Page 34 - RTH3BA Preview

35 - ������� �������������

Any spectral content higher than half the sampling frequency must be sufficiently
attenuated prior to sampling and quantisation otherwise, it will alias and corrupt the
signal we are trying to sample.
Okay, so how do we remove frequencies from the signal, greater than half the
sampling rate before digitising?

It is not as hard as you might think. We just need a lowpass filter. We call this a low
pass filter and in this application it is known as an anti-aliasing filter. Before we do
any digitising (sampling & quantisation) we must attenuate frequencies above the
sampling rate.

Figure 35-20 An anti-aliasing lowpass filter before any digitisation

QUANTISATION

Figure 35-21
Figure 35-21 shows an analogue signal being quantised. There are eight levels of
quantisation. These levels correspond to the voltages on the left-hand side of figure
35-21. On the right-hand side, the voltage values are coded to binary values.

I like to think of quantisation as forcing the continuous analogue signal to “snap-and-
hold” to the nearest quantisation level. We have eight levels, and each portion of the
analogue signal is forced to snap to the nearest of those levels.
PREVIEW
443