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[filmscanners] Re: Archiving???!!!



Hi Brad,

Interesting posting, and something most of us can certainly relate to.

A small bit of technological information to perhaps clarify some issues.

The CDs you get which are pre-written with things like software (and
music or images, for that matter) are not at al the same process as the
ones you burn.  Mass-produced CDs are actually press molded using glass
plates that have the "mirror image" pits and bumps cut into them with a
special machine.  These CDs are more similar to an LP record, an that
molten plastic is poured into a glass die, and an impression is created.
  This disk is then coated on one side (the side with the pits and bumps
on it) with vaporized aluminum (or sometimes other metals) and then a
clear varnish, followed by a label (made of quick drying enamels or
other paints) is applied on top of that.  These are CD-ROM disks in the
sense that they are "READ ONLY", they cannot be written to and never
were in the usual sense of the word.

Although any CD is vulnerable to damage, these are pretty stable.  In
years passed, there have been problems with pinholes in the aluminized
coating, or the coating applied to thinly, or poorly varnished CDs,
allowing the aluminum to corrode.  Sometimes the paints used to make the
label have migrated into the varnish and damaged the aluminum.  The
purpose of the aluminum is act as a reflective surface to reflect the
light that hits the clear portions back to laser/pickup.  The "pit"
areas are more opaque and less reflective, thus the necessary binary
on/off or zero's and ones.

The type of CDs you write to at home are of two basic types.  CD-R and
CD-RW.  CD-R disks can be only written to once in any one location on
the disk.  In other words, although you may be able to use the disk for
a number of writing sessions, each time you do so, you must write to a
new area of the disk.  Once the disk is full, it can never be written to
again.

CD-RW is a different technology.  It has a reversible reaction that can
be erased and rewritten to hundreds or thousands of times.

CD-R technology uses a disk which is constructed similarly to the first
disk discussed, with one main difference.  The disk has no physical pits
and bumps in it.  Instead, under that aluminum, silver or gold
reflective coating, there is a dye layer.  This dye can be made up of a
multitude of different dye formulas, which explains in part the
difference in color of these disks.  I have seen all shades of blue,
yellow, green and what appears to be "clear" to our eyes.  Each of these
different dye formulas has different levels of permanence, just like
dyes in films.  In fact, each disk tells you the dye it uses, which is
encoded on the disk, along with the manufacturer who actually made it.
There are free programs available on the net which can read this
information for you.  CD Identifier is one.

The way these types of disks store information is relatively simple.
These dyes are basically opaque to the laser beam.  The write laser
burns the dye off or the laser shuts off and leaves the dye alone.  This
creates the zeros and ones again.  These types of disks are more
vulnerable to damage than the previous CD-ROM type.  All the same
physical damage is possible, (scratches, gouges, etc) and the reflective
surface can also oxidize (which is why silver/gold or pure sputtered
gold is best).  However, being dyes, they are also vulnerable to light,
humidity, gases, high temperatures, etc.  Some are more vulnerable than
others.  If the dye begins to fade, the laser starts to be unable to
read the zeros and ones anymore.  These disks general do best kept in
the dark, kept in cool surroundings and, if possible kept in a holder
that doesn't off gas, and give the disk some breathing space. (Similar
to photo film).

Other things that can harms them are adhesives from labels, and dyes and
solvents in marking pens, as well as physical damage from pressure from
pen tips, etc.  In general, the best way to mark them is on the inside
spindle ring which has no CD information (the totally clear part next to
the hole in the center).  They are best stored in the plastic jewel
boxes, or some way where they has a bit of air circulation, but are kept
as dark as possible.  Even reading these disks does some degrading to
them, since the read laser, although much less intense, is a light source.

Misumi and Kodak gold sputtered disks are some of the better types for
archival storage.  However, another problem is storing unwritten disks.
Recently, there have been discussions about how long the dyes remain
usable before they are written to, and some are suggesting 18-24 months
before the dyes may not respond properly to the laser (for burning).

You should definitely keep unused disks dark, cool and sealed until needed.

