Sound card mixer: Difference between revisions

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 (with variable gain) these signals, adds then 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.^[1]

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 representing each individual source, 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 filters and distortion.

The following schematic shows how 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 Figure 17 of the AC'97 spec.

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

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

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.

• Convert MIDI into WAV: Play a MIDI file with SW synth or Wave (depending on the playing software) controls at high volume. In record control select mono mix or stereo mix (depends of whether you want a mono or stereo output). The sound can be recorded using any recording software like Windows Sound Recorder.
  In order to avoid interference, all other controls in play control should be muted. If you want to hear the sound while being recorded, care should be taken to avoid feedback, either by lowering speaker volume or repositioning the microphone.
• Record streaming sound (i.e. sound from an internet-radio station): As this sound is controlled through wave control, you have to set this volume in the play control (maybe it's convenient to mute all others, except volume control if you want to hear it meanwhile) In record play select stereo mix or mono mix.
  In this way you can also record any other internal sound of your PC, i.e. voices of TTS engines, sounds of the operating system, ...
• A sound coupling can be easily produced by setting microphone and volume control controls to high volume, especially if the microphone's mic boost setting is turned on.