On Sun, 07 Sep 2014 14:29:07 +0100
Jim Lesurf wrote:

I've recently been experimenting with using Audacity to deal with clicks in
digital recordings made from old LPs. I suspect I'm not the first to do
this or encounter the following! Hence I'd be interested in feedback on
what follows...

LPs in very good condition only have a few clicks, and these can be easy
enough to find and fix. Particulary if they are loud 'rifle shots' that
stick out clearly on something like Audacity's waveform plots!

However other LPs can have many many clicks per LP side. This can make
finding and fixing most of them fairly time-consuming. In particular when a
small 'tick' is hiding as a small alternation to a larger and complex audio
waveform. It becomes a bit like looking for a sapling in a forest! For some
old classical LPs there may be lots of these which are audible as the music
can have long low-level sections, meaning that clicks it would be
impossible to hear with loud Jazz, say, show up against quiet classical.

Because of this I've been experimenting with ways to scan a wave file
looking for clicks. Using tricks like looking at the first or second
derivative of the waveforms which appear rise and fall quckly to emphasise
short sharp clicks out of the steady music background. However I'm
wondering about two things.

1) Anyone know of decent free software that already does something like
this well and can list a good set of 'click candidate' times in a wave
file. i.e. low levels of 'misses' and 'false alarms' even with classical
music.

2) To what extent this is simply a waste of effort beyond finding the most
obvious clicks. i.e. That there isn't a simple and reliable algorithm for
this and it ends up being quicker and better to use ears and eyes and
Audacity.

So far I have the impression that (2) comes into force pretty quickly as
the clicks vanish into the waveforms. But I thought I'd ask as I suspect
others have explored this already. :-)

BTW At present simply using ear/eye/Audacity I seem to find that the 'hard
cases' where I'm searching for many tiny 'ticks' can mean about 0.1 rate
working. i.e. About 200 - 300 mins of work per LP side for classical if I
really want to clear even the faintest ticks I hear. Fortunately, LPs that
tend to spend most of the time at higher levels are much quicker as the
music drowns out the smaller ticks.

BTW2 Having experimented I haven't found the declicking 'effect' of
Audacity to be much use. I've just been using the 'repair' instead. But
maybe I'm missing something here...

Jim

Many years ago the BBC (I think) developed a system that worked by playing a
track *backwards*. The clicks still presented themselves as sharp edged
pulses, while the music was a slowly rising signal.


--
W J G