2003 Oct 01 33
Philips Semiconductors Product specification
CD audio decoder, digital servo and filterless
DAC with integrated pre-amp and laser control
SAA7826
In the sledge jump mode maximum power (user
programmable) is applied to the sledge in the correct
direction while the actuator becomes idle (the content of
the actuator integrator leaks to zero just after the sledge
jump mode is initiated). The actuator can be electronically
damped during sledge jump. The gain of the damping loop
is controlled via the hold_mult parameter.
The fast track jumping circuitry can be enabled or disabled
via the xtra_preset parameter.
7.16.4.5 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.
7.16.5 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 ±0.25 of the track pitch. In combination
with the Radial Polarity flag (RP) the relative spot position
over the tracks can be determined.
These signals can have 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 0.5πradians
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.
7.16.6 TRACK COUNTING MODES
Fast counting mode is auto-selected for track-crossing
speeds above 1200 tracks/s. In this case the off-track
counting decrements occur only for effect of the RP signal.
The direction of the jump is already known because the
Slow counting mode occurs prior to entering the Fast
counting mode.
When the Slow counting mode is selected the maximum
track-crossing speed that can be reached is 12 kHz
(providing that the maximum value for rad_pole_lead is
used). In this case the direction of the jump is given by the
phase shift between RP and TL(+90
o
for outward jumps
and -90
o
for inward jumps). The number of pulses in the TL
signal gives the number of tracks crossed.
When Fast counting mode is enabled, whenever the
track-crossing speed falls below 12 kHz, the counting
mode is automatically changed to Slow counting mode.
7.16.7 DEFECT DETECTION
A defect detection circuit is incorporated into the
SAA7826. If a defect is detected, the radial and focus error
signals may be zeroed, resulting in better playability. The
defect detector can be switched off, applied only to focus
control or applied to both focus and radial controls under
software control (part of foc_parm1).
The defect detector (see Fig.22) has programmable set
points selectable by the parameter defect_parm.
7.16.8 OFF-TRACK DETECTION
During active radial tracking, off-track detection has been
realised by continuously monitoring the off-track counter
value. The off-track flag becomes valid whenever the
off-track counter value is not equal to zero. Depending on
the type of extended S-curve, the off-track counter is reset
after 0.75 extend or at the original track in the 2.25 track
extend mode.
