In article ,
Stewart Pinkerton wrote:

On 11 Jan 2005 01:26:54 GMT, "Timothy A. Seufert"
wrote:

CD drives are designed to deal with this since even near-perfect discs
have some vibration and/or warpage. The laser and its focusing optics
are mounted in a linear voice coil motor, allowing the drive to adjust
the distance of the laser from the disc. This serves as the focus
adjustment -- the lens itself has a fixed focal point and the entire
lens/laser assembly is moved up and down to place the focal point on the
surface of the disc. This assembly is controlled by a servo system
which keeps the laser focused at all times.

On a point of information, one of the strengths of CD is that the
focus point is *not* on the surface of the disc, it's on the underside
of the metallised layer, more than 1mm beneath the reading surface.
This puts surface scratches out of focus and improves legibility of
the pits and lands.

Yes, I glosssed over that, but it is definitely a big part of why CD
works as well as it does. A further benefit is that even if surface
scratches are so bad as to impact playback, it's often possible to
return the disc to life by careful polishing with appropriate plastic
polishing / filler compounds, etc.

About the only thing arguably wrong with this aspect of CD is that the
metal layer is much less protected on the other (non-optical) side --
there, it's covered by a coat of lacquer and a screen printed label,
neither of which are particularly thick. A scratch which breaks through
the lacquer can do serious damage to the metal. Even if it doesn't
cause immediate badness, it allows a path for oxygen to begin attacking
the metal layer.

DVDs solve that problem with sandwich construction; even single-layer
DVDs are constructed from two polycarbonate layers glued together with
the metal layer in the middle. This is visible on the inner surface of
the hub.

--
Tim