Making Compact Discs
The compact disc has become one of the most familiar
objects in electronics, but although the iridescent little discs are everywhere, few
people really know how the shiny things come to be.
Or so it appears. I read an explanation recently, in which
the writer had a laser burning data into a thin metal disc, which was then bonded between
a couple of sheets of plastic to make the final product. It is true that lasers are used
to make the original master, and certainly lasers are used to play CDs, but they
form no part of the process of manufacturing a commercial compact disc.
So here’s how it’s done. The same techniques are
used with other optical media, such as the various forms of DVD.
A CD starts out as a digital tape recording, in which
strings of 1s and 0s -- the binary digits, or bits -- are recorded as on-off pulses. The
master CD is created by focusing a very fine laser onto a glass disc coated with
photosensitive material. As the laser is moved gradually across the surface of the
spinning disc, from center to edge, the pulses on the tape switch the laser on and off.
When the laser is on, it exposes the surface of the disc like a piece of photographic
film.
After the completed disc has been "developed"
like a photo negative it is given a chemical wash that has no effect on the areas that
were not exposed by the laser, but which dissolves the coating where there was exposure.
What's left are a series of depressions, or pits, where the laser hit the surface during
the mastering process. These microscopic pits form the digital code your CD player will
eventually read.
A thin coating of nickel is now plated onto this pitted
surface. When it is sufficiently thick to hold together, it is carefully peeled off the
glass disc. Its surface is the reverse of the master: where there were pits, there are now
bumps or ridges representing the digital data. This metal "father" could be used
as a mold to make the final discs, but this is usually only done for very small runs.
Instead, the negative father is used to create a number of plastic positive
"mothers" that go through the same plating-and-peeling that the master did. The
result is a number of negative "stampers" that are used to press the final
discs. Each stamper can make between 5000 and 10,000 compact discs.
The process, if you hadn’t noticed, is in essence the
same as for making vinyl LPs, even down to the terminology.
Stampers are tested in various ways before they are used.
They are spun on special turntables to remove any eccentricities before the central hole
is punched. They are inspected microscopically to detect any physical flaws. They are even
played on special machines that can read the data in this reverse form.
If all is well, the stamper is inserted into a special
injection-molding press, which forces melted polycarbonate into the surface under very
high pressure: 25 tons. The result is a transparent disc perfectly flat on one side, and
with an impression of the original master's pits on the other.
At this stage, all the digital information is present, but
the disc can't be played because there's nothing for the CD player's laser to bounce off.
To remedy that, a very thin coating of reflective metal is applied to the pitted surface.
Usually it's aluminum, but some specialty labels use gold because, unlike aluminum, it
will not oxidize and become cloudy and non-reflective even if exposed to air. To protect
the reflective layer, a thin coating of liquid lacquer is applied to the surface and then
dried and hardened under ultraviolet light.
Finally, the label is silk-screened onto the lacquer layer,
and the disc placed in its package. All these operations are done in a super-clean,
dust-free environment.
One of the main misconceptions about the CD, even among
quite knowledgeable users, is about where the digital information actually is on the disc.
Intuitively we would expect it to be on the "playing side" but in fact it's
impressed into the label side and sealed under the lacquer coating. The laser reads the
data by passing through the transparent disc and reading what's on the other side.
Because of this, it's very important that you protect the label side of a CD when handling
it; the smooth playing side is much more robust.
Many people ask why CDs are only recorded on one side.
There's no technical reason a two-sided disc couldn't be produced; the DVD uses exactly
the same technology and there are indeed dual-sided discs available (achieved by gluing
two single-sided discs together back-to-back).
One reason is that the second side is needed for the label.
The other is that, at 70-plus minutes playing time, there's no reason to make it any
longer. There's a (possibly apocryphal) story that the 73-minute nominal length was chosen
by a Sony executive because that's the length of Beethoven's 9th Symphony, and he felt
that it should fit on a single disc.
...Ian G. Masters
ian@mastersonaudio.com
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