SAA7370 Datasheet, PDF (26/60 Pages) NXP Semiconductors – Digital servo processor and Compact Disc decoder CD7

Philips Semiconductors
Digital servo processor and
Compact Disc decoder (CD7)
Product specification
SAA7370
7.14.3 FOCUS SERVO SYSTEM
7.14.3.1 Focus start-up
Five initially loaded coefficients influence the start-up
behaviour of the focus controller. The automatically
generated triangle voltage can be influenced by
3 parameters; for height (ramp_height) and DC offset
(ramp_offset) of the triangle and its steepness
(ramp_incr).
For protection against false focus point detections two
parameters are available which are an absolute level on
the CA-signal (CA_start) and a level on the FE_n signal
(FE_start). When this CA level is reached the FOK signal
becomes true.
If the FOK signal is true and the level on the FE_n signal is
reached, the focus PID is enabled to switch on when the
next zero crossing is detected in the FE_n signal.
7.14.3.2 Focus position control loop
The focus control loop contains a digital PID controller
which has 5 parameters which are available to the user.
These coefficients influence the integrating (foc_int),
proportional (foc_lead_length, part of foc_parm3) and
differentiating (foc_pole_lead, part of foc_parm1) action of
the PID and a digital low-pass filter (foc_pole_noise, part
of foc_parm2) following the PID. The fifth coefficient
foc_gain influences the loop gain.
7.14.3.3 Drop-out detection
This detector can be influenced by one parameter
(CA_drop). The FOK signal will become false and the
integrator of the PID will hold if the CA signal drops below
this programmable absolute CA level. When the FOK
signal becomes false it is assumed, initially, to be caused
by a black dot.
action of the PID can be switched at the same time as the
gain switching is performed.
7.14.3.6 Focus automatic gain control loop
The loop gain of the focus control loop can be corrected
automatically to eliminate tolerances in the focus 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).
7.14.4 RADIAL SERVO SYSTEM
7.14.4.1 Level initialization
During start-up an automatic adjustment procedure is
activated to set the values of the radial error gain (re_gain),
offset (re_offset) and satellite sum gain (sum_gain) for TPI
level generation. The initialization procedure runs in a
radial open loop situation and is ≤300 ms. This start-up
time period may coincide with the last part of the motor
start-up time period.
Automatic gain adjustment: as a result of this
initialization the amplitude of the RE signal is adjusted to
within ±10% around the nominal RE amplitude.
Offset adjustment: the additional offset in RE due to the
limited accuracy of the start-up procedure is less than
±50 nm.
TPI level generation: the accuracy of the initialization
procedure is such that the duty factor range of TPI
becomes 0.4 < duty factor < 0.6 (definition of duty
factor = TPI HIGH/TPI period).
7.14.4.2 Sledge control
The microcontroller can move the sledge in both directions
via the steer sledge command.
7.14.3.4 Focus loss detection and fast restart
Whenever FOK is false for longer than approximately
3 ms, it is assumed that the focus point is lost. A fast
restart procedure is initiated which is capable of restarting
the focus loop within 200 to 300 ms depending on the
programmed coefficients of the microcontroller.
7.14.3.5 Focus loop gain switching
The gain of the focus control loop (foc_gain) can be
multiplied by a factor of 2 or divided by a factor of 2 during
normal operation. The integrator value of the PID is
corrected accordingly. The differentiating (foc_pole_lead)
7.14.4.3 Tracking control
The actuator is controlled using a PID loop filter with user
defined coefficients and gain. For stable operation
between the tracks, the S-curve is extended over ±0.75 of
the track. On request from the microcontroller, S-curve
extension over ±2.25 tracks is used, automatically
changing to access control when exceeding those
2.25 tracks.
Both modes of S-curve extension make use of a
track-count mechanism. In this mode, track counting
results in an ‘automatic return-to-zero track’, to avoid
major music rhythm disturbances in the audio output for
improved shock resistance.
1998 Feb 26
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