A sound card mixer is the analog part of a sound card

Sound card

A sound card is an internal computer expansion card that facilitates the input and output of audio signals to and from a computer under control of computer programs. The term sound card is also applied to external audio interfaces that use software to generate sound, as opposed to using hardware...

 that routes and mixes sound signals. This circuit receives inputs from both external connectors and the sound card's digital-to-analog converter

Digital-to-analog converter

In electronics, a digital-to-analog converter is a device that converts a digital code to an analog signal . An analog-to-digital converter performs the reverse operation...

s. It selects or mutes, amplifies (with variable gain

Gain

In electronics, gain is a measure of the ability of a circuit to increase the power or amplitude of a signal from the input to the output. It is usually defined as the mean ratio of the signal output of a system to the signal input of the same system. It may also be defined on a logarithmic scale,...

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

Analog-to-digital converter

An analog-to-digital converter is a device that converts a continuous quantity to a discrete time digital representation. An ADC may also provide an isolated measurement...

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 Audio Component Specification.

Mixer controls

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

Operating system

An operating system is a set of programs that manage computer hardware resources and provide common services for application software. The operating system is the most important type of system software in a computer 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

Mixing console

In professional audio, a mixing console, or audio mixer, also called a sound board, mixing desk, or mixer is an electronic device for combining , routing, and changing the level, timbre and/or dynamics of audio signals. A mixer can mix analog or digital signals, depending on the type of mixer...

. 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

Low-pass filter

A low-pass filter is an electronic filter that passes low-frequency signals but attenuates signals with frequencies higher than the cutoff frequency. The actual amount of attenuation for each frequency varies from filter to filter. It is sometimes called a high-cut filter, or treble cut filter...

s, distortion

Distortion

A distortion is the alteration of the original shape of an object, image, sound, waveform or other form of information or representation. Distortion is usually unwanted, and often many methods are employed to minimize it in practice...

 and boost

Boost

-Science, technology and mathematics:* Automotive:** Boost, positive manifold pressure in cars, see Turbocharger#Pressure increase.*** a loose term for turbo or supercharger** A slang term meaning to start a vehicle, see jump start...

 controls.

List of packages that provide sound card mixer controls for the Unix console

• setmixer
• aumix
• alsamixer
  Alsamixer
  alsamixer is a graphical mixer program for the Advanced Linux Sound Architecture . It is used to configure sound settings and to adjust the volume. It has an ncurses user interface and does not require the X Window System...
  
  

List of packages that provide sound card mixer controls for X11

• aumix-gtx
• asmixer
• ermixer
• gamix
• gdam
• gnome-alsamixer
• gmixer
• kalsamixer
• kamix
• knob
  Knob
  Knob may refer to:* A rounded handle that one pulls or twists:** Doorknob, a round handle one turns to open any door** Control knob, controls a device** Brodie knob on the steering wheel...
  
  
• miqser
• mix2000
• mix2005
• mixmos
• mmix
  MMIX
  MMIX is a 64-bit RISC instruction set architecture designed by Donald Knuth, with significant contributions by John L. Hennessy and Richard L. Sites...
  
  
• opendjmix
• opmix
• ossmixer
• qamix
• qtmixer
• qtrayvolman
• rteq
• sbam
• sgmixer
• smix
• sounddesk
• tclmix
• tcmixer
• tkmixer
• umix
• volumecontrol
• wmixer
• wmmixer
• xamixer
• xmix
• xmmix
• xtmix
• xvolume
• yamp
• zmixer

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 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:

