OT: labelling CD/DVD disks

[Mike McCarty]

Dotan Cohen wrote:
> On 23/10/06, Carroll Grigsby <cgrigs at earthlink.net> wrote:
> 
>> At some point in time it was reported that the inks commonly used in
>> marking pens contained solvents that could degrade the disk material
>> over time, which led the Sharpie people to market a special CD/DVD
>> marker for the purpose. Yeah, I bought one.
> 
> 
> I'm not contradictining you, but recently I had a disk that was
> scratched on the label side and would not read. I could literally see
> through the scratch. I took a Black marker -Artline 70 - and coloured
> over the scratch. Lo and behold- I could read the disc.

This makes some sense, when you realize what is physically going
on. CDROMs use forward error correction (FEC). When the number of
bits in error on any given "track" are less than the critical
number, then they can be corrected. What likely happened was that
with a hole all the way through the metalization layer, the optics
couldn't "see" the disc, and went into auto-focus, which caused the
next bits not to be read due to being out of focus, effectively
widening the scratch beyond the FEC capability to correct. Putting a
backing on the disc may have made it so that the optics remained 
focussed, resulting in correctable errors.

Mike
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