In the simplest case, the word digital refers to the representation of a quantity in numerical form and analog refers to a continuous physical quantity. To digitize means to convert an analog physical quantity into a numerical value. For example, if we represent the intensity of a sound by numbers proportionally related to the intensity, the analog value of the intensity has been represented digitally. The accuracy of the digital conversion depends upon the number of discrete numerical values that can be assigned and the rate at which these numerical measurements are made. The difference between the instantaneous analog signal and de digital representation is digital error.
Conversion introduces a digital error in the signal - digital noise.
We will separately consider the consequences of discrete time and discrete amplitude coordinates on the representation of the analog signal. Discrete Time Nyquist theorem . This is a remarkable result. Thus, any digital system with just two levels has a 1 bit resolution. Generally, the logarithm to the base 2 is used to convert the number of available quantization levels to number of bits.