.1" surround sound most commonly available on DVDs.)