Technics SL-P202A - Page 43

SL-P202A
The
processing
formula
produced
by
the
radial
error
processor
is
RE
=
k d
(411412413414)
-
k(il+i2).
The
factors
k
and
d
are
detected
from
the
loop
itself
by
processing
a
low
frequency
wobble
fault
signal,
which
is
injected
into
the
loop.
Injection
of
this
causes
a
displacement
of
+/-
0.05
um
of
the
arn.
The
k
is
again
factor
and
has
to
compensate:
eThe
tracking
angle
fault
(varies
between
90
degrees,
smallest
radius,
and
45
degrees,
largest
radius).
eThe
pit
geometry
of
disc
(causing
the
amplitude
of
radial
error
signals
to
vary
up
to
100%
from
disc
to
disc).
@Deterioration of the laserdiode.
3-2-2 Automatic
(k-factor)
gain eontrol
Detection of the desired value is done
by detecting the phase angle of an
injected wobble signal in the radial
loop.
The closed loop transfer function has
a
phase
shift
of
135
degrees
at
650
Hz
(the wobble frequency).
To
maintain
the
loop
gain,
the
phase
of
the
closed
loop
signal
of
RE
is
compared
with
a
-45
degrees
shifted
signal
derived
from
the
650
Hz
wobble
injection
oscillator.
If
the
gain
is
nominal
the
output
of
this
phase
comparator
is
zero.
In
other
cases
a
positive
or
negative
phase
error
signal
is
detected.
The detected signal is integrated by
C2538.
The
value
of
this
integrated
signal
sets
the
gain.
This
completes
the
set
up
for
the
k
factor
in
TDA8809.
Note:
The
phase
comparator
is
a
linear
multiplier
which
ensures
that
the
a.g.c.
loop
is
unaffected
by
spurious
signals
near
the
odd
harmonics
of
the
wobble
frequency.
The open-loop gain control is
clamped to an internal reference
level during sstarting-up and
searching.
t<+—
3-2-3 Automatic asymmetry control
(d-factor)
The radial error signal in a
single-beam pick-up is derived froma
comparison of the light intensity in
each pupil half.
Errors in this signal can be caused
by:
eAsymmetry in the far-field pattern of
the laser.
eLight path to the disc.
eImperfections in the beam splitter.
Errors can be corrected by bringing
the d-factor in the total transfer
function.
The wobble used to measure’ the
k-factor
is
also
used
to
measure
the
asymmetry.
The
signal
causes
the
beam
to
wobble
with
an
amplitude
of
0.05
um.
When
the
reading
spot
is
to
the
right
of
the
track
the
total
amount
of
L.F.light
(iTOT
=
i14+12+i3+i4
=
RE1
+
RE2)
is
in
phase
with
the
wobble
in
the
radial
error
signal.
When the reading spot is to the left
the I.f. sum signal (RE1 + RE2) is 180
degrees out of phase with the wobble.
The sum signal is in practice
multiplied by a 90 degrees shifted
version of the wobble signal. The
multiplier is again linear giving the
above-mentioned benefits. The loop has
a filter which is set by the value of
C2537.
The imperfections mentioned above make
the beam follow the track at one of
its edges. The wobble component in the
RE signal for such a _ situation is
visualised below.
Note: C2537 is the capacitor which
integrates the d-factor.
—sum signal
track
(
in phase
opposition
as deviation