As long as they are clean and quite sharp in their sound,
clicks are
relatively easy to remove. Before electronic methods were
available it
was common practise to physically cut clicks out of tape
recordings: of
course you remove a small section of the sound in doing
this, but as
long as you use a high tape speed such as 15 inches per
second the
effect is minimal.
Early electronic methods simply detected the sharp edge of
the click
and reduced the level for a fraction of a second, either
holding it
down to the level of the surrounding audio, or muting it all
together.
Neither method is entirely satisfactory, though the results
are usually
acceptable: modern digital systems can remove the click and
interpolate
the audio by looking at the audio either side of where the
click was
and filling in the gap. Results can be very good, though
there is
always a danger of instruments with sharp transients such as
piano or
percussion triggering the click removal and producing
distortion: so as
with any noise reduction methods it has to be applied with
discretion.
This example shows both the advantages and the limitations
of click
reduction. This LP has picked up some clicks, one of them
being quite
loud and another being unusually so:
Click reduction using Click Repair
(no longer available) has successfully
removed the first click, but the second
was loud enough to vibrate the pickup arm, and this has
caused a
'thunk' which the program cannot recognize as a click. This
may not be
particularly audible on small speakers, but it can be
clearly seen in
these expanded waveforms:
In this particular instance I could have removed it by
simply
cutting it, and making the joint a rapid crossfade: but of
course if
the music had been rhythmic this wouldn't be possible.
Cutting the bass
just for the duration of the thunk would help.
This 78 has a
typical repeating click caused by a very fine split running
across the
opening section of the recording:
It's also possible to remove
crackle, which is in effect a
very high number of clicks close together. This record
isn't worn, but
suffers from granulation - bacterial attack has caused a
large number
of very small hard lumps in the shellac, causing a level
of crackle
which makes
the record quite difficult to listen to. This effect is
very common on
inter-war HMVs (which this record is), much less so on
Columbias, or
indeed on HMVs pressed in India or Australia.
Removing the crackle has to
be
carefully done, because in the case of
musical instruments with sharp transients, such as piano
or percussion,
the transients can be mistaken for crackle and distorted;
this can
also happen with trumpets. Speech is always easier for any
form of
noise reduction, and in this case almost all the crackle
can be
successfully removed.
This still leaves a
considerable
amount of hiss, but at least it's a
continuous sound which can be reduced by the use of a
different
technique, which will be discussed on the next
page