AV News 196 - May 2014
Q & A o n F ilte rs
Howard Gregory
When recording outdoors, it's often necessary to use a filter to reduce wind
noise. I have a filter on the mic and a filter on the recorder. Does it matter which
I use? Presumably if things are really bad I could (should?) use both.
Dyl.
Let's start by reminding ourselves what filters do - they allow part of the audio
spectrum to pass through more or less unaltered, but attenuate (reduce in
amplitude) another part of the audio spectrum. The filters most commonly
encountered, and the ones Dyl is referring to here, reduce the amplitude of
low frequencies but allow mid and high frequencies to pass through. Such
filters are variously called 'high pass' or 'low cut' or 'bass cut' or 'L.F. (low
frequency) cut', 'L.F. roll-off' or, sometimes, with a throwback to vinyl record
days, 'rumble' filters. All mean the same thing, low frequencies are reduced;
everything else, hopefully everything that is really wanted, is allowed through.
In the real world, nothing is perfect, and any filtering will affect the wanted
audio to some degree. A balance has to be struck between removing enough
wind rumble and not affecting the wanted audio too much. Always err on the
side of not removing enough rumble when recording. You can always
remove more at the editing stage. If you start to filter out wanted audio when
recording, it's pretty well impossible to put it back again later without things
starting to sound unnatural. All you need when recording is to ensure that the
(unwanted) rumble is never the loudest sound. Then it can't overload
anything unexpectedly and it won't affect your (if you really must use it!) AGC.
Needless to say there are two long sections on the tutorial CD 'All About
Microphones' dealing with this, complete with recorded examples.
Even before filtering, there is a lot you can do to reduce wind rumble.
First, put the mic as close to the sound source as you sensibly can. Second,
use a decent windshield. Third is to control the recording level manually so
the AGC doesn't keep winding up the gain in every quiet moment. If, after
doing all this, you are still getting objectionable wind rumble, then, and only
then, do you filter.
The severity of a filter is characterised by two figures - turnover frequency
and slope. Turnover frequency is the frequency at which the filter has
reduced the signal by 3dB. ie the frequency at which it starts to have a
noticeable effect. So a 200Hz low cut filter will have more effect than a
100Hz one. Slope is a measure of how rapidly the signal diminishes as the
frequency changes. A 'first order' filter which has a slope of 6dB/octave will
have less effect than a 'second order' filter sloping at 12dB/octave. And so
on. Almost all filters of this type that you encounter on mics and recorders
will be simple first order ones. These figures are often quoted in instruction
books and can give you an idea of the severity of a filter. If no figures are
available, or even if they are, do some test recordings so you know what the
effect of the filter sounds like before you need to use it for something
important.
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