Mixing console

The channel input strips are usually a bank of identical monaural or stereo input channels arranged in columns. Typically, each channel's column contains a number of rotary potentiometer knobs, buttons, and faders for controlling the gain of the input preamplifier, adjusting the equalization of the signal on each channel, controlling routing of the input signal to other functional sections, and adjusting the channel's contribution to the overall mix being produced.

The types of inputs that can be plugged into a mixer depend on the intended purpose of the mixer. A mixer intended for a live venue or a recording studio typically has a range of input jacks, such as XLR connectors for microphones and the outputs from DI boxes, and 1/4" jacks for line level sources. A DJ mixer typically has RCA connector inputs for pre-recorded music being played back on turntables or CD players, and a single mic input.

Depending on the mixer, a channel may have buttons which enable the audio engineer to reroute the signal to a different output for monitoring purposes, turn on an attenuator pad (often reducing the signal by 15 or 20 dB to prevent audio clipping), or activate other features, such as a high-pass filter. Some higher-priced mixers have a parametric equalizer or a semi-parametric equalizer for one or more of the equalizer frequency bands, often the middle range.

The channel strips are typically numbered so that the audio engineer can identify the different channels. For each channel input, a mixer provides one or more input jacks. On mid- to- large-sized live venue and sound recording consoles, these input jacks are numbered as well and consolidated in a patch bay. On smaller mixers, the input jacks may be mounted on the top panel of the mixer to facilitate the connection and disconnection of inputs during the use of the mixer.

The input strip is usually separated into sections:

• Input jacks
• Microphone preamplifiers
• Equalization
• Dynamics processing (e.g. dynamic range compression, gating), if supported
• Routing, including direct outs, auxiliary-sends, panning control, and subgroup assignments
• Level-control faders (on small mixers, these may be rotary knobs to save space)

On many consoles, these sections are color-coded for quick identification by the operator. Each signal (e.g., a singer's vocal mic, the signal from an electric bass amp's DI box, etc.) that is plugged into the mixer has its own channel. Depending on the specific mixer, each channel is stereo or monaural. On most mixers, each channel has an XLR input, and many have RCA or quarter-inch TRS phone connector line inputs. The smallest, least expensive mixers may only have one XLR input with the other inputs being line inputs. These can be used by a singer-guitarist or other small acts.

The first knob at the top of an input strip is typically a trim or gain control. The input/preamp conditions the signal from the external device and this controls the amount of amplification or attenuation that is applied to the input signal to bring it to a nominal level for processing. Due to the high gains involved (around +50 dB, for a microphone), this stage is where most noise and interference is picked up. Balanced inputs and connectors, such as XLR or phone connectors that have been specifically wired as balanced lines, reduce interference problems.

A microphone plugged directly into a power amplifier would not produce an adequate signal level to drive loudspeakers, because the microphone's signal is too weak; the microphone signal needs a preamplifier to strengthen the signal so that it is strong enough for the power amplifier. For some very strong line level signals, the signal that is plugged into the mixer may be too strong, and cause audio clipping. For signals that are too strong, a 15 dB or 20 dB pad can be used to attenuate the signal. Both preamplifiers and pads, and the controls associated with them, are available in the input section of most mixing consoles.

Audio engineers typically aim at achieving a good gain structure for each channel. To obtain a good gain structure, engineers usually raise the gain as high as they can before audio clipping results; this helps to provide the best signal to noise ratio.

A mixing console may provide insert points after the input gain stage. These provide a send and return connection for external processors that only affect an individual channel's signal. Effects that operate on multiple channels connect to auxiliary sends (below).

The auxiliary send routes a split of the incoming signal to an auxiliary bus, which can then be routed to external devices. Auxiliary sends can either be pre-fader or post-fader, in that the level of a pre-fader send is set by the auxiliary send control, whereas post-fade sends depend on the position of the channel fader as well. Auxiliary sends can send the signal to an external processor such as a reverb, with the return signal routed through another channel or designated auxiliary return. Post-fader sends are normally used in this case. Pre-fade auxiliary sends can provide a monitor mix to musicians on stage (which they hear through monitor speakers pointing at the performers or in-ear monitors); this mix is thus independent of the main mix produced by the faders.

