In article 20140907144744.351a420b@debian,
Folderol wrote:
On Sun, 07 Sep 2014 14:29:07 +0100
Jim Lesurf wrote:
[snip]

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.

In effect, that's what I'm experimenting with at present.

My first experiments scan though looking at the level. Then finding places
where the peak levels drop a great deal in a short time. Thus picking up
events with a sharply falling trailing edge. I've tried combining this with
the peak level and crest factors.

It works for the most obvious clicks. But not for the small ones whose size
is *not* much bigger than the musical waveforms. So it shows clicks that
are also clear to see with Audacity, but misses the smaller hard-to-see
examples. So it is useful, but limited in value.

Hence I'm thinking of trying the same approach as above, but to the first
or second differential of the waveforms to change the relative scaling of
quick events (with a lot of HF) to the surrounding music.

BTW I also recall the old Hi Fi News cover showing some LP replay systems
at their pressing factory. These looked strange because they were playing
the LPs 'backward'. They were being used to look for faults (clicks) so
went backwards for the same reason as above.

Anyone buying EMI classical LPs at the time may not have been astonished
that 2 out of 3 of the decks shown had a big red 'fault detected' light lit
up. 8-] That seemed about right to me at the time. About 2/3rds of the EMI
classical LPs I bought then had to be returned due to the added rifle
shots!

Jim

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