Sometimes playing it backwards for detection actually works better than
forwards.
Brian

--
From the Sofa of Brian Gaff Reply address is active
"William Unruh" wrote in message
...
On 2014-09-07, 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...

Apparently my response never got posted. So try again.

Clicks are caused by defects on the record. beween the cartridge and the
output is the RIAA filter, which is essentially an integrator (actually
and integrator followed by a high frequency single pole boost). This
means that a sudden displacement of the stylus back and forth, gets
converted into what is essentially a step funtion-- ie the effect of the
click gets distributed in time.

Thus what one wants to do is to apply an inverse RIAA curve to the
output and look at the the result. The clicks should now be far more
localised-- ie their effect should be far more concentrated, and
removeable. Ie, apply the inverse RIAA (essentially a differentiation
followed by a bass boost-- Ie, flat to 500 Hz, then a fall at 6dB/octave
to 2000 Hz, and then flat again above that if I remember the RIAA
correctly.) note that this means that there is a total of about 50dB
change from low freq to high, which means that you have to be using at
least 24bit, and preferaqbley 32 bit processing of the signal in order
not to get clipping, or introduce excess noise.

So use sox say to impliment the inverse RIAA, then use audacity to look
for those spikes, and remove them, then use the RIAA on the result.
Note that one could just take the derivative, but that would still leave
a finite spreading due to the treble/bass boost.


Jim