The last technology is that of the CD-RW.  This is again quite different
from the CD-ROM or CD-R.  Again we start with a plastic disk with a
reflective coating.  And again, this disk does not have pits and bumps.
However, this media uses yet another method to record those ones and
zeros.  These disks have a coating under that aluminum that is called a
"Phase changing" material.  This has a type of structure that allows it
to go from a non-crystalline to a crystalline form each time it is
heated to a certain temperature.  So, the disk starts out with the whole
disk coating transparent and non-crystalline (amorphous).  When it is
written to, the areas that are to not reflect the light back to the
laser pickup are heated, and they become crystalline in structure and
opaque.  Areas to be left reflective, are untouched.

Since each time this layer is reheated it reverts from one form to the
other, if an area that has been made opaque needs to be made reflective
again, it is just reheated, if an are that is reflective needs to be
made opaque it is heated.  This reversible response to heat can usually
be done thousands of times.

For a long time I was told CD-RW technology was fugitive and not to be
trusted, but intuitively, I felt otherwise.  Certainly, these disks are
vulnerable to heat, if it gets high enough, but unlike the dyes, this
phase change reaction will not occur unless the temperature hits beyond
a certain threshold, and quite honestly, if they get that hot in
storage, they will likely warp before they will become unreadable
because of phase change data changes.

As a result, I have used mainly CD-RW as my backup.  The disadvantage is
that the technology has changed over the years and some are not at all
compatible with some drives, and they have their write speed
pre-determined at the factory, unlike CD-R.

Two other considerations.  Both the frequency range and intensity of
lasers has changed over the years, and I would bet the old 2X HP was
pretty intense, so it may tend to read older disks better.  However,
supposedly error correction and tracking has improved in newer models.

Lastly, some older CD writing software used standards of the day in
terms of data construction which has changed.  A lot of hacks and tricks
were incorporated into software to get extra time out of disk, or other
features that were not standardized and may have changes.  I have some
disks that will not read at all after I tried to add a session to them
using the exact same drive, but a newer version of the burning software.

I suspect data recovery is going to be a continually growing industry.

Art



Brad Davis wrote:

>
> Archiving:
>
> I've been using CD's for archiving for at least 6 years.  When I started, I
> used an HP burner that worked at 2X.  It still works.  In fact, if a CD
> won't read on another burner or CD drive, it may read on the old HP.  This
> doesn't surprise me, running slower would seem likely to be more robust.
>
> But, as I try to access older CD's, I consistently find files that I can't
> open - with any CD reader, even the HP.  While CD's written by the HP are
> likely to have fewer bad files, it seems that virtually all of the older
> CD's have some files that are unreadable, or if read, can't be opened by
> photoshop for one reason or another.  It seems that the question isn't if I
> am going to lose files, but how many on a given CD.
>
> Now, I may be doing things that increase my chances of losing a file, or
> even an entire CD, but I haven't been able to identify what I might be
> doing.  I pretty successfully avoid scratches, and beyond that, I keep the
> CD's in books that have sleeves in them.  They are stored at room
> temperature which is never above 75 degrees, nor below 60 and the humidity
> remains in a range around 40% - not a lot higher or lower.
>
> I've always purchased the more expensive name brand CD's, even though I am
> somewhat suspicious that on occasion what I got was no better than the no
> name sold by Fry's out here.  In talking to others, I hear the same stories
> irrespective of brand of CD used.
>
> CD's written by companies (that contain software, such as my Photoshop CD)
> seem to do better, I rarely have any trouble, and on the rare occasion I do,
> putting it in the old HP has always taken care of it.  I've never had to
> request a replacement CD and I don't back them up - I probably should.
>
> I have been in the habit of making multiple backups, so I haven't lost
> anything of value - yet.
>
> I've been considering DVD's, but reading about the problems they many have,
> they seem to be an even more fugitive medium.
>
> Someone must have a solution, must have found way to reduce the losses.  The
> only way I can see to reduce my losses is to write everything on my old HP
> burner and make multiple copies - perhaps 4 copies each.  That seems a bit
> much as it reduces the effective capacity of a CD to about 160 megabytes.
>
> Suggestions?
>
>
> Brad
>
> --
> Science is built up with facts, as a house is with stones. But a collection
> of facts is no more a science than a heap of stones is a house.
>                                 Henri Poincare  --Science and Hypothesis
>


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