Components used to combine audio sources are called mixers because they mix multiple audio sources into a combined output. An audio mixer can include dozens of inputs, with advanced features to provide sophisticated signal routing to a variety of destinations. A smaller 4-channel mixer typically has only four channels to handle incoming audio sources, but can have multiple audio output channels. Generally, a 4-channel mixer is designed to solve more basic audio problems.
Disc Jockeys (DJs) typically use a 4-channel mixer to combine two stereo audio sources so music can be cross-faded from one song to another. Mobile audio production crews for film or television may use a 4-channel mixer to combine audio from two actors while also capturing the ambient sound from the location. The mixers are frequently used in conjunction with a public address (PA) system where solo performers need to blend a vocal microphone with an instrument. A 4-channel mixer might also be used in an auditorium where a group of speakers each require their own microphone.
Most 4-channel mixers include design features for specific applications. A mixer used by DJs typically will feature slider controls that are well suited for smooth transitions between sources. Mobile audio production units might use rotary knobs so an accidental bump will not cause an unwanted adjustment of the volume level. Depending on the application, mixers may also provide light-emitting diode (LED) displays that warn when input levels are too high, or a larger multi-color LED display that shows output levels.
Regardless of the number of inputs provided, mixers often include additional features to further control the audio signal. These features might include equalization for each channel. Equalization allows the user to adjust different parts of the audio spectrum. Reducing low, or bass, frequencies can help reduce feedback problems. Reducing high, or treble, frequencies reduces hiss or noise created by the input source.
Some mixers include the ability to send a separate audio level to additional outputs. A musician may need to hear themselves through a stage monitor, as well as send a separate audio level to the PA system. Mixers can allow performers to adjust both the stage monitors and the sound heard by the audience by having separate controls for each output. The cost of the mixer typically will increase with the number of added features.
A well-designed audio mixer generally will operate without adding additional noise or distortion from its internal electronic circuits. Mobile mixers used by musicians or mobile applications are designed to endure the bumps and scrapes that happen with frequent movement. As needs grow, 4-channel mixers can be replaced by models that feature more inputs for additional audio sources.