A sound card (also referred to as an audio card) is a peripheral device that
attaches to the ISA or PCI slot on a motherboard to enable the computer to
input, process, and deliver sound. 

The sound card's four main functions are: as a synthesizer (generating
sounds), as a MIDI interface, analog-to-digital conversion (used, for example,
in recording sound from a microphone), and digital-to-analog conversion (used,
for example, to reproduce sound for a speaker). The three methods of sound
synthesis are through frequency modulation (FM) technology, wavetable, and
physical modeling. 

FM synthesis is the least expensive and least effective method. Sounds are
simulated by using algorithms to create sine waves that are as close to the
sound as possible. For example, the sound of a guitar can be simulated,
although the result does not really sound very much like a guitar. Wavetable
uses actual, digitally recorded sound samples stored on the card for the
highest performance. Physical modeling is a new type of synthesizing, in which
sounds are simulated through a complex programming procedure. Some sound cards
can also have sounds downloaded to them. 

Sound cards were once all connected to the ISA slot. However, because
connection to the PCI bus offers advantages such as improved signal-to-noise
ratio and decreased demand on the CPU, sound cards being produced today are
intended for use with a PCI bus. 

In the beginning, Sound Cards included a Power Amp at the Output stage, that
allowed to drive non-powered speakers in the days of Windows 3.1 However, 
this Amp was small by nature and design (usually 2X4 Watts) yielding a lot of
distortion as well as adding noise to the Computer System.  Nowadays, Powered
Speakers have come down dramatically, therefore, no Sound Card Manufacturer
incorporates these low quality 'noisy' Amps. So Powered Speakers are a must
these days. They also come in various combinations as far as Sound
Distribution is concerned (Surround etc) as well as with on board 'decoders'
for handling of Dolby Digital, DTS and other formats.

Some sound cards have 3-D capabilities enabled by processors on the card that
use mathematical formulas to create greater depth, complexity, and realism of 
sound. High quality audio can be produced through a system that uses the 
Universal Serial Bus (USB) and does not require a sound card. Processing is 
left to the CPU, and digital-to-audio conversion to the speakers. 

Current Sound Cards offer capabilities such as: Multichannel Support, Dolby
Digital, DTS, 24/96kHz Audio Quality, Optical S/PDIF Digital In and Out, EAX,
3D Support, etc. etc.
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