Digital Mixing Console Sony DMX R-100 used in project studios

In audio recording1audio mixing is the process by which multiple recorded sounds are combined into one or more channels, most commonly 2-channel stereo2. In the process, the source signals' level, frequency content, dynamics, and panoramic position are manipulated and effects such as reverb3 may be added. This practical, aesthetic, or otherwise creative treatment is done in order to produce a mix that is more appealing to listeners.
Audio mixing is done in studios as part of creating an album or single. The mixing stage often follows a multitrack recording4. The process is generally carried out by a mixing engineer5, though sometimes it is the musical producer6, or even the artist, who mixes the recorded material. After mixing, a mastering engineer7 prepares the final product for reproduction on a CD, for radio, or otherwise.
Prior to the emergence of digital audio workstations8 (DAWs), the process of mixing used to be carried out on a device known as an audio mixersound boarddesk, or mixing console9. Currently, more and more engineers and independent artists are using apersonal computer10 for the process (commonly referred to as mixing in-the-box). Mixing consoles still play a large part in the recording process. They are often used in conjunction with a DAW, although the DAW may only be used as a multitrack recorder and for editing or sequencing, with the actual mixing being performed on the console.

The role of audio mixing







An audio production facility at An-Najah National University11

The role of music producer is not necessarily a technical one, with the physical aspects of recording being assumed by the audio engineer, and so producers often leave the similarly technical mixing process to a specialist audio mixer. Even producers with a technical background may prefer that a mixer comes in to take care of the final stage of the production process. Noted producer and mixer Joe Chiccarelli12 has said that it is often better for a project that an outside person comes in because:
"when you're spending months on a project you get so mired in the detail that you can't bring all the enthusiasm to the final [mixing] stage that you'd like. [You] need somebody else to take over those responsibilities so that you can sit back and regain your objectivity."[1]
However, as Chicarelli explains, sometimes limited budgets dictate that a producer takes care of the mixing as well.[1]

History

Before the introduction of multitrack recording, all the sounds and effects that were to be part of a recording were mixed together at one time during a live performance.[2] If the recorded blend (or mix, as it is called) wasn't satisfactory, or if one musician made a mistake, the selection had to be performed over until the desired balance and performance was obtained.[2] However, with the introduction of multitrack recording, the production phase of a modern recording has radically changed into one that generally involves three stages: recording, overdubbing, and mixdown.[2]
Mixing as we know it today emerged with the introduction of commercial multitrack tape machines13, most notably the 8-track recorders that were introduced during the 1960s. The ability to record sounds into a multitude of channels meant that treating these sounds can be postponed to a later stage – the mixing stage.
In the 1980s, home recording and mixing began to take market share from recording studios. The 4-track Portastudio14 was introduced in 1979. Using one, Bruce Springsteen15 released the album Nebraska16 in 1982. The Eurythmics17 topped the charts in 1983 with the song "Sweet Dreams (Are Made of This)18", recorded by bandmember Dave Stewart19 on a makeshift 8-track recorder.[3] In the mid-to-late 1990s, computers replaced tape-based recording for most home studios, with the Power Macintosh20 proving popular.[4] At the same time, digital audio workstations21 (DAW), first used in the mid-1980s, began to replace tape in many professional recording studios.

Equipment

Mixers

A mixer, or mixing console23, or mixing desk, or mixing board, or software mixer is the operational heart of the mixing process.[5] Mixers offer a multitude of inputs, each is fed by a track from a multitrack recorder; mixers would normally have 2 main outputs (in the case of two-channel stereo mixing) or 8 (in the case of surround).
Mixers offer three main functionalities[5][6]:
  • Mixing – summing signals together, which is normally done by a dedicated summing amplifier or in the case of digital by a simple algorithm.
  • Routing – allows the routing of source signals to internal buses or external processing units and effects.
  • Processing – many mixers also offer on-board processors, like equalizers and compressors.






