A sound card mixer is the analog part of a

sound card A sound card (also known as an audio card) is an internal expansion card that provides input and output of audio signals to and from a computer under the control of computer programs. The term ''sound card'' is also applied to external audio i ...

that routes and mixes sound signals. This circuit receives inputs from both external connectors and the sound card's

digital-to-analog converter In electronics, a digital-to-analog converter (DAC, D/A, D2A, or D-to-A) is a system that converts a digital signal into an analog signal. An analog-to-digital converter (ADC) performs the reverse function. There are several DAC architec ...

s. It selects or mutes, amplifies (with variable

gain Gain or GAIN may refer to: Science and technology * Gain (electronics), an electronics and signal processing term * Antenna gain * Gain (laser), the amplification involved in laser emission * Gain (projection screens) * Information gain in de ...

) these signals, adds them together, and finally routes the result to both external output connectors and the sound card's

analog-to-digital converter In electronics, an analog-to-digital converter (ADC, A/D, or A-to-D) is a system that converts an analog signal, such as a sound picked up by a microphone or light entering a digital camera, into a digital signal. An ADC may also provide ...

s. 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 AC'97 (''Audio Codec '97;'' also MC'97 for ''Modem Codec '97'') is an Sound reproduction, audio codec standard developed by Intel Architecture Labs in 1997. The standard was used in motherboards, modems, and sound cards. The specification covers ...

Audio Component Specification.AC'97 Component Specification
Revision 2.3, Intel Corporation, April 2002. Figure 17: AC ’97 mixer functional diagram (section 5.5, page 43)

Mixer controls

Sound card mixer controls are provided through the GUI interface in the computer's

operating system An operating system (OS) is system software that manages computer hardware, software resources, and provides common services for computer programs. Time-sharing operating systems schedule tasks for efficient use of the system and may also in ...

. On most

desktop environments In computing, a desktop environment (DE) is an implementation of the desktop metaphor made of a bundle of programs running on top of a computer operating system that share a common graphical user interface (GUI), sometimes described as a graphica ...

, the mixer can be accessed via a Volume icon in the

system tray A taskbar is an element of a graphical user interface which has various purposes. It typically shows which programs are currently running. The specific design and layout of the taskbar varies between individual operating systems, but generally a ...

. Mixer controls are similar to that of a

mixing console A mixing console or mixing desk is an electronic device for mixing audio signals, used in sound recording and reproduction and sound reinforcement systems. Inputs to the console include microphones, signals from electric or electronic inst ...

. 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 (output) sources and recording (input) sources. Additional hardware or software may add other effects such as

low-pass filter A low-pass filter is a filter that passes signals with a frequency lower than a selected cutoff frequency and attenuates signals with frequencies higher than the cutoff frequency. The exact frequency response of the filter depends on the filter des ...

distortion In signal processing, distortion is the alteration of the original shape (or other characteristic) of a signal. In communications and electronics it means the alteration of the waveform of an information-bearing signal, such as an audio signal ...

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: soundcard mixing schema

A more accurate depiction of the mixing scheme used in AC'97 compatible sound cards can be seen i
Figure 17 of the AC'97 spec

Typical input channels and controls

Each of the following signal sources has its own gain and mute control in a typical mixer scheme:

Typical output channels and controls

Each of the following signal destinations has its own gain and mute control in a typical mixer scheme: {, class="wikitable" , - ! Control ! channels ! Controlled source , - , Line out ,

stereo Stereophonic sound, or more commonly stereo, is a method of sound reproduction that recreates a multi-directional, 3-dimensional audible perspective. This is usually achieved by using two independent audio channels through a configuration ...

, Audio signal provided to the LINE OUT jack of a sound card (colored ''lime green'' in

PC99 The PC System Design Guide (also known as the PC-97, PC-98, PC-99, or PC 2001 specification) is a series of hardware design requirements and recommendations for IBM PC compatible personal computers, compiled by Microsoft and Intel Corporation duri ...

). This can be connected to headphones or a HiFi amplifier, etc. , - , Aux out ,

, Audio 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 speaker , mono , Audio signal provided to the MONO connection of the mixer. Some PCs connect this signal to an internal

PC speaker A PC speaker is a loudspeaker built into some IBM PC compatible computers. The first IBM Personal Computer, model 5150, employed a standard 2.25 inch magnetic driven (dynamic) speaker. More recent computers use a tiny moving-iron or pie ...

. , - ,

SPDIF S/PDIF (Sony/Philips Digital Interface) is a type of digital audio interface used in consumer audio equipment to output audio over relatively short distances. The signal is transmitted over either a coaxial cable (using RCA or BNC connectors) ...

, 7.1 , Digital interface

Typical record controls

In a typical AC’97 style mixer scheme, the

s 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.

References

Sound cards