Program channels on a radio sound board

Most live radio broadcasting sound boards send audio through program channels. Most boards have 3-4 program channels, though some have more options. When a given channel button is selected, the audio will be sent to that device or transmitter. Program 1 is typically the on-air live feed, or what those listening to the broadcast will hear. Other program channels may feed one or more computers used for editing or sound playback. Another program channel may be used to send audio to the talent's headset if they are broadcasting from a remote area.

Further channel controls affect the equalization of the signal by separately attenuating or boosting a range of frequencies. The smallest, least expensive mixers may only have bass and treble controls. Most mid-range and higher-priced mixers have bass, midrange, and treble, or even additional mid-range controls (e.g., low-mid and high-mid). Many high-end mixing consoles have parametric equalization on each channel. Some mixers have a general equalization control (either graphic or parametric) at the output, for controlling the tone of the overall mix.

The cue system allows the operator to listen to one or more selected signals without affecting the console's main outputs. A sound engineer can use the cue feature to, for instance, get a sound recording they wish to play soon cued up to the start point of a song, without the listeners hearing these actions. The signal from the cue system is fed to the console's headphone amp and may also be available as a line-level output that is intended to drive a monitor speaker system. The terms AFL (after-fader listen) and PFL (pre-fader listen) are used to describe respectively whether or not the level of the cue signal for an input is controlled by the corresponding fader. Consoles with a cue feature have a dedicated button on each channel, typically labeled Cue, AFL, PFL, Solo, or Listen. When cue is enabled on multiple channels, a mix of these signals is heard through the cue system.

Solo in place (SIP) is a related feature on advanced consoles. It typically is controlled by the cue button, but unlike cue, SIP affects the output mix; It mutes everything except the channel or channels being soloed. SIP is useful for setup of a mixing board and troubleshooting, in that it allows the operator to quickly mute everything but the signal being adjusted. For example, if an audio engineer is having problems with clipping on an input, they may use SIP to solely hear that channel, so that the problem can be diagnosed and addressed. SIP is potentially disastrous if engaged accidentally during a performance, as it will mute all the channels except one, so most consoles require the operator to take very deliberate actions to engage SIP.

Each channel on a mixer has a volume control (fader) that allows adjustment of the level of that channel. These are usually sliders near the front of the mixing board, although some smaller mixers use rotary controls to save space. The signals are summed to create the main mix, or combined on a bus as a submix, a group of channels that are then added as a whole to the final mix. For instance, many drum mics could be grouped into a bus, and then the proportion of drums in the final mix can be controlled with one bus fader.[3] A bus can often be processed just like an individual input channel, allowing the engineer to process a whole group of signals at once. Once again using the drum kit example, the use of bus-processing can enable the sound engineer to run all of the drum kit through an audio compressor effect to reduce unwanted signal peaks, rather than having to route all of the 10 or more mic signals on the drum kit individually. There may also be insert points for a certain bus, or even the entire mix.

Some higher-end consoles use voltage-controlled amplifier (VCA)[lower-alpha 1] VCAs function somewhat like a submix but let the operator control the level of multiple input channels with a single fader. Unlike subgroups, no sub-mix is created. The audio signals from the assigned channels remain routed independently of VCA assignments. Since no sub-mix is created, it is not possible to insert processing such as compressors into a VCA/DCA group. In addition, on most VCA-equipped consoles, post-fader auxiliary send levels are affected by the VCA master. This is usually desirable, as post-fader auxiliary sends are commonly used for effects such as reverb, and sends to these effects should track changes in the channel signal level.

The master control section is used to adjust the levels of the overall output of the mixer. The master control section on a large live venue or sound recording mixer typically has sub-group faders, master faders, master auxiliary mixing bus level controls and auxiliary return level controls. On most mixers, the master control is a fader. However, on some small mixers, rotary knobs are used instead to save space.

