A sound mixing console is an electronic device for combining sounds of many different audio signals. Inputs to the console include microphones or electronic instruments. Depending on the type, a mixer is able to control analog or digital signals.
In practice, mixers do more than simply mix signals. They can provide phantom power for condenser microphones; pan control, which changes a sound's apparent position in the stereo soundfield; filtering and equalization, which enables sound engineers to boost or cut selected frequencies to improve the sound; dynamic range compression.
A mixing console is also known as an audio mixer, audio console, mixing desk, sound mixer, sound board, or simply as board or mixer.
An analog mixing board is divided into functional sections. Some of the more important functional sections are subdivided into subsections.
Channel input strip
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/or faders for controlling the gain of the input preamplifier (which strengthens the audio signal), adjusting the equalization (e.g., bass and treble) of the signal on each channel, and changing the volume of the overall channel. Each channel can have one input (if it is a monaural channel) or two inputs for stereo channels.
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 attenuation "pad" (often reducing the signal by 15 dB or 20dB to prevent unwanted distortion or audio clipping). 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 ("mids").
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 smaller mixers, the input jacks may be mounted on the top panel of the mixer, near the top, to facilitate the connection and disconnection of inputs during the use of the mixer.
The input strip is usually separated into these sections:
• Input jacks
• Microphone preamplifiers
• Dynamics processing (e.g. dynamic range compression, gating)
• Routing including direct outs, auxiliary-sends ("aux-sends"), panning control and subgroup assignments
• Level-control faders (on some very small micro-mixers, these may be rotary knobs, to save space)
Basic input controls
Below each input, there are usually several rotary controls (knobs or "pots"). The first knob is typically a trim or gain control. The inputs buffer the signal from the external device and this controls the amount of amplification (boosting) or attenuation (turning down of gain) needed to bring the signal to a nominal level for processing. This stage is where most noise of interference is picked up, due to the high gains involved (around +50 dB, for a microphone). Balanced inputs and connectors, such as XLR or phone connectors, reduce interference problems.
A microphone that was plugged directly into a power amplifier would not produce sound, because the microphone's signal is too weak; the microphone signal needs to be plugged into a preamplifier to strengthen the signal so that it is strong enough to be amplified by the power amplifier. For some very strong line level signals, the signal that is plugged into the mixer might be too strong, and it might be causing audio clipping. For signals that are too strong, a 15 dB or 20 dB pad can be used to attenuate the gain. 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 (S/N ratio).
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 a parametric equalizer 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.
Master output controls
The master control section is used to adjust the levels of the overall output of the mixer. On most mixers, the master control is a fader. However, on some mini-mixers, rotary knobs are used instead to save space.
Finally, there are usually one or more VU or peak meters (peak meters often use LEDs) 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" (unwanted distortion) or causing audio feedback "howls". Having the gain set as high as possible improves the signal to noise ratio.
As the human ear experiences audio level in a logarithmic fashion (both amplitude and frequency), mixing console controls and displays are almost always in decibels, a logarithmic measurement system. Since it is a relative measurement, and not a unit itself, the meters must be referenced to a nominal level.
The "professional" nominal level used on professional mixers is considered +4 dBu. The "consumer grade" level is −10 dBV.