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The Representation of Digital Audio
Most people know that real-world sounds are analog--this means simply that they do not have discrete values, but rather are continuous in nature. When you hear someone blow a whistle, for example, the sound is continuous, and if they start blowing more strongly, the sound increases from quieter to louder in a gradual, smooth way. Computers are of course digital: they work with discrete values. Computerized sound therefore uses discrete approximations to represent the analog world of "real" audio. There are also advantages to storing information digitally, as explained here.
Since analog information and digital information are certainly not the same, this raises an important question: how is the analog sound converted to digital form? The process used to do this conversion is called sampling. When an analog signal is sampled, an electronic device measures the level of the signal periodically, and then records it in digital form. Then, by playing back the samples at the same rate, an approximation of the original signal is produced. This is in fact what happens with compact disk audio, with "wave" files in games, etc. Each sample represents the amplitude (loudness, in essence) of the signal at that moment in time.
Next: Sample Rate