Simple mixing console

Outboard gear and plugins

Outboard gear (analog) and software plugins (digital) can be inserted to the signal path in order to extend processing possibilities. Outboard gear and plugins fall into two main categories[5][6]:
  • Processors – these devices are normally connected in series to the signal path, so the input signal is replaced with the processed signal (e.g. equalizers).
  • Effects – while an effect can be considered as any unit that affects the signal, the term is mostly used to describe units that are connected in parallel to the signal path and therefore they add to the existing sounds, but do not replace them. Examples would include reverb and delay.
Common classes:
  • Processors:
    • Faders – used to attenuate or boost the level of signals.
    • Pan pots – used to pan signal to the left or right and in surround also back and front.
    • Equalizers – used to manipulate the frequency content of signals. Most commonly used are high-pass, low-pass, band-pass, shelf and notch filters.
    • Compressors24 – used to manipulate the dynamic content of signals. Among many applications they can even out the level fluctuations of a vocal or bass track, or reshape dynamic envelopes of instruments (e.g. shortening the sustain of tom-toms).
    • Gates – used to attenuate signals that fall below a certain dynamic threshold, for example, the kick drum bleed on a snare track, or unwanted buzz on guitar tracks. They can also be triggered by sources other than the target track, so as to synchronize the dynamics of two tracks. For example, a kick drum could be used to trigger the gate on a bass track, thus making it so the bass is only heard when the kick drum is struck.
  • Effects:
    • Reverbs25 – used to simulate boundary reflections created in a real room, adding a sense of space and depth to otherwise 'dry' recordings.
    • Delays26 – most commonly used to add distinct echoes as a creative effect.

Mixing in Surround

Mixing in surround27 is very similar to mixing in stereo except that there are more speakers, placed to "surround" the listener. The same mixing domains mentioned above are involved, but instead of stereo's horizontal panoramic aspects, and depth's front-back aspects, mixing in surround lets the mix engineer28 pan sources within a much more 2-dimensional environment. In a surround mix, sounds can appear to originate from any direction.
There are two common ways to approach mixing in surround:
  • Expanded Stereo – With this approach, the mix will still sound very much like an ordinary stereo mix. Most of the sources such as the instruments of a band, the vocals, and so on, will still be panned between the left and right speakers, but lower levels might also be sent to the rear speakers in order to create a wider stereo image, while lead sources such as the main vocal might be sent to the center speaker29. Additionally, reverb and delay effects will often be sent to the rear speakers to create a more realistic sense of space. In the case of mixing a live recording that was performed in front of an audience, signal recorded by microphones aimed at, or placed among the audience will also often be sent to the rear speakers to make the listener feel as if he or she is in the crowd.
  • Complete Surround/All Speakers Are Treated Equally – Instead of following the traditional ways of mixing in stereo, this much more liberal approach lets the mix engineer30 do anything he or she wants. Instruments can appear to originate from anywhere, or even spin around the listener. When done correctly, interesting sonic experiences can be achieved, as was the case with James Guthrie31's 5.132 mix of The Dark Side of the Moon33.[7]
Naturally, these two approaches can be combined any way the mix engineer34 sees fit. Recently, a third approach, or method of mixing in surround was developed by surroundmix engineer35 Unne Liljeblad.
  • MSS – Multi Stereo Surround[8] – This approach treats the speakers in a surround sound system as a multitude of stereo pairs. For example, a stereo recording of a piano, created using two microphones in an ORTF configuration36, might have its left channel sent to the Left Rear Speaker and its right channel sent to the Center Speaker. The piano might also be sent to a reverb having its left and right outputs sent to the Left Front Speaker and Right Rear Speaker respectively. Additional elements of the song, such as an acoustic guitar recorded in stereo, might have its left and right channels sent to the Left Front Speaker and the Right Rear Speaker with a reverb returning to the Left Rear Speaker and the Center Speaker. Thus, multiple clean stereo recordings surround the listener without the smearing comb filtering effects that often occurs when the same or similar sources are sent to multiple speakers.