Sound card
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A sound card is a computer expansion card that can input and output sound under program control.
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General characteristics
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 (earphone-type) connector where a cable to an amplifier or similar sound destination can be plugged in.
More advanced designs usually include more than one sound chips, and separate between synthesized sounds (usually for real-time generation of music and special effects with little amounts of data and CPU time and perhaps MIDI compatibility) and digital sound reproduction.
The latter is usually achieved by multi-channel DACs, able to play multiple digital samples at different pitches and perhaps even applying real-time effects to them, like filtering or distortion. Sometimes, 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.
"Audio codecs" on the other hand rely heavily on software for music synthesis, MIDI compliance and even multiple-channel emulation, their purpose being to simplify the design of costs of the sound card itself.
Also, most sound cards have a "line in" connector where the sound signal from a cassette tape recorder or similar sound source can be connected to. The sound card can digitize this signal and store it (controlled by the corresponding computer software) on the computer's hard disk.
The third external connector a typical sound card has, is used to connect a microphone directly. Its sound can be recorded to hard disk or otherwise processed (for example, by speech recognition software or for Voice over IP).
There are however some sound cards aimed specifically at music synthesis and MIDI interfacing which lack any "real" sound recording capabilities, such as some Turtle Beach and Roland MT32 products, which were however aimed at a specific market and are designed for professional high-quality MIDI music playback.
One of the first manufacturers of sound cards for the IBM PC was AdLib, who produced a card based on the Yamaha YM3812 sound chip, aka the OPL2. This set the de facto-standard until Creative Labs produced the Sound Blaster card, which had a YM3812 plus a sound coprocessor (presumably an Intel microcontroller) which Creative unproperly called a "DSP" which suggested it was a digital signal processor; several years passed before Creative released a card which could even record and playback sound at full-duplex mode, without however any real-time sound processing capabilities.
Contrary to popular belief, the Sound Blaster didn't overtake AdLib due to the inclusion of digital sound as a feature. Support of Sound Blaster's digital sound didn't occur until a few years later after the release of the Sound Blaster Pro. In fact, it was the inclusion of a basic game port that provided the key difference. At the time both the AdLib card and the Sound Blaster sold for $289. AdLib owners who also wanted to play games requiring a joystick had to purchase a separate dedicated game port card that generally cost about $50. The dedicated game port also came at the further cost of another precious expansion slot at a time when many PC clones commonly included only three slots. Once Creative Labs proved that the Sound Blaster was fully "AdLib compatible", consumers were left with little reason to choose the less-functional, but equally priced AdLib card.
The Sound Blaster, in tandem with the first cheap CD-ROM drives and evolving video technology, ushered in a new era of computer capabilities, in which they could play back CD audio, add recorded dialogue to computer games, or even play movies (but only short clips and in a very low quality form, incomparable with modern digital video).
Early soundcards could not record and play simultaneously. Most soundcards are now full-duplex.
Also, for years soundcards had only one or two channels of digital sound (most notably the Soundblaster series and their compatibles) with the notable exception of the Gravis Ultrasound family, which had hardware support for 16 or 32 independent channels of digital audio, and early games and MOD-players had to fully emulate multiple channels by software downmixing.
Today, most good quality sound cards have hardware support for at least 16 channels of digital audio but others, like cheap Audio codecs, still rely partially or completely on software, either their drivers or the operating system itself to perform a software downmix of multiple audio channels.
In the late 1990s, many computer manufacturers began to replace plug-in soundcards with a "codec" (actually a combined audio AD/DA-converter) integrated into the motherboard. Many of these used Intel's AC97 specification. Others used cheap ACR slots.
As stated before, these "codecs" usually lack the hardware for direct music synthesis or even multi-channel sound, with special drivers and software making up for these lacks, at the expense of CPU speed (e.g. midi reproduction takes away 10-15% CPU time on an Athlon XP 1600+ CPU).
Driver architecture
To use a sound card, a certain operating system typically requires a specific device driver.
- Microsoft Windows uses proprietary drivers supplied by sound card manufacturers and supplied to Microsoft for inclusion in the distributions. Sometimes drivers are also supplied by the individual vendors for download and installation.
- The Linux kernel used in the Linux distributions have two different driver architectures, the Open Sound System and ALSA (Advanced Linux Sound Architecture). Both include drivers for most cards by default. Sound card manufacturers seldom produce stand-alone drivers for Linux.
- The Universal Serial Bus (USB) specification defines a standard interface for sound cards to adhere to, the USB audio device class, allowing a single driver to work with the various USB sound cards on the market.
See also
References
- This article was originally based on material from the Free On-line Dictionary of Computing, which is licensed under the GFDL.
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