Owner`s manual

22
CHAPTER 3 -
The Craft of audio Synthesis
CHAPTER 3 -
3.6.4.1
Low-Pass Filters
Any device or mechanism that passes along slower motions better
than faster ones can act as a lowpass  lter.
Picture yourself stirring a cup of tea with one of those little
wooden paddles they hand out in the coffee shops. Stir it back and
forth, fast. Now slow down. Now imagine the tea has turned to
syrup. You can still stir it slowly, but if you try to go fast the stick
will simply not move.
That’s a lowpass  lter. You can see the effect of this on a signal
quite easily.
For audio signals, you will usually be more interested in the
frequency-domain effects of  ltering. For subaudio signals, it is
usually the time-domain effects – changes in waveshape – that
we care about. In the time domain, a low-pass  lter rounds off
any sharp transitions in the signal. A good example of this is the
lag processor
described
lag processor described lag processor
in section 3.6.4.5 below.
In the frequency domain, it weakens spectral components that are higher than the  lter
cutoff frequency
– the frequency at which the  lter begins to have an effect on the signal.
cutoff frequency – the frequency at which the  lter begins to have an effect on the signal.cutoff frequency
3.6.4.2
High-Pass Filters
Any device or mechanism that passes along faster motions better
than slower ones is a highpass  lter.
Take the drinking straw from your water glass. Seal the end of it
with your thumb and dip it back into the water. Notice that you
can pump it up and down in the glass, fast or slow, but the water
never leaks into the straw as long as you hold your thumb over the
end. Think of the up and down motion of the water at the bottom
end of the straw as “the signal”.
Now start letting a little air leak into the straw as you move it
up and down. The water level at the bottom end of the straw no
longer stays down when the straw “signal” goes down – it starts
to come up again. And then when you draw the straw back up,
the water leaks back down more or less rapidly depending on the
position of your thumb at the top of the straw.
This is a high pass  lter. In the time domain, it constantly “leaks” its output signal level
back to zero, at a rate related to the cutoff frequency and slope. In the frequency domain,
it passes all spectral components higher than the cutoff frequency, and attenuates those
below the cutoff frequency by an amount proportional to the cutoff slope.
A lowpass filter in
the time domain . . .
f 4f 8f 16f 32f
| | | |
. . . makes it difficult
for the medium to
change value.
. . . and in the frequency
domain, this cuts off
higher-frequency
components.
T=1/f
f 2 4 8 16 32
A highpass filter makes
it difficult for the medium
to keep a new level - it
"leaks" away . . .
. . . and so lower-
frequency components
are weakened.