For an audio board to function, it must have an incoming signal. The mic and line inputs at the top of the board allow you to connect XLR microphone cables and phone plugs from other audio devices directly into the mixing console. These inputs feed into the channel below them, so that if a microphone is plugged into the input on top of channel one, that channel's dials and sliders affect the signal. There are also audio outputs that allow you to send signals to auxiliary devices or to a recording deck, CD player or speakers.
The columns of knobs and sliders that make up the main portion of the board are called channels. Each channel controls the signal that is connected to it through its audio inputs. The trim knob effects the amount of signal coming into the board. The AUX, or auxiliary knobs, affect the strength of the signal leaving the board to be processed by an effects device before it re-enters for further manipulation. EQ knobs change the strength of the high, mid-range and low frequencies. The pan knob directs the signal towards the left or right speaker, or sends it evenly to both. The level knob affects the strength of the incoming audio signal, similar to the trim knob. The large slider at the bottom effects the volume of the outgoing signal. Various buttons along each channel provide other options, such as assigning a channels to a group or submix.
The AUX Send portion of the mixer controls the level of the signals being sent out to the auxiliary devices. Most often, these are effects, such as a reverb device or a deesser, that reduce abnormally strong "s" sounds. The controls here affect how much processing is added to the sound before it is re-enters the board to continue normal mixing.
The output section contains controls that affect what signals leave the board and how loud those signals are. Solo buttons allow you to isolate certain channels and turn off others. Other buttons select certain groups. Sliders control the relative volume of individual submixes, or composite signals from other channels, while a master slider controls the overall output volume.
An effective sound mixer must wear headphones so that he can accurately hear the outgoing audio signal. The monitor section of the board allows him a place to plug these in and control what he hears and at what volume.
Sometimes your ears are not enough to accurately judge the signal strength. This is why various meters are placed on the board. As the signal strength fluctuates, the meters rise and fall to indicate decibel levels. Some of the most common meters contain colored lights. When the signal is adequate, it falls within the green range. When it is in danger of peaking, or going beyond the limits of what the board can handle, it falls within the yellow range. A peaking signal lights up red.