Desks

The mixing desk is the hub of the system, the bridge of the ship. Good ones cost lots, like, as much as three houses lots, while crappy ones are cheap.

BASIC DESCRIPTION

(Channels):(Groups):(Sends):(Outputs)
For example, 32:4:8:2. Often on cheaper desks there are no sends, so only three numbers are seen.

CHANNELS

Really basic, a channel is a standard input on the desk, and it's associated EQ, fader, etc. 

MONITOR INPUTS

Separate from 'main' inputs, often with reduced facilities. By having monitor inputs, you get facilities for multitrack recording, like allowing the recording to be fed into the Aux network of the desk (typically so the musicians can 'play along' with the recorded sounds, while being recorded themselves on the main desk channels).
Always only line inputs, monitor inputs can be used as further desk channels if not needed as monitors (eg: as FX returns).

GROUPS

Rather than just going into the main Left and Right outputs, 'mixed' channels can be assigned to go to feed other pairs of channels, called groups (or sub-mixes) so they can then be processed or leveled as a stereo pair (eg: an 8 mic drum kit can be set to one group, so the level of the whole kit is then set by the group level).
The 'pairs' on groups are assigned so that a desk channel panned LEFT goes to the ODD number in the group; so strictly, the group isn't a stereo pair; just two additional channels, which can, if wanted be panned to be stereo.
Another use is rather than mixing groups into the master output, each group routes to a separate multitrack recorder input channel. For when you need to mix more channels than you can record.
Assigning desk channels to groups is called ROUTING.

AUXILIARIES (Aux)

Used for effects etc; each aux. is an output, which can be fed a signal from any un-muted input channel on the desk. The level of the signal is set by potentiometers on the channel controls (below the EQ). 
With effects, normal operation is to feed the output to an effect, then return the 'effected' (wet) signal into another desk channel.
When using large desks, buss noise from the many potentiometers connected to the output can present a problem, so where possible, route unused auxs to a different output (therefore buss).

FX (or AUX) RETURNS

imagine they're just another line-level input channel, without EQ or mic preamp; saves an FX using a main desk channel and often feed straight into masters.

The MATRIX

Think of the matrix system as a set of Aux sends, but on the GROUP channels. Used mostly on live desks configured as on-stage monitordesks. Groups are configured as vocals, backing, drums, etc. etc. then the matrix is used to feed an amount of each into a musicians monitor (on the matrix output). Sometimes used for delay stacks on FOH desks.

PFL button

(Pre-Fade Listen/Level), Sends the (mono) signal of the channel, before the fader, to both the control monitor system (headphones/dedicated output) AND to a VU meter, allowing you to check if the initial level is excessive and adjust the channel gain accordingly. 

SOLO button

Similar to PFL, found mostly on install desks, sends the signal, this time in stereo and at POST fade level through the control monitor. Despite the name, numerous tracks can be solo'd at once.

PHANTOM POWER

Method of sending power down balanced cables to power DI boxes and capacitor-mics. 48vDC is fed +ve down the signal pins (2&3), -ve returned via the shield (pin1).

IN LINE MIXER

Where, on recording desks, the main inputs and monitor inputs are controlled by the same "strip" (line of faders/buttons) on the desk. The origins of the 'channel flip' switch.

SPLIT MIXER

More traditional; input channels on the left, monitor and group channels on the right separated by the master controls in the centre. Seen on the best recording desks.

HOUSECLOCK

With digital sources, they all HAVE to be sync'd, at the same sample rate to have any remote hope of mixing together. The Houseclock is found in digital studios and is the master reference code that all the digital sources and recorders are fed with (into the "wordclock" input) to ensure they're sync'd.

DIM button

Again, mostly on recording desks, it attenuates the 'control monitor (headphone) output', to allow the engineer to talk to others, make phonecalls etc. without risking changing the level of their monitor signal.

MUTE AUTOMATION

Essentially a 'programmable' mute, where required channels can 'unmuted' and set to a 'mute state', so the setting is simply recalled by one or two button presses, rather than re-muting the entire desk.
Digital consoles take this a step further where EQ, FX and all other settings (in fader levels) can be 'saved' if desired.

PINK NOISE

Random signal with equal energy per octave; use with specky-analyser and graphic to 'flat' a room. If the audio signal was said to represent light, it would appear 'pink'.

TEST OSCILLATOR

generates a tone to test routing and effects.
Both the pink noise and test tone generators are situated close together in the 'mater' section of recording desks (often with a basic oscilloscope to analyse the sine wave when it returns).

The PAD button

A 'buffer' usually around -20dB, to reduce the level of a strong input signal to a more manageable amplitude.

The PHASE button

Reverses the phase of the input channel.... useful for problem-solving if bass is lacking etc.

PARAMETRIC EQs

like conventional 'bandpass' EQs, but filter width can be adjusted. Width is the 'Q' = divide filter frequency by it's bandwidth. 'Sweep' = Frequency control. High Q has sharp effect, low Q gives smoother sound. 
Parametric can be used for 'spectral mixing' essentially using the possible width to lift the entire tonal range of the instrument / voice on a channel to 'pick it out' or 'set it back' in the overall mix balance.

'LOUDNESS'

remember how a hi-fi loudness button works?: by emphasising bass and treble extremes; one trick is to EQ a sub-mix to make it sound more powerful, rather than the easy option of upping the overall level, where the midrange then begins to muddle the lead vocals / rhythm.

DISTANCE

sounds are instinctively quieter the further away they get, but emphasise this by dropping off the top-end a little; likewise, bring it up to make things sound closer. Good for backing-off support singers etc.

GROUND COMPENSATION

Really shite attempt at a balancing system used on crappy desks. Avoid.

Digital desks usually have onboard gates, compressors, EQ etc. because insert channel connectors don't physically fit onto the desk.