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Part VII: Appendices > What is Digital Audio?

What is Digital Audio?

Sound is simply a variation in air pressure within a range of frequencies and amplitude our ears can perceive. Digital audio is a method of describing that variation numerically. The most common type of digital audio, pulse code modulation (PCM), measures the air pressure many times each second and assigns a binary number to that measurement. On playback, these measurements describe the variation of air pressure over time that recreates the sound. There's more to the process, of course—the air pressure needs to be converted to voltage first, and then the measurements really describe a voltage pattern that moves a speaker that moves the air—but this translation of air pressure to binary numbers is the essence of PCM digital audio.

Digital audio can capture and reproduce sounds quite realistically if the measurements are taken often enough and accurately enough. The speed of measurement is called the sample rate, and it is expressed in kilohertz (kHz)—or thousands of samples per second. Typical sample rates for professional audio production are 44.1 kHz (the sample rate of CDs), 48 kHz, and 96 kHz. The accuracy of measurement is determined by the bit depth, which determines how many units are available to measure the amplitude. Professional recording is done at 16 bits (the bit depth of CDs) or 24 bits. In general, the higher the sample rate and bit depth the more realistic the reproduction.


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