From ECDL.web

A sound card (also known as an audio 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.

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) and video projection. Many computers have sound capabilities built in, while others require additional expansion cards to provide for audio capability.

Sound cards usually feature a digital-to-analog converter (DAC), which converts recorded or generated digital data into an analog format. The output signal is connected to an amplifier, headphones, or external device using standard interconnects.

Most sound cards have a line in connector for signal from a cassette tape recorder or similar sound source with higher voltage levels than a microphone. The sound card digitizes this signal and stores it (under control of appropriate matching computer software) on the computer's hard disk for storage, editing, or further processing. Another common external connector is the microphone connector, for use by a microphone or other low level input device. Input through a microphone jack can then be used by speech recognition software or for Voice over IP applications.

[edit] Sound channels and polyphony

An important characteristic of sound cards is polyphony, which is more than one distinct voice or sound playable simultaneously and independently, and the number of simultaneous channels. These are intended as the number of distinct electrical audio outputs, which may correspond to a speaker configuration such as 2.0 (stereo), 2.1 (stereo and sub woofer), 5.1 etc.

The first soundcard solutions were mono. Stereo sound was introduced in the early 90s, and quadraphonic sound came in 1989. This was shortly followed by 5.1 channel audio. The latest soundcards support up to 8 physical audio channels in the 7.1 speaker setup.

[edit] Integrated sound hardware on PC motherboards

In the late 1990s, many computer manufacturers began to replace plug-in soundcards with a "codec" chip integrated into the motherboard. Many of these used Intel's AC'97 specification. From the mid 2000s most motherboards were manufactured with integrated "real" (non-codec) soundcards, usually in the form of a custom chipset providing something akin to full Sound Blaster compatibility; this saves an expansion slot while providing the user with a (relatively) high quality soundcard.

[edit] USB sound "cards"

USB sound "cards" are mostly external boxes that plug into the computer via USB. They are sometimes called audio "interfaces" rather than sound-cards, however, this is misleading, since an "audio interface" is a term also used to describe the interface between a computer's sound-card and an external device, e.g., a standard audio socket. And thus, a USB audio interface may describe a device allowing a computer which has a sound-card, yet lacks a standard audio socket, to be connected to an external device which requires such a socket, via its USB socket.

The USB specification defines a standard interface, the USB audio device class, allowing a single driver to work with the various USB sound devices and interfaces on the market.