Any sound intended to be stored as a sound recording--analog or digital--must first be captured. Sounds that are produced naturally--a child's voice, a piano, the rustling of leaves, a brass band--are captured using a microphone. The variations in air pressure that make up the sound hit a diaphragm in the microphone and cause it to vibrate sympathetically. The vibrating diaphragm creates a weak series of voltage pulses that are transmitted through the wires of the microphone to its plug.
The sound captured using the microphone can be stored in a number of ways. The microphone input can be routed to a recording component, such as a tape deck, where the voltage pulses are stored as magnetic patterns on tape, or to an analog-to-digital converter, which converts the voltage pulses into a series of binary digits that can be stored on a compact disc, DVD, DAT, or hard drive.
Most digital-audio projects do not involve capturing natural sound.1.1 Instead, they take existing recordings--commercial recordings, recordings of local concerts and lectures, field recordings captured by researchers--and convert them into digital audio files, which can then be stored, duplicated to media for playback, or delivered over networks.
For audio that is already in a digital format, no capture or conversion is needed. The process involves simply reading the binary digits on the source recording and copying them to another medium.
The sound on an analog recording, however, must be converted to digital audio. A microphone is not used to capture the audio, because the sound waves are already represented as a series of voltage pulses on the analog recording. The analog sound is reproduced on a traditional audio component (typically a turntable or tape deck), and the resulting analog signal is routed directly to a computer or digital audio recorder for conversion and storage as digital audio.