Let us study quantization concept by following example. Fig. 1 shows the electronic waveforms of a typical ADC. Here, figure 'a' is the analog audio signal to be digitized. The block diagram includes two sections, namely the sample-and-hold (S/H), and the analog-to-digital converter (ADC). In fact, the sample-and-hold is required to keep the voltage entering the ADC constant while the conversion is taking place. Moreover, breaking the digitization process into these two stages is an important theoretical model for understanding digitization (Smith 1999).
As shown by the difference between 'a' and 'b', the output of the sample-and-hold is allowed to change only at periodic intervals, at which time it is made identical to the instantaneous value of the input audio signal. Changes in the input signal that occur between these sampling times are completely ignored. That is, sampling converts the independent variable (i.e. time) from continuous to discrete.
Then, as shown by the difference between 'b' and 'c', the ADC produces an integer value for each of the flat regions in 'b'. So, quantization converts the dependent variable (i.e. voltage) from continuous to discrete. ...