Sony DVP S530D Bedienungsanleitung PDF herunterladen (Seite 54)

KHM-220A DVD Optical Block

Sony DVD players had two independent laser assemblies. One was used

to play DVD and the other used to play CDs. In 1999, a single DVD laser

in the KHM-220A block was used to play both discs. There were two

additional G & H photo detectors added to the optical assembly, but they

are not used.

By examining disc focusing, we can see why the lens was altered to play-

back a CD with a DVD laser.

DVD Focus

Focusing the DVD laser on the information layer of the disc is not simple.

The DVD laser must pass through the objective lens before reaching the

disc. Like a magnifying glass, this convex lens focuses the beams of light

into a point above the lens. As the beam leaves the lens, it passes from

air into the polycarbonate CD. This change in medium (density) refracts

or bends the light.

DVD

Info Layer

Objective Lens

DVD PLAYBACK

Focus Points

Close To

ether

DVD Laser

0.6 cm

CD Focus

The information layer within a DVD is at the center of the disc. The infor-

mation layer in a CD is at the top. The laser beam has to go further into

the CD disc.

1.1 cm

Focus

Points

Info Layer

Refraction

Objective Lens

Laser

DVD Laser

CD FOCUS - NO CORRECTION

Because the beam must go further, the focus points are spread into a

longer line perpendicular to the CD’s information layer. Any point along

this line results in a correct focus point, causing focus variations and RF

eye pattern fluctuations.

This focusing aberration was corrected by reshaping the objective lens to

read the CD. The lens changes the angle of the light as it enters the CD,

reducing the number of focus points. The beam can now focus on the CD

layer, but the objective lens must move slightly higher to compensate for

the angle change.

The frequency of the laser, the transition medium and the angle of light

entry determines the amount of refraction. Bending of the light as it en-

ters the disc causes a few focus points to form within. The focus points

form a short line perpendicular to the DVD’s information layer. Because

the laser beam only travels a short 0.6cm in a DVD disc before arriving at

the information layer, the number of valid focus points is limited.

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