CEDAR Duo – declickle and auto dehiss
21
An Introduction to Click and Crackle Removal
Clicks
Clicks are impulsive noises of short duration.
The simplest audio declicker is an attenuator which, at the precise moment that a click is
detected, mutes the audio, thereby reducing the impact of the click. The minimum duration of
the mute (actually a high speed fade-out and fade-in) is typically 2.5ms, and even a small
number of mutes seriously affect the perceived sound quality. Also, since the method only seeks
to make the clicks less obtrusive, it does not restore the underlying signal. In addition, it is only
applicable when the energy contained within a click is very much greater than the energy within
the signal.
A more sophisticated analogue click-removal algorithm is used in a device known as a switcher.
Using two sources of nearly identical signals (the opposite groove walls of a monaural record
replayed using a stereophonic cartridge) this monitors for the cleaner signal and switches
between them as appropriate. This removes large clicks but, like all non-digital solutions, is
unable to distinguish small ticks from genuine signal. Also, the switcher assumes a perfect
monaural source. If the groove walls differ significantly, or suffer degradation simultaneously,
then the assumption (and therefore the restoration) fails.
Digital technology has made it possible to implement ideas that could not be realised using
analogue electronics. The first of these is Sample & Hold (S&H) which, in many ways, is the
same algorithm as used in a perfect muting system. However, instead of creating a signal
plateau at zero amplitude, this method assumes that a plateau at the level of the most recent
valid signal will be closer to the true signal. S&H removes the largest manifestations of clicks
and scratches, but the resulting audio contains unpleasant distortion and many audible 'bumps'
and 'pops'. While low amplitude thumps may be preferable to the high amplitude clicks of
untreated data, the signal will show signs of severe break-up if the click density is high. Many
listeners complain that these artefacts and side effects are more unpleasant than the clicks that
they replace.
