An audio mixer is designed to create a single control panel that routes audio signals to and from many sources. When multiple instruments are combined with multiple independent effects processors such as reverb or distortion, the result is a complicated matrix of signals that must be clearly organized. The FX send control is one of the features provided on a mixer to facilitate the processing of these signals as they are incorporated in to the final audio mix.
The FX send control is usually a knob located above each track's fader lever. The actual location and type of control may vary between different mixer models, however. Usually there are as many FX send controls as there are tracks on a mixer. Sometimes the mixer support two independent FX audio buses and thus two such knobs are located above each track. They would be labeled as "FX Send 1" and "FX Send 2," for example. Thus a standard eight track mixer may have as many as 16 FX send controls. Additionally, the master audio output may have its own FX as well.
It is not uncommon to have multiple instruments mixed together on a single recording. These instruments would all be routed through the same mixer. However it is inefficient to have a separate effects processor for each instrument. Certain effects, such as reverb, chorus, delay or distortion, are often used on many instruments simultaneously. The FX send controls make it possible to have one effects processor attached to the mixer that all tracks can use. Each track gets a separate control to determine how much, if any, of its audio signal should be sent to the effects processor. This control, the FX send control, is what makes this determination. If the track is not to be processed at all, the FX send control is turned down to zero.
The mixer can route a separate audio signal into the effects processor. It has an audio output for this purpose separate from the mixer's main audio output. Cables connect this to the effects processor, and then the additional cables are routed between the processor's output back into the mixer. The FX input interface is also independent of any other audio inputs on the mixer. When the FX send control is turned up on a track, part or all of that track's audio signal is sent out into the effects process via these cables and then returned to the final mix. The processed audio is then recombined with the full mix before being output through the mixer's main master audio channel.
The advancements in computer technology have made it possible to replace hardware mixers with software equivalents that perform the same functions. These mixers appear on the computer screen using a similar interface as their hardware counterparts. The mixer controls are all placed in recognizable locations that follow decades of mixer manufacturing convention. Thus a software mixer may also offer FX send controls above its track faders. As software is not restricted to the same limitations of hardware mixers, it is possible for a software mixer to have many more FX send controls than are normally expected. Some software mixers may offer unlimited FX send options that are restricted only by the computer's resources.
When only one effects processor is used for an entire multi-track environment, each instrument will not get its own individual effects settings. Rather, all the instruments must utilize one shared effect. Only the amount of that effect influenced on an individual track can be controlled. For this reason, it is often necessary to use simple but high quality effects that work for a variety of sound sources. An additional problem, often experienced by novices, is that a final mix can quickly sound "mushy" when the same effect is applied to multiple tracks simultaneously. It is advisable that the quality of each track's original sound source be carefully tweaked before any effect is applied so that minimal effects are necessary to optimize the sound.