On Sat, 18 Feb 2017 14:21:48 +0000, Eiron
wrote:

On 18/02/2017 12:00, Iain Churches wrote:
"Vir Campestris" wrote in message
o.uk...
On 17/02/2017 09:42, Iain Churches wrote:
A bad pressing? I like that :-))

I did have one once. It so happened we had a CD tester at work, and it
showed multiple errors scattered all over the surface. I contacted the
manufacturer, and they sent me a new disc. Pressed somewhere else... I
don't know if they did anything with the tester report I sent them.

:-))

Did the multiple errors affect the replay, or prevent
the disc from playing? Quite often they do not.

CD plants produce in "lines", so the replacement disc does
not have to be made somewhere else just on a different line.

A Reed Solomon reader (probably like the one you had at
work) is used for QC. One of these two gentlemen
(Reed or Solomon) is quoted as saying "without error
correcting codes digital audio would not be technically
feasible"

Which is complete bollox.

Anyway, a CD will play perfectly even after drilling a few 2mm holes in it.

Yup, and the reason it will is the error correction, specifically
trellis coding that splits a byte up into individual bits then dots
them all over the place so something like a hole won't destroy
complete bytes which would be hard to rebuild. The R&S coding plus a
heap of redundancy means you can reassemble valid data from some
pretty awful damage. They are right that not many CDs would survive
more than a few plays and still be readable without this error
correction.

d

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In article , Don Pearce
wrote:
Which is complete bollox.

Anyway, a CD will play perfectly even after drilling a few 2mm holes in
it.

Yup, and the reason it will is the error correction, specifically
trellis coding that splits a byte up into individual bits then dots them
all over the place so something like a hole won't destroy complete bytes
which would be hard to rebuild. The R&S coding plus a heap of redundancy
means you can reassemble valid data from some pretty awful damage.

Yes. Something I've been confirming over the last few days. c.f 'aside'
below...

They are right that not many CDs would survive more than a few plays and
still be readable without this error correction.

Not sure what you meant there. A conventionally made CD (i.e. not a CD-R/W)
shouldn't be degraded by being played a few times.

Aside:

A couple of days ago a musician we know brought a CDR they'd got from a
recent concert they'd made being recorded. This was covered with gunk which
took some cleaning off before it would play reliable even in some Audio CDR
players I use. It then played despite having an obvious 'hole' in the
polycarb covering one part of the disc. Fortunately the hole is smaller
than a mm or so.

But the disc was clearly still 'difficult' as one of the Audio CDR machines
players still a fuss about reading the TOC and deciding what it was. (The
other was happy.) Similarly, an experimental try and ripping it see what
I'd get using a standard drive in my main Linux box struggled.

However the drive in my RISC OS box read it, no worries. I used two
different ripping processes and the results, when compared, were
bit-identical. So I have now edited one work to have a track per movement.

I suspect the disc was written at far too high a rate as well as then
having been covered in gunk.

FWIW I also noticed that the recording has peaks that go right up to within
a gnat's crotchet of clipping. So I'll try and find out more about who
recorded it and if there is a source version to compare with. Curious to
see what I can find out.

However all the Audio CDRs I'd written a decade or so ago continue to be
playable with no problems - although I have now transferred almost all to
flac files anyay.

Jim

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On Sat, 18 Feb 2017 16:58:17 +0000 (GMT), Jim Lesurf
wrote:

They are right that not many CDs would survive more than a few plays and
still be readable without this error correction.

Not sure what you meant there. A conventionally made CD (i.e. not a CD-R/W)
shouldn't be degraded by being played a few times.

I was really referring to the random little marks and scratches that a
CD accumulates in less-than-careful hands. Without error correction
even minor marks make a CD unplayable.

d

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