Control	channels	Controlled source
Wave / PCM	stereo STEREO STEREO is a solar observation mission. Two nearly identical spacecraft were launched into orbits that cause them to respectively pull farther ahead of and fall gradually behind the Earth...	Audio signal generated by the CPU via the sound card's digital-to-analog converter. (This includes audio produced by games, MP3 MP3 MPEG-1 or MPEG-2 Audio Layer III, more commonly referred to as MP3, is a patented digital audio encoding format using a form of lossy data compression...  or WAV WAV Waveform Audio File Format , is a Microsoft and IBM audio file format standard for storing an audio bitstream on PCs...  players, but also some software playing a CD-DA through the CPU, such as, Windows Media Player Windows Media Player Windows Media Player is a media player and media library application developed by Microsoft that is used for playing audio, video and viewing images on personal computers running the Microsoft Windows operating system, as well as on Pocket PC and Windows Mobile-based devices...  or Media Player Classic Media Player Classic Media Player Classic is a compact media player for 32-bit and 64-bit Microsoft Windows. The application mimics the look and feel of the old, lightweight Windows Media Player 6.4 but integrates most options and features found in modern media players... , as well as TV tuner cards that use the CPU for decoding audio.)
MIDI/SW Synth	stereo STEREO STEREO is a solar observation mission. Two nearly identical spacecraft were launched into orbits that cause them to respectively pull farther ahead of and fall gradually behind the Earth...	Audio signal generated by a synthesizer chip (often described as “MIDI device”) or digital signal processor on the soundcard.
CD playback	stereo STEREO STEREO is a solar observation mission. Two nearly identical spacecraft were launched into orbits that cause them to respectively pull farther ahead of and fall gradually behind the Earth...	Audio signal received from the analog CD input of the mixer, which is usually connected to the analog audio output of a CD drive.
Microphone	mono Monaural Monaural or monophonic sound reproduction is single-channel. Typically there is only one microphone, one loudspeaker, or channels are fed from a common signal path...	Audio signal received through one of the (typically two: MIC1 and MIC2, switchable) analog microphone inputs. MIC1 is typically connected to the external microphone jack (colored pink under PC99). An optional 20 decibel Decibel The decibel is a logarithmic unit that indicates the ratio of a physical quantity relative to a specified or implied reference level. A ratio in decibels is ten times the logarithm to base 10 of the ratio of two power quantities...  (10× voltage) amplifier can be activated for this input in most mixers.
Line in	stereo STEREO STEREO is a solar observation mission. Two nearly identical spacecraft were launched into orbits that cause them to respectively pull farther ahead of and fall gradually behind the Earth...	Audio signal received through the LINE input of the mixer, usually connected to an external jack (colored light blue in PC99), which can be used to connect a HiFi amplifier, tuner, iPod IPod iPod is a line of portable media players created and marketed by Apple Inc. The product line-up currently consists of the hard drive-based iPod Classic, the touchscreen iPod Touch, the compact iPod Nano, and the ultra-compact iPod Shuffle... , television Television Television is a telecommunication medium for transmitting and receiving moving images that can be monochrome or colored, with accompanying sound... , etc.
Aux in	stereo STEREO STEREO is a solar observation mission. Two nearly identical spacecraft were launched into orbits that cause them to respectively pull farther ahead of and fall gradually behind the Earth...	Audio signal received through the AUX input of the mixer. This mixer input is not used by all PCs.
PC speaker	mono	Some soundcards provide an input for the (very crude) audio signal that the Programmable Interval Timer Programmable Interval Timer In computing and in embedded systems, a programmable interval timer is a counter which triggers an interrupt when it reaches the programmed count.- Common features :...  of an IBM PC generates for the PC speaker 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 speaker. More recent computers use a piezoelectric speaker instead. The speaker allows software and firmware to provide... .
SPDIF	mono	Digital 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:

Control	channels	Controlled source
Line out	stereo STEREO STEREO is a solar observation mission. Two nearly identical spacecraft were launched into orbits that cause them to respectively pull farther ahead of and fall gradually behind the Earth...	Audio signal provided to the LINE OUT jack of a sound card (colored lime green in PC99). This can be connected to headphones or a HiFi amplifier, etc.
Aux out	stereo STEREO STEREO is a solar observation mission. Two nearly identical spacecraft were launched into orbits that cause them to respectively pull farther ahead of and fall gradually behind the Earth...	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 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 speaker. More recent computers use a piezoelectric speaker instead. The speaker allows software and firmware to provide... .
SPDIF	7.1	Digital interface

Typical record controls

In a typical AC’97 style mixer scheme, the analog-to-digital converter

Analog-to-digital converter

An analog-to-digital converter is a device that converts a continuous quantity to a discrete time digital representation. An ADC may also provide an isolated measurement...

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.

Some sound recording tips

• 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
  Windows Sound Recorder
  Sound Recorder is an audio recording program included in Microsoft Windows. The Vista version can record for longer durations but contains fewer options, and cannot play back recorded sound....
  
  .
  
  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
  Audio feedback
  Audio feedback is a special kind of positive feedback which occurs when a sound loop exists between an audio input and an audio output...
  
  , either by lowering speaker volume or repositioning the microphone.
  
• Record streaming sound (i.e. sound from an internet-radio station): To record sound that is played on PC there is a special virtual recording source called stereo mix or mono mix. All sounds on your PC are “duplicated” to this recording source by the audio driver or by the sound card hardware.
  
  Just select stereo mix source in your favorite recording tool and record what you hear. Not all the sound cards have stereo mix and it works only on Windows XP and older operating systems.
  
  Starting from Windows Vista and 7 new sound API model was introduced. There is no stereo mix or mono mix. Instead every playback device can be opened in loopback recording mode, so you can record what you hear from your speakers, headphones, digital output etc. Even if your audio card does not support it, Windows will emulate this functionality. HD Audio Recorder is the single tool that records “what you hear” in Windows Vista or Windows 7 using new API.
  
  Few sound cards temporary support stereo mix source in Vista for backward compatibility with old software, but it is not enabled by default and is looking not very stable just like it used to be in Windows XP.
  
  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.
  

Recording sound without Sound card mixer

Software for recording without Sound card mixer on Mac OS

• Soundflower (freeware)

Software for recording without Sound card mixer on Linux

• PulseAudio Volume Control (freeware)
• alsamixer (freeware)

Software for recording without Sound card mixer on Windows

• Freecorder 4 (freeware)
• Replay Music,
• Replay Media Catcher,
• Replay AV,
• Replay Video Capture,
• WM Capture
• VirtualAudioCable