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FIGURE 39.57<br />

quality is far superior to that produced by the earlier FM soundcards, the high price of the early wavetable<br />

implementations was prohibitive to widespread market acceptance. The normal market forces eventually<br />

drove the price down, and soundcards that could only produce FM became obsolete.<br />

An important force in the evolution of both graphics and audio in PCs is the computer game. The<br />

continually increasing realism of game graphics, with players able to navigate through virtual three-dimensional<br />

(3-D) environments, created demand for more realistic game audio. This demand led to the advent<br />

of 3-D positional audio, allowing accurate placement of sound sources within a virtual 3-D environment<br />

rendered on stereo speaker systems. It ultimately led to full environmental simulation, with the ability to<br />

simulate a sound in various environments such as a carpeted room, a large hall, and even underwater.<br />

A 3-D audio experience is difficult to achieve using two speakers. The smallest head movement of the<br />

listener can often destroy the effect. Movie theaters overcame this problem using multi-speaker sound<br />

systems that placed speakers to the sides and rear of the listener. It eventually migrated to home theater<br />

systems and finally to computer gaming systems.<br />

The best systems now have a 5.1 channel audio system such as Dolby Digital ® (Dolby Laboratories)<br />

coupled to a 3-D rendering soundcard with environmental simulation capabilities. These systems provide<br />

an audio experience that immerses the listener in the environment, helping to create the illusion of realism.<br />

Today’s System Requirements<br />

Today’s systems use a layered approach, with applications able to produce audio with little or no knowledge<br />

of the underlying hardware. Layers of software hide most of the hardware-specific features. Applications<br />

use a query mechanism to determine which features are present, enabling considerable freedom<br />

in hardware implementation. Many features of the audio system can also be rendered in software,<br />

guaranteeing the application developer a minimum feature set and performance level, nearly independent<br />

of the installed hardware. Thus, today’s architecture is scalable, allowing the user to choose hardware<br />

acceleration for better performance, or software emulation for lowest cost.<br />

Audio on a PC can be divided into several general categories, including operating system interaction,<br />

music, gaming, and voice applications. Each of these categories has unique properties, but with proper<br />

architecture, a single solution can apply to all of them.<br />

Operating system interaction is generally limited to alerting the user to various events, such as starting<br />

up the system, selecting an invalid choice, or receiving new e-mail messages. In the early days of PCs,<br />

simple beeps communicated all of these items. Now, these events can be associated with any sound<br />

recording, and each association can be unique. Whenever an event occurs, the operating system instructs<br />

the soundcard to play back the associated sound recording.<br />

Music applications are much more complex. The soundcard is required to provide a wavetable synthesizer<br />

responsive to musical instrument digital interface (MIDI) commands [2]. In addition, it must<br />

be able to play back streaming audio in various formats including PCM, MP3 (MPEG-1 Layer 3 Audio),<br />

and Dolby Digital (5.1-channel home theater audio). Finally, it must be capable of recording in CD<br />

quality, or 16-bit stereo PCM at a sample rate of 44.1 kHz. Each of these major features must be<br />

independent and operate simultaneously.<br />

© 2002 by CRC Press LLC<br />

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