Hi Brian
>My understanding up until that point in time was that the recording spots
>were etched / burned into a layer of aluminium or some other metal -
>certainly something more substantial than a layer of dye.
The quick answer :-)
The aluminum is simply there to provide a reflective coating. By
heating the dye up, you pocket the plastic and the dye, changing
the reflective index for the laser to read it as a different bit.
To have a laser available for home use that can pocket aluminum
would be both power consuming, and potentially dangerous the
way some people have to pull things apart.
>Are there ANY systems for recording CDROMs that actually record into
>metal? What is there to say about them?
Yes, the ones you find at Pressing Plants :-(
A bit more on CDR's
There are two primary layers on CDR media. One is the reflective layer. The
other is
referred to as the dye or recording layer. These layers are applied to a
polycarbonate
plastic disk. A CD player has a small laser in the drive that is used to read
or play the CD.
This laser is precisely aimed at the bottom surface of the disk and moves
across the disk
from track to track as it reads or plays it. A sensor in the drive picks up the
laser's light as it
is reflected off of the disk. The data or audio content of the disk is recorded
in a binary
data format. This is a scheme where data is represented in a pattern of ones
and zeros. The
reflected light is different for a one versus a zero. The drive's electronics
detect this
difference in the reflected light and reproduces the ones and zeros pattern
just as it is on
the CD.
When a CDR disk is manufactured, the reflective layer and the dye layer are
applied.
These layers are applied to the top of the plastic disk, not the bottom as is
sometimes
thought. The dye layer goes on first and the reflective layer is applied on top
of the dye
layer. Usually a lacquer layer is applied but this is only to protect the
reflective layer from
being easily scratched or damaged. A CDR recorder has a more powerful laser
than does a
CD-ROM drive or CD player. The purpose of this stronger laser is to enable it
to record
onto the CDR disk. This process is often referred to as "burning" a CD. This
term is used
because the heat of the laser actually creates tiny deformations of the dye
layer during the
recording process. Each deformation represents a bit of recorded data. As the
disk spins in
the CDR recorder, the laser is turned off and on to correspond to the ones and
zeros that
represent the data or bit pattern of the information that is being recorded.
These tiny
deformations are to alter the amount of light that is reflected from the
reflective layer when
the disk is subsequently played on a CD-ROM drive or CD player. This difference
in
reflection represents the pattern of ones and zeros that make up the recorded
data.
Media colors
===========
The ideal reflective layer should be one that reflects the maximum amount of
the laser's
light in the CD-ROM drive or player in which the disk will be played.
Additionally, it
shouldn't shift the color of the laser's light. Any color shift reduces the
readability of the disk
because the sensor in a drive or player may not recognize the altered reflected
light. The
wavelength of the laser's light isn't visible to the human eye, however we
still refer to it as
color; it is just a color that we can't see. A technical problem existed when
CDR media was
initially developed. In order to resist the heat of the recorder's laser, the
reflective layer
had to be made of metal. Shiny plastic film for example couldn't be used
because the
recording process would damage it and destroy the reflective properties
required for the
reflective layer. At that time, silver wasn't suitable because it oxidized
(tarnish) and this
changed the reflective characteristics and made silver unreliable for use as
the reflective
layer. Gold worked well because it doesn't oxidize. This made it the material
of choice for
the reflective layer.
Gold wasn't without its problems though. Since it is golden in color, it shifts
the color of the
laser light. This created a readability problem in some players. They weren't
able to reliably
read the disks because of the color shift. The read sensors in some of these
drives weren't
designed to cope with this color shift. They were designed to read silver
colored pressed
CDs, not gold CDR media. More recently, major technological advancements have
been
made that allow silver to be used as the reflective layer. Actually, it isn't
pure silver that is
used. It is a silver alloy containing other metals that make it no longer
subject to oxidation
and the problems that causes. An analogy could be made to iron versus stainless
steel. Iron
easily rusts when exposed to moisture and air. However, when certain other
metals are
added to iron, an alloy is formed which won't rust at all even though the
primary content of
stainless steel is iron. Because these silver alloys don't cause a color shift
when used as the
reflective layer on CDR media, the media manufacturers have almost entirely
converted to
the use of silver alloys for the reflective layer.
Which material is best suited for the application. The technical jury says
silver alloy is. The
only case where gold media is preferred is for use in some older recorders that
were
designed before the widespread use of silver media. These recorders were
designed to
deal with the gold color shift problem and they don't always handle silver
media reliably.
Most of these recorders are no longer in use due to obsolescence in other
respects as well.
There are two materials that are primarily used for the dye layer (also know as
the
recording layer). These are cyanine, which is light blue and pthalocyanine,
which is nearly
colorless. Both are organic compounds. There is no cyanide in either of them as
is
sometimes mistakenly thought. These materials were chosen for their predictable
and
uniform performance. You may be thinking; what about green dye, I used to buy
gold
media that was green on the bottom. Actually it isn't green it's blue. It only
appears to be
green because it looks green when viewed with the gold reflective layer behind
it. Gold
plus blue equals green. Remember the color shift that the gold reflective layer
causes. In
this case, you are seeing it with your own eye. The dye layer must uniformly
alter the
reflection of light from the reflective layer when bits are recorded by
"burning" tiny spots
on the dye layer. Drive to drive variations in laser power as well as different
recording
speeds are considerations that must be accommodated if the recorded disk is to
be reliably
played in a CD-ROM drive or CD player.
Technological advancements in CDR media technology have blurred the distinction
between the two materials with respect to which is "better". The media
manufacturers have
developed proprietary dye layer formulations that are much more reliable and
predictable
than either cyanine or pthalocyanine were in earlier media manufacturing
processes. Each
media manufacturer can build their case about their product's virtues; however,
from the
user's perspective it is difficult to separate the sales pitches from the
facts. Buying high
quality Grade A media from a reliable source is a safe bet with current
technology media.
As far as IO Products is concerned, if it isn't premium Grade A media, we don't
sell it.
Whether the user's choice is silver/light blue media or silver/silver media is
more an issue
of their color preference than one of functional reliability.
Cheers
Ian Manners
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