SAA7374 Datasheet, PDF (27/60 Pages) NXP Semiconductors – Low voltage digital servo processor and Compact Disc decoder CD7LV

Philips Semiconductors
Low voltage digital servo processor and
Compact Disc decoder (CD7LV)
Product specification
SAA7374
The sledge is continuously controlled, or provided with
step pulses to reduce power consumption using the
filtered value of the radial PID output. Alternatively, the
microcontroller can read the average voltage on the radial
actuator and provide the sledge with step pulses to reduce
power consumption. Filter coefficients of the continuous
sledge control can be preset by the user.
7.14.4.4 Access
The access procedure is divided into two different modes
(see Table 11), depending on the requested jump size.
Table 11 Access modes
ACCESS
TYPE
Actuator jump
JUMP SIZE⁽¹⁾
1 - brake_distance
Sledge jump brake_distance - 32768
Note
1. Microcontroller presettable.
ACCESS
SPEED
decreasing
velocity
maximum
power to
sledge⁽¹⁾
The access procedure makes use of a track counting
mechanism, a velocity signal based on a fixed number of
tracks passed within a fixed time interval, a velocity set
point calculated from the number of tracks to go and a user
programmable parameter indicating the maximum sledge
performance.
If the number of tracks to go is greater than the
brake_distance then the sledge jump mode should be
activated or, the actuator jump should be performed. The
requested jump size together with the required sledge
breaking distance at maximum access speed defines the
brake_distance value.
During the actuator jump mode, velocity control with a PI
controller is used for the actuator. The sledge is then
continuously controlled using the filtered value of the radial
PID output. All filter parameters (for actuator and sledge)
are user programmable.
In the sledge jump mode maximum power (user
programmable) is applied to the sledge in the correct
direction while the actuator becomes idle (the contents of
the actuator integrator leaks to zero just after the sledge
jump mode is initiated).
7.14.4.5 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 optimum
performance, the gain control should only be activated for
a short time (for example, when starting a new disc).
This gain control differs from the level initialization. The
level initialization should be performed first. The
disadvantage of using the level initialization without the
gain control is that only tolerances from the front-end are
reduced.
7.14.5 OFF-TRACK COUNTING
The track position signal (TPI) is a flag which is used to
indicate whether the radial spot is positioned on the track,
with a margin of ±¹â„₄ of the track-pitch. In combination with
the radial polarity flag (RP) the relative spot position over
the tracks can be determined. These signals are, however,
afflicted with some uncertainties caused by;
• Disc defects such as scratches and fingerprints
• The HF information on the disc, which is considered as
noise by the detector signals.
In order to determine the spot position with sufficient
accuracy, extra conditions are necessary to generate a
track loss signal (TL) and an off-track counter value. These
extra conditions influence the maximum speed and this
implies that, internally, one of the following three counting
states is selected:
1. Protected state: used in normal play situations. A good
protection against false detection caused by disc
defects is important in this state.
2. Slow counting state: used in low velocity track jump
situations. In this state a fast response is important
rather than the protection against disc defects (if the
phase relationship between TL and RP of ¹â„_2π radians
is affected too much, the direction cannot then be
determined accurately).
3. Fast counting state: used in high velocity track jump
situations. Highest obtainable velocity is the most
important feature in this state.
1998 Feb 26
27