The process of converting a continuous analogue signal into digital codes is called Analogue to Digital conversion (A/D). The inverse process of re-generating the analogue signal from the digital codes is called digital to analogue (D/A) conversion. Two main processes are deployed in the analogue to digital conversion of a signal: Sampling and quantisation. Sampling denotes registration of the value of the analogue signal at different discrete times, and quantisation denotes the representation of these values using absolute numbers (digital codes).
One of the questions regarding sampling is, how often a signal must be sampled to asure that the analogue signal can be reproduced without changes. The problem was solved by Nyquist and was described in the ‘Nyquist theorem’. Here the minimum sampling frequency for a signal is defined to be at least twice the maximum frequency of the signal.
Quantisation is the representation of the sample values by digital codes. The number of codes used per sample is called the resolution of quantisation. In practice the quantisation process is not an exact process as it is impossible to represent the analogue signal values by digital codes.
The signal will often be rounded up and down to match the chosen codes, generating quantisation noise. On the other hand, it is not necessary to have exact representation of signals as regarding, e.g., audio and video, the sensitivity of the human ear and eye will put a natural minimum for the resolution.