Woody <
[email protected]> wrote:
On Thu 26/05/2022 11:10, J. P. Gilliver (John) wrote:
I saw in another post (Jim LeSurf in the "Speakers to play back voice" thread) the expression "clicks and pops".
This is such a common expression - in relation to cleaning up captures
from LPs anyway - that I normally don't think about it, but for some
reason it suddenly occurred to me to wonder - what's the difference? A click is what I'd normally think of as the unwanted effect of playing
over a scratch, but what's a pop? The word implies perhaps a slight resonance or ringing, maybe?
Could it be that a click is a transient fast-rising noise caused by a
scratch on the disc surface as suggested which produces a lot of high frequency content, and a pop is a drop-out or slow rising edge which
contains very much more of a thump-type sound?
In the world of sound restoration there doesn't seem to be a formal
definition but it is generally understood that a click contains mainly
high frequency energy whereas a pop includes lower frequencies. ...so
it's more-or-less what you said.
With disc playback the scene is complicated by the playback
characteristics. To correct for the 6dB per octave low frequency cut
applied during recording, there has to be a 6dB per octave rising bass characteristic applied during playback. This means that a transient
with equal energy across the spectrum will be replayed with a lot more
energy in the low frequencies.
Some reproducer combinations (amplifier+ loudspeaker or soundbox+ horn)
use resonance to enhance their low frequency response. A transient will
excite these into ringing and will sound like a "pop" with a definite
tone, rather than the toneless thud which it ought to be. Bass reflex
speakers and sharp cutoff rumble filters the main electronic offenders
in this category.
The slew rate of an upwards click on a disc is determined by the
steepness of the sides of the damage (or protruding particle) and the
radius of the stylus tip. Up to a certain point, the slew rate
downwards can be determined by these geometrical factors, but there is
also a limit due to the force of the stylus cantilever suspension acting
on the effective tip mass of the cantilever+jewel assembly.
In both cases, the resilience of the stylus cantilever bar between the
jewel and the moving part of the transducer (magnet, armature or coil)
can also be a slew-limiting factor and can give rise to high frequency resonances.
Clicks are relatively easy to get rid of by a declicker (analogue or
digital) because if the 'persistence of hearing' (analagous to the
persistence of vision). With pops the wanted signal is missing for a
longer period and the question arises of what can be used to fill the
hole.
A few milliseconds of silence results in an audible 'bloop', which is
almost as annoying as the missing pop; software like Cedar attempts to synthesise the missing part of the waveform, but it is not 100%
successful and in the hands of an unskilled operator can make the
recording sound a lot worse than the untreated version.
--
~ Liz Tuddenham ~
(Remove the ".invalid"s and add ".co.uk" to reply)
www.poppyrecords.co.uk
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