Audio mixing (recorded music)
In sound recording and reproduction, audio mixing or mixdown is the process by which multiple recorded sounds are combined into one or more channels, for instance 2-channel stereo. In the process, the source signals' level, frequency content, dynamics, and panoramic position are manipulated and effects such as reverb 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 recording. The process is generally carried out by a mixing engineer, though sometimes it is the musical producer, or even the artist, who mixes the recorded material. After mixing, a mastering engineer prepares the final product for reproduction on a CD, for radio, or otherwise.
Prior to the emergence of digital audio workstations (DAWs), the process of mixing used to be carried out on a mixing console. Currently, more and more engineers and independent artists are using a personal computer for the process. 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
The role of a 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 Chiccarelli 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."
However, as Chiccarelli explains, sometimes limited budgets dictate that a producer takes care of the mixing as well.
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. 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. 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.
Mixing as we know it today emerged with the introduction of commercial multitrack tape machines, 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 Portastudio was introduced in 1979. Using one, Bruce Springsteen released the album Nebraska in 1982. The Eurythmics topped the charts in 1983 with the song "Sweet Dreams (Are Made of This)", recorded by bandmember Dave Stewart on a makeshift 8-track recorder. In the mid-to-late 1990s, computers replaced tape-based recording for most home studios, with the Power Macintosh proving popular. At the same time, digital audio workstations (DAW), first used in the mid-1980s, began to replace tape in many professional recording studios.
A mixer, or mixing console, or mixing desk, or mixing board, or software mixer is the operational heart of the mixing process. Mixers offer a multitude of inputs, each is fed by a track from a multitrack recorder. Mixers typically have 2 main outputs (in the case of two-channel stereo mixing) or 8 (in the case of surround).
- 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.
Outboard gear and plugins
- 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.
- 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.
- Compressors – used to attenuate signals above of a certain dynamic threshold. Among many applications they can even out the level fluctuations of a vocal or bass track, or reshape dynamic envelopes of instruments. They can be triggered by sources other than the target track by using the sidechain input. For example, you could use a voiceover track to trigger the compressor on a background music track so that the music is attenuated when the voiceover is heard, a technique called "ducking".
- 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. Like compressors, they can also reshape the dynamic envelopes of instruments, for example, shortening the sustain of a tom-tom. They can also be triggered by sources other than the target track by using the sidechain input. Among other uses, this method can be used 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.
In multitrack recording, multiple sound sources are recorded from separate microphones to individualized audio channels. Downmixing is the process of combining some of those audio channels. This task, which is also known as fold-down, is performed by a mixing engineer.
Consumer electronics may also downmix automatically. For example, a DVD player or sound card may downmix a surround sound signal (four or more channels) to stereophonic sound (two channels) for playback through two-speakers.
Stereo downmixes / fold-downs
Left total /Right total (Lt/Rt)
Lt = L + -3dB*C + -3dB*(-Ls -Rs)
Rt = R + -3dB*C + -3dB*(Ls + Rs)
(where Ls and Rs are phase shifted 90°)
Left only/Right only (Lo/Ro)
Lo/Ro is a downmix suitable when mono compatibility is required. Lo/Ro destroys front/rear channel separation information and thus a Dolby Pro Logic upmixer will not be able to properly extract 5.1 channels again.
Lo = L + -3dB*C + att*Ls
Ro = R + -3dB*C + att*Rs
(where att = -3dB, -6dB, -9dB or 0)
Mixing in surround
Mixing in surround is very similar to mixing in stereo except that there are more speakers, placed to "surround" the listener. In addition to the horizontal panoramic options available in stereo, mixing in surround lets the mix engineer pan sources within a much wider and more enveloping environment. In a surround mix, sounds can appear to originate from many more or almost any direction depending on the number of speakers used, their placement and how audio is processed.
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 speaker. Additionally, reverb and delay effects will often be sent to the rear speakers to create a more realistic sense of being in a real acoustic space. In the case of mixing a live recording that was performed in front of an audience, signals 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 actually a part of the audience.
- 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 engineer do anything he or she wants. Instruments can appear to originate from anywhere, or even spin around the listener. When done appropriately and with taste, interesting sonic experiences can be achieved, as was the case with James Guthrie's 5.1 mix of Pink Floyd's The Dark Side of the Moon, albeit with input from the band. This is a much different mix from the 1970s quadrophonic mix.
- MSS – Multi Stereo Surround – 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 configuration, 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 a different stereo pair such as the left front speaker and the right rear speaker with its reverb returning to yet another stereo pair, 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.
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