Reading a CD that has no reflective layer
jfoust at threedee.com
Mon Nov 17 14:19:51 CST 2014
A contemporary band recently released a CD without a reflective layer.
My son asked me about ways it might be read.
It looks completely clear but when tilted in the light you can see the
rainbow of recorded material. I believe it's a data CD, not audio,
as statements from the band hint it contains the MP3s of their
previous four albums.
(Why did the band do this? They said they were making an artistic
statement about the nature of and longevity of CDs, and as a puzzle
to their fans. The other half of the package was on a second CD in
an uncommon format known as "Mini-Max", where it is an ordinary-sized
CD but only the center 3" is silvered, giving it a distinctive look.)
I asked a friend who was an engineer at a 3M pressing plant in 1986-88.
He said that early CD-ROM drives could read a CD that didn't have a
reflective layer. He thought these drives had some of the first
Phillips and Sony mechanisms. He said they read discs like this
"all the time" in the production process. He also thought there were
a few "novelty" cealer discs produced this way in those early days.
He thought that by today's standards, these first-gen readers had lasers
that were overpowered and the gain circuitry more sensitive, so they
were able to read the un-aluminumized pits in the pressed polycarbonate.
I believe I first acquired a CD-ROM drive in 1988 or so. I may even
still have it, but I've yet to hunt for it in my storage. I don't
remember the brand of its internals.
On eBay I see old Phillips CD reading mechanisms selling for astounding
prices of more than $500, as they're used by equally astoundingly-priced
($3,000-5,000) high-end CD players such as the Studer brand. Bits is
bits, you'd think, but as usual, not for the high-end audio enthusiast.
The same mechanisms were used in the first CD-ROm readers for PCs
such as the Phillips CM100.
I believe this CD has its top-coat of acrylic / lacquer. I think
that adding a reflective layer at this point (such a shiny paint) wouldn't
affect the readability of the pits (and that spray-paint would probably
melt the lacquer.) As I understand it, the depth of the pits are
about a fifth of the wavelength of the light used to read them,
so the detector sees a phase shift.
Does anyone remember if first-gen CD drives had such super-powers?
Does anyone remember any clear pressed CDs? Any advice on how this
clear CD might be read?
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