Sound card mixer


A sound card mixer is the analog part of a sound card that routes and mixes sound signals. This circuit receives inputs from both external connectors and the sound card's digital-to-analog converters. It selects or mutes, amplifies these signals, adds them together, and finally routes the result to both external output connectors and the sound card's analog-to-digital converters. Different mixing schemes are in use, but the ones implemented in most IBM-PC compatible computers today are variants of a scheme defined in Intel's AC'97 Audio Component Specification.

Mixer controls

Sound card mixer controls are provided through the GUI interface in the computer's operating system. On most desktop environments, the mixer can be accessed via a Volume icon in the system tray.
Mixer controls are similar to that of a mixing console. They consist of volume sliders or rotary controls that represent each individual source, which may be accompanied by balance and mute controls. Most interfaces provide a method of switching between playback sources and recording sources. Additional hardware or software may add other effects such as low-pass filters, distortion and boost controls.

Mixing scheme

The following schematic shows a greatly simplified example of how some sound card's mixers manipulate sound from different sources:
A more accurate depiction of the mixing scheme used in AC'97 compatible sound cards can be seen in .

Typical input channels and controls

Each of the following signal sources has its own gain and mute control in a typical mixer scheme:
ControlchannelsControlled source
Wave / PCMstereoAudio signal generated by the CPU via the sound card's digital-to-analog converter.
MIDI/SW SynthstereoAudio signal generated by a synthesizer chip or digital signal processor on the sound card.
CD playbackstereoAudio signal received from the analog CD input of the mixer, which is usually connected to the analog audio output of a CD drive.
MicrophonemonoAudio signal received through one of the analog microphone inputs. MIC1 is typically connected to the external microphone jack. An optional 20 decibel amplifier can be activated for this input in most mixers.
Line instereoAudio signal received through the LINE input of the mixer, usually connected to an external jack, which can be used to connect a HiFi amplifier, tuner, iPod, television, etc.
Aux instereoAudio signal received through the AUX input of the mixer. This mixer input is not used by all PCs.
PC speakermonoSome sound cards provide an input for the audio signal that the Programmable Interval Timer of an IBM PC generates for the PC speaker.
SPDIFstereoDigital interface, uncommon in the PC world

Typical output channels and controls

Each of the following signal destinations has its own gain and mute control in a typical mixer scheme:
ControlchannelsControlled source
Line outstereoAudio signal provided to the LINE OUT jack of a sound card. This can be connected to headphones or a HiFi amplifier, etc.
Aux outstereoAudio signal provided to the AUX OUT connection of the mixer. Not all sound cards provide this mixer channel on an external connector.
Mono out / PC speakermonoAudio signal provided to the MONO connection of the mixer. Some PCs connect this signal to an internal PC speaker.
SPDIF7.1Digital interface

Typical record controls

In a typical AC’97 style mixer scheme, the analog-to-digital converters that allow the CPU to receive audio signal can either be connected directly to one of each of the inputs, or they can be connected to the same summation result that the mixer can provide via the line, aux and mono analog outputs. Therefore, in addition to the above input and output gain controls, a mixer also provides a number of controls for selecting the recording source. The result of this selection is again subject to a mute and gain control before it is digitized.