VERTICAL
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OF 1V
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•
VERTICAL
MOTOR SPEED
1800RPM • 900
60
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1"-oFlV
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BOTTOM VERTICAL
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STANDARD PLAY DISC
CONS'TANT MOTOR SPEED
VARIABLE VERTICAL·FIELD TRACK LENGITH
EXTENDED PLAY DISC
CONSTANT VERTICAL FIELD TRACK LENGITH
VARIABLE MOTOR SPEED
CAV
CLV
FIBURE 1 - COMPARISON OF CAV (STANDARD PLAY) AND CLV (EXTENDED
PLAY) DISC FORMATS
selected by a set of collating
lenses and a grating lens, which
breaks to single laser beam into a
number of separate beams of
particular qualities. From the
output of the grating lens, a main
and two first order beams are
selected.
Signal reproduction is
accomplished by converting the
reflected main order beam signal
to an RF video signal. The two
first order beams are responsible
for maintenance of the tracking
position on the disc. Each first
order beam returns from the disc
to land on four photo-diodes.
When all four (eigh"t, for both
beams) of the photo-dk>de
electrical outputs are fhe same,
the main order beam is perfectly
on track. By tying the photo-
diode circuit into the mirror and
focus lens feedback circuits, the
process becomes automatic, and
if the laser can focus, and all is
lined up correctly, then the disc
player can track on its own.
To focus on the disc, PR821 O
type disc players wrap the
objective lens with a coil, not
unlike a piezo transducer. The
coil is driven by an op-amp bridge
circuit with feedback from the
photo-diodes. The feedback
allows the lens, and the tracking
mirrors that move the two first
order beams, to compensate for
eccentricities in the disc surface.
If the surface of the disc is within
3mm of being flat, the disc player
can read it. In the PR8210, about
7mm of lens travel is available,
and when focused typically the
lens is up 3mm, leaving 3mm up
and 3mm down as the available
travel range. The main order
beams and the photo-diodes
generate two different tracking
error signals, the radial error,
which is the error referenced to
the radius of the disc, and the
tangential error, the error
referenced to the tangent of the
spiral of pits. See Figure 3.
A custom microprocessor similar
to the Intel 8049 keeps tabs on
this entire sequence, and the
initial power-up focus routine is
software generated. On PR82 1 O
type disc players, the PLAY
command is followed by four
attempts at focus, the objective
lens being energized to rise until
it focuses, and if it does not focus
before it reach es the end of its
travel path, the lens will descend
and try again until all four
attempts have been made. If
focus was successful, the spindle
motor will be activated and the
disc will spin, coming up to speed
within 40 seconds. If focus was
not successful in four attempts,
the disc player will give up and
set itself to the idle state. When
focus is not achieved, the first
suspects are dirt on the objective
lens, the disc, or elsewhere in the
optical path. When the disc
player reaches the age cf 3000·
4000 hours, a focus problem
indicates a need for alignment.
Alignment of PR821 O type disc
players is covered in a separate
article.
Figure 4 shows the overall block
diagr11m of the PR821 O type disc
player: Note the rad ial and
tangential error sections of the
diagram. Note also that the signal
takes many fo rms before final
output to the video monitor; an