In a typical live sound mixing context, with a band playing at a venue, consisting of a rhythm section, solo instrumentalists and singers, the master control section allows the audio engineer to control the volume of the entire group with just one fader (for monaural mixers) or a pair of left and right faders (for stereo mixers).

Subgroup and main output fader controls are often found together on the right hand side of the mixer or, on larger consoles, in a center section flanked by banks of input channels. Matrix routing is often contained in this master section, as are headphone and local loudspeaker monitoring controls. Talkback controls allow conversation with the artist through their monitors, headphones or in-ear monitor. A test tone generator might be located in the master output section. Aux returns such as those signals returning from external processors are often in the master section.

Meter bridge on API Legacy Plus console

⇑ VU meter

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⇐ Peak program meter

There are usually one or more VU or peak meters[lower-alpha 2] to indicate the levels for each channel, for the master outputs and to indicate whether the console levels are clipping the signal. The sound engineer typically adjusts the gain of the input signals to get the strongest signal that can be obtained without causing clipping. Having the gain set as high as possible improves the signal to noise ratio. Most mixers have at least one additional output besides the main mix. These are either individual bus outputs or auxiliary outputs, used, for instance, to output a different mix to onstage monitors.

The meters may be above the input and master sections or they may be integrated into the input and master sections themselves. Meters may have needles or LEDs. On meters using LEDs, there may be different colored LEDs to indicate when there is signal present in the channel's input; the audio level of the channel, typically by lighting up more LEDs; and clipping, which may be indicated using a different colored LED. In one popular color-coding system, green LEDs indicate signal presence and the audio level; one or more amber LEDs indicate that the channel is approaching clipping; and one or more red LEDs indicate clipping.

As the human ear experiences audio level in a logarithmic fashion,[lower-alpha 3] mixing console controls and displays are almost always labeled in decibels, a logarithmic measurement system. Since the decibel represents a relative measurement, and not a unit itself, the meters must be referenced to a nominal level. Most professional audio equipment is referenced to a nominal level of +4 dBu, while semi-professional and domestic equipment is usually referenced to a nominal level of −10 dBV.[4]

For convenience, some mixing consoles include inserts or a patch bay or patch panel. Patch bays are more common in recording mixers than live sound mixers. In live sound, the cables from the onstage microphones and instrument outputs are not typically plugged directly into the mixer, because this would require a large number of individual cables to go from the stage to the mixer. Instead, the onstage mic and instrument cables are typically plugged into the stage box of a snake cable which runs from the stage to the mixer. The snake is then plugged into the mixer.

Some mixing consoles, particularly those designed for broadcast and live sound, include facilities for mirroring two consoles, making both consoles exact copies of each other with the same inputs and outputs, the same settings, and the same audio mix. There are two primary reasons for doing this; one, in the event of a hardware failure, a second redundant console is already in place and can be switched to (an important feature for live broadcasts); second, it allows the operators to set up two identical mix positions, one at front of house — where the audio will be mixed during a performance — and the other at some other location within the theater (e.g., with the broadcasting equipment); this way, if the acoustics at front of house are unfavorable, a mix can be programmed at an acoustically better position in the room, and the presets (on the faders and knobs) can be accessed from the front of house console during the performance.

Digital mixers have an unavoidable amount of latency, ranging from less than 1 ms[6] to as much as 10 ms, depending on the model of digital mixer and what functions are engaged. This small amount of latency is not a problem for loudspeakers aimed at the audience an not necessarily a problem for monitor wedges aimed at the artist, but can be disorienting and unpleasant for in-ear monitors where the artist hears their voice acoustically in their head and electronically amplified in their ears but delayed by a couple of milliseconds.[7]

Every analog to digital conversion and digital to analog conversion within a digital mixer introduces latency. Audio inserts to favorite external analog processors make for approximately double the usual latency. Further latency can be traced to format conversions such as from ADAT to AES3 and from normal digital signal processing steps.

Within a digital mixer, there can be differing amounts of latency, depending on the routing and on how much DSP is in use. Assigning a signal to two parallel paths with significantly different processing on each path can result in comb filtering when recombined. Some digital mixers incorporate internal methods of latency correction so that such problems are avoided.

