Mixing Boards/Consoles
Mixing boards or mixing consoles (also called mixing desks, soundboards, or just mixers) allow you to adjust, mix, and route audio streams. They can be analog or digital, and can be physical or virtual (software).
Contents
Channels
Each input and its associated controls are referred to as a channel or channel strip. The term also sometimes covers outputs when they have a similar appearance. The channels are typically arranged in single row, with the inputs on the left and outputs on the right, although larger mixers may place the outputs in the middle. This is why they are called channel strips - each channel looks like a vertical strip of tape.
Channels often feature numerous controls that affect only that channel. They may have different types of input jacks depending on what instruments or gear are designed to input into that channel, and may accept mono or stereo input. They may contain mic pre-amps and phantom power.
Buses
The term bus is very generic and can refer to any sort of connection to transfer audio in the mixer. However, it generally refers to groups of channels. For example, the master output is often called the master or main bus, as all the other channels are routed through it before being sent out of the mixer. There may also be one or several auxiliary (or "aux") buses that carry the signal from a subset of the available channels.
When multiple channels are all routed into a single channel, that channel is called a bus. For instance, drums typically require numerous microphones to capture the entire kit's sound - all those mics are fed into separate input channels which are all routed to a drum bus, where they can be processed together before being output to the master bus. This makes it much simpler and quicker to adjust the volume of the drum kit as a whole, for example. Types of instruments can be routed this same way - guitars to a guitar bus, background vocals to a vocals bus, brass instruments to a brass bus, etc.
Volume Faders
The most obvious and common element of each channel is a volume fader. This is the thing you slide up and down to adjust the volume of the channel, consuming more of the space for the strip than any other single control. The reason such large space is devoted to the volume faders is because the most instrumental part of the mixing board is to ensure all the instruments are mixed at the correct relative volumes. Sliding faders give the user instant visual feedback on the mixer's volume setting for each channel and allow fine adjustments. That said, some mixers are designed to be smaller and may forego faders in place of simple knobs.
Trim/Gain
Since mixing boards are typically working with microphones and a wide range of instruments, incoming volumes can be radically different from channel to channel. A trim or gain pot/control is used to attenuate the inbound signal before it hits the pre-amp or fader. This helps avoid clipping as well as maintaining good gain staging. It can be used to roughly level or normalize all the channels, while the faders are used to make some instruments more prominent or otherwise create a pleasing mix.
It may seem a bit counterintuitive to have two ways to control volume, but remember that certain processing occurs before the signal is affected by the fader, such as amplification by a microphone pre-amp and adjustments on the channel strip itself. Too hot of a signal can result in unwanted distortion. Furthermore, an extremely hot signal would require sliding the volume fader very low to normalize the instrument with others. This greatly reduces the volume fader's adjustment range, making it more difficult to fine-tune that channel's volume.
Phantom Power
Microphone inputs often feature a button to enable phantom power. This provides a +48V DC current through the 2 signal lines of the balanced cable connected to the microphone, essentially powering the microphone. While most dynamic and ribbon microphones do not require this, condenser microphones typically do. It can also be used to power active electronics on other types of devices. Since balanced connections operate on the voltage difference between the 2 signal lines, constant DC power in both lines should not have any effect on the signal itself. Toggling phantom power on/off should have no noticeable effect on a microphone that does not require it, hence the name phantom power.
Pad
A pad button or switch is used to attenuate the signal level by a large amount as soon as it enters the mixing board, before hitting any pre-amp (for example -26 db). This is useful for high-volume signals, such as direct feeds from electric guitars with high-output humbucker pickups. The trim/gain knob may not provide enough range to attenuate the signal to the point where it will not clip, or this will help normalize the signal to a level, where it is easier to fine-tune the trim/gain knob to achieve the best gain staging.
Mute
Most channels feature a mute button that does exactly that - it mutes the channel, preventing it from being output to any buses. In other words, you will no longer hear it in the mix. This can be useful if an instrument starts feeding back, or a guitar amp malfunctions, or something similarly-awful happens that starts causing the channel to fill the mix with unwanted noise. It can also be used to compare a mix with and without a certain instrument to determine if the instrument is helping or hurting the mix.
Solo
A solo (or cue) button on the channel mutes all other channels, unless they also have their solo button pushed. This allows you to quickly monitor a particular channel (or several) apart from the full mix. It saves you the trouble of having to push the mute button on every other channel.
Balance/Pan
Mono channels typically have pan controls, while stereo have balance controls. A mono channel is typically defaulted to the center of the stereo mix, so that it is equally loud on each side of the stereo spectrum. However, you can use pan to move it off-center, so it is stronger on the left or right side. This has numerous applications - you may want each of the drums to be a bit off-center, resembling their placement in the actual drum kit, giving the drums a wider or more 3-dimensional sound. Or when multi-tracking guitars, you may want to pan one track to the left and the other to the right. If the tracks were performed in-time with each other, this gives the illusion that there is one guitar but it sounds wider and bigger than a single guitar track panned to center.
Stereo inputs can also have pan controls - in this case, the sound that enters one side will be "pushed" increasingly into the other. A full left pan essentially mixes the incoming left and right fields and outputs them into the left field. More often, stereo inputs have balance controls, which control the relative volume between the incoming stereo fields. A center balance leaves each part of the signal untouched, while shifting balance to the right attenuates the left field and vice versa.
Aux Send/Receive
A channel may feature the ability to route to one or more aux buses. These can be used in various ways, possibly exporting an alternative mix of all the instruments, such as to stage monitors. Or they can be routed into a single channel, creating a channel bus, as described above for drums or other instrument types.
They are commonly used to feed external processors and called sends when used in this way. The return feed from the processor(s) is input back into a channel, called a receive where it can then be routed to the main mix. For example, you may want all the instruments that are close-mic'ed (channels 1-4) to be run through the same reverb processing. You can route all those channels to Aux bus 1, which is output to a reverb processor. The reverb outputs to another mixer channel (channel 8) where it can be mixed with other instruments (channels 5-7) into the main mix. If the mixer cannot prevent the original close-mic'ed channels from being sent to the main mix, you could run the reverb processor 100% wet, then use the fader on channel 8 to get the correct wet/dry mix between channels 1-4 and channel 8.
EQ
A channel may feature relatively simple EQ controls, typically Bass, Mids, Treble, or a low-pass and/or high-pass. See the EQ page for more about EQ.
Compression/Limiting
Channels often feature rudimentary compression/limiting to prevent strong transients from clipping the mix. This is typically a single knob or button. See Compression for more info.
Level Meters
Channels often have level meters to view the current volume of that channel's output. These are typically measured in decibels and consist of a series of colored LED's. The number of LED's lit up indicates the volume, with lower volumes only lighting up the lowest LED's and high volumes lighting up all of them. Most of them are colored green, indicating that the signal has no danger of clipping. Towards the top, the LED's change to yellow, with the topmost LED being red. Red indicates clipping, requiring compression/limiting or lowering the volume fader. Yellow indicates that there is a danger of clipping on a strong transient.
Sometimes meters appear as gauges, similar to a sector of an analog clock. The needle resting fully to the left indicates minimum volume and pressed fully right means maximum. There are numbers to indicate the decibel level of the signal, with a red strip at the largest ones indicating clipping.
Many meters provide two sets of data simultaneously - the current volume, and the maximum volume encountered over a larger time-frame, or the peak. Both provide relevant data - the peaks indicate whether you have and can expect to hit clipping levels, even if only momentarily, while the current volume gives an idea of how loud the instrument is in general. You may use the volume fader to avoid clipping, but then find that your current volume is too low, indicating a need to apply some compression.