Specifications

6.4 Time dep endent signal shaping 67
terminology and talk about bands which are passed or stopped. A band has
a center frequency, specified in Hz, the middle of the range where it has the
most effect, and also a bandwidth which is the range of frequencies it operates
over. Narrow bands affect fewer frequencies than wider bands. In many filter
designs you can change the bandwidth and the frequency independently. Four
commonly encountered filters are the low pass, high pass, band pass, and band
cut or notch filter shown in Fig. 6.19 The graphs show the spectrum of white
noise~
bp~ 5000 9
$0-spectrum1
$0-spectrum2
pd spectrum1
noise~
pd spectrum2
lop~ 1000
$0-spectrum3
$0-spectrum4
pd spectrum3
pd spectrum4
Common filter spectrums
low passband pass
noise~
hip~ 7000
high pass
noise~
biquad~
notch 5000 456
band reject
fig 6.19: Common user friendly filter shapes.
noise after it’s been passed through each of the filters. The noise would normally
fill up the graph evenly, so you can see how each of the filters cuts away at a
different part o f the spectrum. The high pass allows more signals above its
centre freq uency through than ones below. It is the opposite of the low pass
which prefers low frequencies. The notch filter carves out a swathe of frequencies
in the middle of the spectrum, which is the opposite o f the band pass that allows
a group of frequencies in the middle through but re jects those either side.
Integration
Another way of looking a t the behaviour of filte rs is to consider their effect on
the s lope or phas e of moving signals. O ne o f the ways that r e c ursive (IIR) filters
can be used is like an accumulator. If the feedback is very high the current input
is added to all previous ones. Integration is used to compute the area under
a curve, so it can be useful for us to work out the total energy contained in a
signal. It can also be used to shape waveforms.