In digital recording, an audio or video signal is digitized, converting 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. The digitized number streams themselves are never actually heard or seen, being hidden by the process.
In a properly matched analog-to-digital converter (ADC) and digital-to-analog converter (DAC) pair, there is one and only one analog output which must, by definition, exactly match the analog input. Because the signal is stored digitally, the recording is not degraded by copying, not degraded by storage (assuming proper error detection and correction), and not degraded by interference.