The Sound Card: A Microchip Miracle? (page 2)

From Sounds to Bits

At the heart of a sound card is the audio digitizer. A digitizer turns an analog waveform--in this case, sound--into a series of numbers. The computer can then manipulate these numbers, store them on a drive and copy them without quality loss. On playback, the card converts the numbers back into an audible signal (see figure 1).

Sound cards allow you to adjust the fidelity of the recording--the tradeoff is hard-drive storage space. For example, CD-quality stereo audio takes up many times the drive space of telephone-quality mono sound. If you can get by with lesser-quality audio, you can fit more of it on your hard drive. And your computer will have an easier time processing and playing back these smaller audio files.

There are three parameters you can adjust on a sound card to tailor your audio quality. The first step is to choose whether you're digitizing in stereo or mono. Capturing in stereo doubles the file size, and increases the demand on your computer. The second parameter is sampling rate. This tells the computer how often to "measure" the analog signal and store a numerical value. Audio sampling rate has a direct impact on the frequency response of your recordings; the sampling rate must be double that of the highest frequency you want to capture. An 8kHz sampling rate, for example, will only capture frequencies up to about 4kHz. Hence, the lower the sampling rate, the more "dull" your audio sounds.

The last parameter is bit depth or "word length." This sets how many bits the computer uses to store each measurement of the audio signal. The more bits used, the more accurate the recording (and the larger the resulting file). Using fewer bits can make for a coarse-sounding, noisy recording. For more on these quality tradeoffs, see figure 2.

How you set your sampling rate and word length are the greatest factors in digital audio quality, but they're not the only ones. The design of the card itself plays a large role in how it sounds. Poorly designed cards can introduce unwanted noises into your audio due to inadequate shielding. The analog-to-digital (A/D) and digital-to-analog (D/A) converters used in the card also contribute to its sound quality. In general, though, even inexpensive sound cards should deliver sound quality on par with VHS-family hi-fi audio and 8mm-family AFM and PCM au…