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".