What is a Sound Card?

Sound card

Typical uses of sound cards include providing the audio component for multimedia applications such as music composition, editing video or audio, presentation/education, and entertainment (games). Many computers have sound capabilities built in, while others require these expansion cards if audio capability is desired.
General characteristics

Close-up of a sound card PCB, showing electrolytic capacitors (most likely for AC coupling), SMT capacitors and resistors, and a YAC512 two-channel 16-bit DAC.A typical sound card includes a sound chip, usually featuring a digital-to-analog converter, that converts recorded or generated digital waveforms of sound into an analog format. This signal is led to a (typically 1/8-inch earphone-type) connector where an amplifier, headphones, or similar sound destination can be plugged in. More advanced designs usually include more than one sound chip to separate duties between digital sound production and synthesized sounds (usually for real-time generation of music and sound effects utilizing little data and CPU time).
Digital sound reproduction is usually achieved by multi-channel DACs, able to play multiple digital samples at different pitches and volumes, optionally applying real-time effects like filtering or distortion. Multi-channel digital sound playback can also be used for music synthesis if used with a digitized instrument bank of some sort, typically a small amount of ROM or Flash memory containing samples corresponding to the standard MIDI instruments. (A contrasting way to synthesize sound on a PC uses "audio codecs", which rely heavily on software for music synthesis, MIDI compliance and even multiple-channel emulation. This approach has become common as manufacturers seek to simplify the design and the cost of the sound card itself).
Most sound cards have a line in connector where the sound signal from a cassette tape recorder or similar sound source can be input. The sound card can digitize this signal and store it (controlled by the corresponding computer software) on the computer's hard disk for editing or further reproduction. Another typical external connector is the microphone connector, for connecting to a microphone or other input device that generates a relatively lower voltage than the line in connector. Input through a microphone jack is typically used by speech recognition software or Voice over IP applications.
Connections
Most sound cards since 1999 conform to Microsoft's PC 99 standard for color coding the external connectors as follows:
Color Function
  Pink Analog microphone input.
  Light blue Analog line level input.
  Lime green Analog line level output for the main stereo signal (front speakers or headphones).
  Black Analog line level output for rear speakers.
  Silver Analog line level output for side speakers.
  Orange S/PDIF digital output (sometimes used as an analog line output for a center speaker instead)
Voices vs channels
Another important characteristic of any sound card is the number of distinct voices (intended as the number of sounds that can be played back simultaneously and independently) and the number of channels (intended as the number of distinct electrical audio outputs).
For example, many older sound chips had three voices, but only one audio channel (mono) where all the voices were mixed into, while the AdLib sound card had 9 voice and 1 mono channel.
For a number of years, most PC sound cards had multiple FM synthesis voices (typically 9 or 18) which were mostly used for MIDI music, but only one (mono) or two(stereo) voice(s) and channel(s) dedicated to playing back digital sound samples, and playing back more than one digital sound sample required performing a software downmix at a fixed sampling rate. Modern low-cost integrated soundcards using an audio codec like the AC'97 still work that way, although they may have more than two sound output channels (surround sound).
Today, a sound card having hardware support for more than the two standard stereo voices, is likely to referred at as "providing hardware audio acceleration".