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Philips SACD 1000/001 Service Manual Page 61

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In the sledge jump mode maximum power (user
programmable) is applied to the sledge in the correct
direction, while the actuator becomes idle.
Radial Automatic Gain Control loop:
The loop gain of the radial control loop can be corrected
automatically to eliminate tolerances in the radial loop. This
gain control injects a signal into the loop which is used to
correct the loop gain. Since this decreases the optimal
performance the gain control should only be activated shortly
(for instance when starting a new disc). This gain control
differs from the earlier mentioned level initialisation. This
level initialisation should be done first. The level initialisation
without the gain control reduces tolerances from the front-
end only.
The radial PID.
Since we are dealing with a big variety of applications and
drives, the servo controllers in MACE2 should be adjustable
within a large frequency range.
In order to read out the track properly -a track consists of
sequential ordered data pit's which hold audio, video or ROM
data - the focus and radial position controls must follow the
moving track within some tenths of a micrometer, despite of
disc imperfections and external disturbances.
For instance, a rotating disc causes, due to track eccentricity,
track unroundness, or disc skew, track movements up to
some millimetres. The control loop reduces this to about one
tenth of a micrometer.
Initialisation control.
Due to optical, electrical and mechanical tolerances in CD
players, properties of the servo signals such as offset and
gain can vary. In general, without proper signal processing, a
simple PID controller function cannot cope with this relatively
large offset and gain spreads. Therefore, gain and offset
adjustments during manufacture or active control, to
compensate for these signal imperfections, become
Adjustment procedures in the factory are expensive. So,
automatic adjustment procedures have been implemented in
order to avoid most of the potentiometer adjustments. In the
MACE2 servo automatic adjustments are applied to the
radial error signal only.
9.10 The AGC.
The Automatic Gain Control is used in the MACE2 digital
servo to adjust the radial and focus bandwidths to a nominal
value. Injecting a signal in the loop and measuring the phase
of its resulting signal (wobble method) does this.
Principle of the Automatic Gain Control.
The principle of the Automatic Gain Control (AGC) circuit, as
used in MACE2, is drawn in the next figure.
Figure 9-3 Principle of the automatic gain control.
A sine wave signal with a certain frequency is injected in the
control loop at the summation point. The resulting signal is
Circuit-, IC descriptions and list of abbreviations
SACD 1000
measured before the injection point. The injected signal is
shaped by H/1+H . This measured signal is multiplied by a
phase shifted version of the injected signal. The result of the
multiplication is low pass filtered and integrated. (The
integrator is started at the nominal gain of the control loop).
The result of the integration is fed to the adjustable loop gain.
This principle, synchronous detection, is also known as
'wobble method'.
9.11 The Fast brake.
The fast brake is an aid to speed up radial capture after a
high-speed jump. It is a separate radial control with a much
higher bandwidth. The radial control output can be switched
between fast brake mode and original radial control mode.
The fast brake helps the radial actuator at the end of a jump
to "stick" to the right track. In fast brake mode, the actuator
starts to follow the track movements. It's a bit like jumping on
a moving train. If you run as fast as the train, you can just step
in. After a radial open loop jump the tracks are moving (as a
result of eccentricity) at a very high speed underneath the
radial actuator. This speed is too high for a normal radial
control loop to do radial capture. When the radial control is
switched over to fast brake mode for a short term, this moving
of the tracks underneath the actuator becomes much slower,
(because the actuator follows the track movement), so when
you switch back to original radial control, it's much easier to
do radial capture.
9.12 The Defect Detector.
Because of the possible earlier mentioned defects
(fingerprints, etc) a defect detection circuit is incorporated
into the MACE2 servo. If a defect is detected, the radial and
focus error signals may be zeroed, resulting in better
9.12.1 Operation:
The defect detector prevents the light spot from going out of
focus and going off track due to disc dropout excitations. The
defect detector can be switched on and off under software
control and can be applied to the focus control only, or both
to the focus and radial control.
Whenever this circuit detects a defect, it will hold all radial
and focus controls.
The hold signal is generated whenever the reflected light
intensity drops rapidly (< 1:5 ms) down to roughly 75% of the
actual intensity level. In that case the output of the
comparator becomes active and controls the focus and radial
signal switch.
This circuit improves the playability of the application (black
dot performance, etc) and is programmable to optimise it for
specific disc defects. The actions of this circuit can be
monitored on the DEFO pin (active high).
An external defect detection circuit can be added by
removing the connection between DEFO and DEFI (normal
operation) and inserting the external circuitry.
These signals are afflicted with some uncertainties caused
Disc defects like scratches and fingerprints
The HF information on the disc, which is considered as
noise by the detector signals.
9.13 Laser Drive On.
The LDON pin is used to switch the laser drive off and on. It
is an open drain output. In case the laser is on, the output has
a high impedance. The pin will be automatically driven if the
focus control loop is switched on.
GB 105


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