Cedar AZX+, Руководство пользователя

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PAGE 22
These filters allow you to remove signal components at a number of regularly-
spaced frequencies. Furthermore, the tighter the ‘teeth’ of the comb are, the more
precise the removal can be. This looks, at first sight, like an ideal solution, but it
isn’t. For one thing, the filter exhibits the same behaviour as all other static filters:
if, at a given frequency, you remove an offending noise, you also remove any
genuine signal that may exist at the same point in the spectrum. (Figure 4).
Secondly, comb-filters introduce a characteristic ‘boxiness’ that makes the
genuine signal sound distant and indistinct. This problem becomes worse the
more precise the ‘teeth’ become so, in limiting the unwanted side effects caused
by the first problem, you exacerbate the second. Thirdly, the offending
frequencies may not be entirely constant, so the filter’s ‘teeth’ may not always be
appropriately placed, and you will hear the buzz come and go during the course
of the material you are playing.
These problems make comb-filters – which are, nonetheless, implemented on a
number of audio processing platforms – unacceptable for the highest quality
removal of buzzes and hum. A far better solution is an algorithm that identifies the
offending frequencies, tracks any fluctuations that they experience, and then
removes the noise without destroying the underlying signal. (Figure 5).
CEDAR
’s Debuzz process precisely fulfils this specification. It will track buzz
frequencies within a range of 2% at 50Hz, and will retain genuine signal
components while removing any buzz harmonics that exist at the same
frequencies.