CEDAR Duo – declickle and auto dehiss
34
A Short History of Noise Reduction
It would be best to describe first what we mean by the term 'broadband' noise, defining it to be
an effect that adds (or subtracts) a random amplitude at all times to (or from) all frequencies
within the audio spectrum. Thus, we do not include artefacts of limited duration such as clicks or
crackles, both of which are removed by quite different methods to those described below.
Next, let's dispel any illusions regarding
dual-ended processes
that encode during recording
and decode upon playback. These limit the accumulation of extra noise added by the limitations
of analogue recording tape, but do nothing to remove noise from a signal that already contains
it - they simply limit the amount you add when you commit that signal to tape and play it back
again.
A
single-ended process
removes noise from your audio prior to committing it to tape, or at
the very least, improves the signal to noise ratio without affecting the signal adversely. Which
brings us neatly to the volume control... stunningly effective at removing noise, it does nothing
to improve the S/N ratio, and has an all-too-noticeable side-effect. No noise, No signal.
Since broadband noise is most intrusive at high frequencies, the first stage in our evolutionary
tale is the
Low-pass Filter
. Less damaging than the volume control, this removes a proportion
of the signal above its cut-off frequency. Unfortunately, if, at any given frequency, you reduce
the amplitude of the noise by, say, 6dB, you also reduce the desired signal by the same
amount. So the low-pass filter will clean your antique '78s (which have little or no high
frequency content) but even then, only at a cost.
Dynamic Filters
are devices in which the cut-off frequency moves dynamically according to the
signal content, thus removing high frequencies when there is no signal present, but leaving
them untouched when the noise is being masked by genuine high frequencies. But such devices
are limited because they only remove the noise that exists above the cut-off, which is itself an
inaccurate representation of the highest frequencies contained in the genuine signal. Secondly,
and in common with the simple filter, they have roll-offs of the order -12dB/octave or -
6dB/octave, so they allow some high frequencies through. And thirdly, even though the filters
are designed to track quickly, they still round off transients and dull the genuine signal.
Now, instead of altering the frequency response of the signal, how about changing the level in
some way? Consider: if noise of relatively constant amplitude is always present then, if the total
amplitude drops to the noise level, we can assume that no genuine signal is present. While
there are many flaws in this argument, it suggests a device which will eliminate some noise: a
Noise Gate
. This detects when the signal drops below a 'threshold' set by the user, and then
cuts off the signal entirely. There are many enhancements to the idea (added to limit damaging
side-effects) but the principle remains the same: if the total signal drops below the threshold,
the gate shuts and removes all the noise. Unfortunately, an 'open' gate removes no noise
whatsoever.
