When data are transmitted, or indeed handled at all, a certain amount of noise enters into the signal. This can have several causes: data transmitted by radio may be received inaccurately, suffer interference from other radio sources, or pick up background radio noise from the rest of the universe. Microphones pick up everything—signal as well as background noise—without discriminating between signal and noise, so when audio is encoded digitally, it already includes noise.
Electric pulses being sent via wires are attenuated by the resistance of the wire, and changed by its capacitance or inductance. Temperature variations can increase or reduce these effects. While digital transmissions are also degraded, slight variations do not matter since they are ignored when the signal is received. With an analog signal, variances cannot be distinguished from the signal and so are a kind of distortion. In a digital signal, similar variances will not matter, as any signal close enough to a particular value will be interpreted as that value. Care must be taken to avoid noise and distortion when connecting digital and analog systems, but more when using analog systems.
