BUZZ REMOVAL
Buzz is a word that we use to refer to many audio degradations. It sometimes
describes the noises produced by, for example, problems with lighting rigs and
faders. These noises often take the form of closely spaced, regular ‘ticks’ in the
signal. On other occasions, ‘buzz’ is used to describe the sounds produced by
electrical faults such as earthing problems. These can lead to a variety of
unwanted noises such as simple mains hum or harmonically complex tones.
(Figure 1).
You can view and modify audio signals in two principal ways – in the time domain
(in which you consider the waveform of the signal), and in the frequency domain (in
which you consider the spectral content of the signal). It is important to remember
that these domains are just different ways of considering the same information.
Nevertheless, it is most effective to handle the audio and modify it in the domain
that is most appropriate to the problem. Therefore, for example, a buzz that is
composed of closely spaced, regular ‘ticks’ is best restored using one of
CEDAR
’s
various decrackle (time domain) processes, and we will not consider it further
here. On the other hand, a buzz that has an indistinct time domain structure may
be more easily characterised by its harmonic content and then analysed and
treated in the frequency domain.
Historically, audio engineers have used a number of conventional filtering
techniques to eliminate buzzes and hums. The simplest of these is the basic
high-pass filter, which attenuates the signal at all frequencies below a specified
cut-off point. (Figure 2). This can be used to eliminate harmonically simple hums
which, because of the mains frequencies of most countries, have fundamentals of
50Hz or 60Hz. Unfortunately, such a filter will also eliminate some of the desired
sound as well as the unwanted tone, resulting in a gutless signal that lacks bass.
Some complex buzzes and hums exhibit harmonics reaching up to many kHz.
But these too have significant energy at lower frequencies, so a low-pass filter is
completely inappropriate for removing buzz. A more sophisticated approach
involves the use of comb-filters – so-called because the response of the filter
resembles the teeth of a comb. (Figure 3).
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