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Computer Audio Adapters

Standalone sound cards are a dying category, killed by the ubiquity and high quality of integrated audio. Of the many companies that formerly specialized in sound cards, only Creative Labs, M-AUDIO, and Voyetra/Turtle Beach retain a significant retail market presence.

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Figure 12-1 M-AUDIO Revolution 5.1 (left) and 7.1 sound cards

Still, there is a place for standalone sound cards. Gamers use them, because they provide features not available with integrated audio, such as hardware acceleration of gaming audio standards like EAX, and because their on board processors relieve the main system processor of processing audio data streams. Audiophiles use them, because good sound cards provide better audio quality than even the best integrated audio. Upgraders use them, because adding an inexpensive sound card is an easy way to add functions, such as surround sound, that are not provided by the integrated audio in some older motherboards. Figure 12-1 shows a pair of M-AUDIO Revolution sound cards, a popular (and good) choice among both gamers and audiophiles.

Audio adapter components

The key function of an audio adapter is playback accepting a digital data stream from the PC and converting it to an analog audio signal that can be reproduced on speakers or headphones. Most audio adapters can also do the converse accept an analog audio signal and convert it to a digital data stream that can be stored on a PC. Audio adapters use the following components to provide these functions:


Audio adapters contain at least one Digital-to-Analog Converter (DAC) and one Analog-to-Digital Converter (ADC) for each of the two stereo channels, and at least one DAC for each additional audio channel. (Many audio adapters can output 5, 6, 7, or 8 channels, but few can record more than two-channel stereo sound.) A DAC converts a digital audio stream into the analog audio delivered to the Line-out port. An ADC digitizes analog sound received from the Line-in or Microphone port. CD-Audio sound, generally the highest quality supported by audio adapters, requires 16-bit resolution. The converters used in better-quality audio adapters usually support higher resolution, typically 18- or 20-bit. Some expensive cards use 24-bit or higher resolution for both recording and playback. Resolution some times differs between the DAC and ADC. For example, a card might use an18-bit DAC and a 20-bit ADC. Internal resolution is often higher than that supported by the DAC/ADC, typically 24- or 32-bit.

Sample rate generator

The sample rate generator provides the clock for the converters under the control of the PC. While nothing prevents using arbitrary or continuously variable sample rates, most sample rate generators instead support discrete sample rates, which are usually even fractions of 44,100 Hz and 48,000 Hz. A sample rate generator might support sample rates of 48,000, 44,100, 32,000, 24,000, 22,050,12,000, 11,025, and 8,000 Hz. Many audio adapters support differing rates for recording versus playback. For example, a card may support playback rates of 48,000, 44,100, 22,050, 11,025, and 8,000 Hz, but record only at 44,100 Hz. High-end cards may support sampling rates as high as 96 KHz in Dolby Digital 5.1 mode and 192 KHz in stereo mode.


The processor (also called the sound generator or synthesis engine)performs general audio processing tasks and creates analog output from MIDI input by reading, interpolating, and combining wavetable samples into the composite audio waveform represented by the MIDI instructions. Most audio adapters use a custom digital signal processor (DSP) like the E-mu Systems EMU10K1 or EMU10K2, or the Crystal/Cirrus Logic CS4630 or CS8420. The processor used directly or indirectly determines several key capabilities of the audio adapter, including how many MIDI channels, voices, hardware-accelerated sound streams, and so on it supports. DSPs provide useful supplementary capabilities in hardware, such as reverb and chorus effects, text-to-speech processing, and compression. Because a DSP is programmable, some DSP-based audio adapters support related functions, such as fax modem or telephone answering machine functions.


Audio adapters typically provide the following connectors:


Line-out is a line-level (unamplified) stereo output intended to be connected to line-in on amplified speakers, headphones, home audio equipment, or a recorder. Most audio adapters provide one stereo Line-out port, but some provide two mono Line-out ports, designated left and right. Audio adapters that support four speakers usually have two stereo Line-out ports, one each for front and rear speakers. Audio adapters that support surround sound may provide three or four Line-out connectors to support six or eight speakers. The standard color code for Line-out is lime, although for this and other color codes makers often pay scant attention to the exact hue. The standard icon usually stamped into the card bracket is three concentric circle segments (to represent audio vibrations) with an outward-pointing arrow anchored in the center.


Line-in is a line-level stereo input intended to be connected to Line-out of external analog audio sources such as a CD player or VCR. Some microphones can also be connected to Line-in. The standard color is light blue. The standard icon is the same as for Line-out, but with the arrow head pointing to the center.


Microphone-in, sometimes labeled Mic, is a monaural input that supports in expensive microphones for recording voice. The standard color is pink, although red is commonly used, and the standard icon resembles a microphone.


Subwoofer is a line-level monaural output intended to be connected to a powered subwoofer, which may also be called an LFE (Low-Frequency Emitter). Standard color is orange.


Sony-Philips Digital InterFace (S/PDIF) is an RCA coax jack or optical jack that provides a direct digital connection between the audio adapter and an external device with an S/PDIF jack. S/PDIF is a standard feature on most high-end audio adapters, and may be an option on midrange audio adapters. Some audio adapters have both S/PDIF input and output ports, but others have only an S/PDIF output. Because of limited room on the expansion bracket of the audio adapter, S/PDIF ports are often present as a header connector on the audio adapter, which uses an extender cable to a cliffhanger bracket where the S/PDIF connectors reside. Some audio adapters use a proprietary connector that joins the audio adapter to a remote head, which often contains S/PDIF connector(s), Line-in connector(s), and MIDI connector(s).

Figure 12-2 shows the audio connectors on an M-AUDIO Revolution 7.1, which are typical for a premium sound card. From left to right are a coax S/PDIF jack, Microphone-in, and Line-in, followed by four Line-out jacks to connect the eight speakers supported by this card.

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Figure 12-2: M-AUDIO Revolution 7.1 audio connectors

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