16-channel mixing console with compact short-throw faders

Analog consoles have a column of dedicated, physical knobs, buttons, and faders for each channel, which is logical and familiar to generations of audio engineers who have been trained on analog mixers. This takes more physical space but can accommodate rapid responses to changing performance conditions.

Most digital mixers use technology to reduce physical space requirements, entailing compromises in user interface such as a single shared channel adjustment area that is selectable for only one channel at a time. Additionally, most digital mixers have virtual pages or layers that change fader banks into separate controls for additional inputs or for adjusting equalization or aux send levels. This layering can be confusing for some operators. Many digital mixers allow internal reassignment of inputs so that convenient groupings of inputs appear near each other in the fader bank, a feature that can be disorienting for persons having to make a hardware patch change.

On the other hand, many digital mixers allow for extremely easy building of a mix from saved data. USB flash drives and other storage methods are employed to bring past performance data to a new venue in a highly portable manner. At the new venue, the traveling mix engineer simply plugs the collected data into the venue's digital mixer and quickly makes small adjustments to the local input and output patch layout, allowing for full show readiness in very short order. Some digital mixers allow offline editing of the mix, a feature that lets the traveling technician use a laptop to make anticipated changes to the show, shortening the time it takes to prepare the sound system for the artist.

A studio engineer at a Control 24 mixing surface

Both digital and analog mixers rely on analog microphone preamplifiers, a high-gain circuit that increases the low signal level from a microphone to a level that is better matched to the console's internal operating level. In this respect, both formats are on par with each other. In a digital mixer, the microphone preamplifier is followed by an analog-to-digital converter. Ideally, this process is carefully engineered to deal gracefully with overloading and clipping while delivering an accurate digital stream. Further processing and mixing of digital streams within a mixer need to avoid saturation if maximum audio quality is desired.

Analog mixers, too, must deal gracefully with overloading and clipping at the microphone preamplifier and as well as avoiding overloading of mix buses. Very high-frequency background hiss in an analog mixer is always present, though good gain stage management and turning unused channels down to zero minimizes its audibility. Idle subgroups left "up" in a mix add background hiss to the main outputs. Many digital mixers avoid this problem by low-level gating. Digital circuitry is more resistant to outside interference from radio transmitters such as walkie-talkies and cell phones. Hiss can be reduced with electronic noise reduction devices or with an equalizer.

Many electronic design elements combine to affect perceived sound quality, making the global "analog mixer vs. digital mixer" question difficult to answer. Experienced live sound professionals agree that the selection and quality of the microphones and loudspeakers (with their innate higher potential for creating distortion) are a much greater source of coloration of sound than the choice of mixer. The mixing style and experience of the person mixing may be more important than the make and model of audio console. Analog and digital mixers both have been associated with high-quality concert performances and studio recordings.

Hip hop producer Chilly Chill behind a large audio console in a recording studio

Analog mixing in live sound has had the option since the 1990s of using wired remote controls for certain digital processes such as monitor wedge equalization and parameter changes in outboard reverb devices. That concept has expanded until wired and wireless remote controls are being seen in relation to entire digital mixing platforms. It is possible to set up a sound system and mix via laptop, touchscreen or tablet. Computer networks can connect digital system elements for expanded monitoring and control, allowing the system technician to make adjustments to distant devices during the performance. The use of remote control technology can be utilized to reduce the amount of venue space used for the front of house mixing console, nicknamed "seat-kills" in the music industry. As such, using remote control technologies in a venue can enable them to fit more paying customers into the venue.

For recorded sound, the mixing process can be performed on screen, using computer software and associated input, output and recording hardware. The traditional large control surface of the mixing console is not utilized, saving space at the engineer's mix position. In a software studio, there is either no physical mixer fader bank at all or there is a compact group of motorized faders designed to fit into a small space and connected to the computer. Many project studios use such a space-efficient solution, as the mixing room at other times can serve as the business office, media archive, etc. Software mixing is heavily integrated as part of a digital audio workstation.