J said:
I've seen this problem with car cd players and portables - pretty much
anything that gets bounced around. Any solution for this besides that
cheesily-named d-skin ("duh skin")?
My friend says turn the laser up ( by turning the pot down).
Not a good idea, I think.
The distance between the lens assembly, and the surface of the disc is
set by the basic optical (focusing) behavior of the system. The beam
is cone-shaped - much wider at the lens than at the (tiny) focus spot
on the disc. If the lens is too far away, or too close to the disc
surface, the beam won't focus properly on the data layer and the data
won't read.
This focusing process really has little or nothing to do with the
strength of the laser beam. Boosting the laser signal strength won't
change the distance which the system tries to maintain between the
lens surface and the data layer, and it'll tend to age the laser diode
more rapidly.
I would question whether any but the multilayer machines even have a
vertical lens adjustment let alone active control over it. most just have a
spring holding it all the way up, no?
He says the laser is moving in closer to the disk to get a better
reflection... again, I doubt it. I can see that perhaps a machine would be
built so that reading problems result in a panic behavior rapid refocusing -
but I don't think they're built that way - CD players or regular dvds.
Well, he's sorta right in one respect. CD players *do* move the lens
vertically - but they usually do not move the whole lens *assembly*.
There's the coarse distance-setting, and the fine distance-setting to
be considered. The coarse (overall average) distance between the lens
assembly and the disc is usually fixed by the mechanism... it assumes
that the CD is of the standard/average thickness and is sitting
flat-and-square in the spindle. This average distance doesn't change
as the CD plays and the head assembly tracks it. The lens assembly is
usually free to move in only one direction (inwards and outwards on a
rail or "sled") and does not move vertically.
Then, there's the fine-scale distance changes which are needed to keep
the beam focused on the data layer. A CD player *does* have to track
these... it must focus its beam to within a tolerance of +/- .5
micrometer, while the disc's flatness and eccentricity have a much
looser tolerance (about .5 millimeter). That's a thousand-to-one
difference in precision!
CDs are often manufactured with the center hole a bit off-center
(causing an eccentric wobble in the track position under the laser)
and they often don't sit quite flat in the CD player's
spindle-and-clamp mechanism (causing a bit of vertical wobble).
There's no way to manufacture CDs, economically, so that their data
track's position would be reproducibly precise within .5 micrometer.
So, the CD player's laser-head assembly contains a focus-and-tracking
mechanism, which can actually move the position of the lens up and
down (to track vertical variations), and inwards and out (to track
horizontal ones), as the disc rotates and wobbles. This
tracking-and-autofocus process is usually done magnetically, using
voice coils (not unlike the voice coils used to move a speaker cone in
and out). The basic position of the lens "sled" assembly doesn't
change, but the lens moves inside of it as it "floats" on its voice
coils.
The trick to all of this, really, is that the lens-and-laser must be
in the *correct* position - not too close to the disc, not too far
away - in order to get a clean reflection from the data layer.
"Closer" doesn't necessarily mean "a better reflection" - if the lens
gets too close (physically or optically) its theoretical point of
focus will be on the other side of the data layer, and its beam will
actually reflect back from a wide patch on the disc rather than from a
narrow point and it won't be able to read data from any single track.
Hence, boosting the strength of the laser signal will *not* allow the
system to operate with the lens further away from the disc.
And, since the discs are probably not being scratched by the lens
itself, but by the whole lens-assembly (which is moved or rotated by
an entirely different motor system than the one used for fine
focusing), the position of the lens itself probably isn't relevant.
Then again, I don't know where to find cd lens guide design documents.
I'd recommend picking up any of Ken Pohlmann's books on CD, DVD,
and/or digital audio. He gives a good basic description of the
process.
The other thing I'd suggest, is that you make sure that your CDs are
sitting flat in the spindle. Scratching/scraping may indicate that
the CD isn't being held flat, and is wobbling vertically as it rotates
(a sort of drunken-flying-saucer wobble). If it wobbles more than a
millimeter or so, it might wobble down into the fixed-position or
spring-loaded lens assembly or into some other fixed portion of the
drive mechanism, and be scratched. Check the rim of the hole in each
CD to make sure that there isn't a bit of plastic sprue left over from
the molding/stamping process - the inner rim should be clean and smooth.
Careless use of slot-loading CD players can also cause scratches, as
can the lamentable habit of tossing CDs down on a table without
putting them into a jewel case first. I dislike Digipack and similar
packaging styles, as the CD is often stored in a paper or cardboard
sleeve and it's relatively easy to abrade them when putting them in or
taking them out.
Polycarbonate (the plastic of which CDs are made) is tough - it's
often used for bulletproof windows - but without a special
hard-coating it's pretty easy to scratch. I've never heard of CDs
being manufactured with a hard-coating.
