Digitalisation - Signals, Processes, and Systems - page 271

The gateway to the digital world: the A/D converter
If DSP opens up such impressive perspectives, we still have to enter this territory. The
central issue is to make equivalent digital signals - strings of numbers - from analog
signals. The term A/D conversion has come to be used to designate this process.
There are many different processes and variants for this conversion process. At this point
the main thing is to understand the principle of the A/D converter.
A/D (and D/A) converters are always hardware components on
which a series of analog and digital signalling processes take
place.
In contrast to practically all other signalling processes in DSP
systems A/D (and D/A) conversion cannot be carried out using
a computer by means of a virtual system - i.e. a program.
At the input of the A/D converter lies the (real) analog signal and
at the output the digital signal appears as a string of numbers.
The numbers are outputted in the dual number system and are
further processed.
The conversion process takes place in three steps: sampling ,
quantization and coding .
The principle of an A/D converter is now to be simulated using DASY Lab . The process
selected works according to the „counter coding principle“ (see Illustration 184).
In the top series a small segment from an LF signal (for example, a speech signal) is to be
seen, below it the time pulse with which “ measurement samples” can be taken. These and
two other signals are produced by the component and the module “function generator”.
The two upper signals are connected with the module “Sample & Hold”. The features of
this process can be seen in the third row - the measurement at the point of time of a needle
pulse (see time pulse in the second series) is calculated and stored, that is, “held” until the
next measurement is sampled.
Each of these measurement values must be converted into a discrete number in the time
between two measurements. In the fourth series a periodic sawtooth curve is to be seen
which runs in synchrony with the time pulse (sampling frequency). In the subsequent
“comparator” the step-like Sample & Hold signal is compared with this periodic
sawtooth. At the output of the “comparator” there is a signal of level 1 („high“) as long as
this sawtooth voltage is smaller than the instantaneous Sample & Hold value. If the
sawtooth voltage exceeds the instantaneous Sample & Hold value, the output signal of the
output signal of the comparator jumps to 0 (“low”).