Digital recording

In digital recording, an audio or video signal is converted into a stream of discrete numbers representing the changes over time in air pressure for audio, or chroma and luminance values for video. This number stream is saved to a storage device. To play back a digital recording, the numbers are retrieved and converted back into their original analog audio or video forms so that they can be heard or seen.

In a properly matched analog-to-digital converter (ADC) and digital-to-analog converter (DAC) pair, the analog signal is accurately reconstructed, within the constraints of the Nyquist–Shannon sampling theorem, which dictates the sampling rate and quantization error dependent on the audio or video bit depth. Because the signal is stored digitally, assuming proper error detection and correction, the recording is not degraded by copying, storage or interference.

Recording

Playback

For digital cassettes, the tape head moves as well as the tape, typically in a helical scan configuration, in order to maintain a high enough speed to keep the bits at a manageable size.

For optical disc recording technologies such as CD-R, a laser is used to alter the optical properties of the dye layer of the medium. A weaker laser is used to read these patterns.

The number of bits used to represent an audio signal directly affects the resulting noise or distortion in a recording.

As stated by the Nyquist–Shannon sampling theorem, to prevent aliasing, the audio signal must be sampled at a rate at least twice that of the highest frequency component in the signal. For music-quality audio, 44.1 and 48 kHz sampling rates are the most common.