Re: [ARSCLIST] Memorex CDs - long reply on CD methodology

[Mike Richter <mrichter@xxxxxxx>]

steven c wrote:

> Isn't what we actually have in a CD a "sandwich" of three layers?
> There is a polycarbonate layer which is actually formed by a
> negative stamper...a very thin layer of metal to enhance the
> reflectivity of the readable surface...and a thicker acrylic
> layer to protect the playing surfact.

No, that's not what we have in a recordable disc but it is similar to 
that in a pressed disc. Considering the latter first, the acrylic 
lacquer is thicker than the metallizing, but hardly 'thicker' in any 
usual sense unless an overcoating has been applied.

Most significantly, the metallizing does not "enhance" the reading; it 
is the source of the information. Laser illumination goes through the 
thick polycarbonate layer and is reflected from the metal to be 
detected. Oversimplifying madly, assume that only the lands reflect 
significant signal back to be read; the pits and transitions reflect and 
scatter in other directions. If one disturbs the metal on even a 
microscopic level, scattering rules and signal is lost.

Now for the recordable disc in its two flavors: write-once and 
rewritable. One may think of a CD-R (write-once) as a two layer disc of 
polycarbonate with the upper 'layer' having the recording dye dispersed. 
Here the metal again reflects the signal back to the detector with 
information contained in the attenuation of the illumination in its 
round trip from emitter to detector. In theory, one could peel off the 
metal and replate it if the surface can be kept perfectly intact. 
However, see the paragraph after next for complications.

An erasable disc uses yet a third mechanism. The metal is an alloy with 
two physical states having different reflectivity. Thus the plastic is 
incidental when recording to such a disc and the information is in the 
metal layer - not in its shape but in its (microscopic) state. I suppose 
one might in some ideal world dissolve away the polycarbonate and put a 
fresh layer under the alloy, but not in the real world.

There are complications on this, notably the presence of the spiral 
groove which guides the laser as it tracks the disc. That is impressed 
into the plastic and the metallic layer overlays it. So a write-once 
disc must have the metal conform to the plastic in that sense just as 
the pressed disc does.

Beyond that, this all gets complicated.

Mike
--
mrichter@xxxxxxx
http://www.mrichter.com/