(A brief introduction for analog dummies)
Sound and Light are basically composed of many sinusoidal waves (electric/magnetic signals), a time/space continous signal who can be stored into a magnetic support like audio tape if we're talking for example about sounds.
Digital tecnology basically converts a real thing like an analogue sound (wave(s)) to a sequence of numbers and to do so, the calculation process devides waves into thousand of small trapeziums with a curved broad. (0-a / a-b etc etc)
Unfortunately, the mathematical process to calculate the area of every single trapezium described in the figure above implies an error in the aproximation of the final value of it.
The error accours everytime you go from analog to digital then necessary from digital to analogue.
Because of this, despite the high accurancy of digital converters, any digital sound is not as good as the original analog one and obviously because of this any digital sound is not perfect and pure.
In the following graph the sinusoidal wave after digital convertion.
