saa7372 NXP Semiconductors, saa7372 Datasheet - Page 26

• Manufacturer Part Number: saa7372
• Description: Digital Servo Processor And Compact Disc Decoder Cd7
• Manufacturer: NXP Semiconductors

Philips Semiconductors
7.14.3
7.14.3.1
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
(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
reached, the focus PID is enabled to switch on when the
next zero crossing is detected in the FE
7.14.3.2
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
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.
7.14.3.4
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
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)
1998 Feb 26
Digital servo processor and Compact Disc
decoder (CD7)
F
OCUS SERVO SYSTEM
Focus start-up
Focus position control loop
Drop-out detection
Focus loss detection and fast restart
Focus loop gain switching
n
signal.
n
n
signal
signal is
26
action of the PID can be switched at the same time as the
gain switching is performed.
7.14.3.6
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
7.14.4.1
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
time period may coincide with the last part of the motor
start-up time period.
7.14.4.2
The microcontroller can move the sledge in both directions
via the steer sledge command.
7.14.4.3
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
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.
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
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).
50 nm.
R
ADIAL SERVO SYSTEM
Focus automatic gain control loop
Level initialization
Sledge control
Tracking control
2.25 tracks is used, automatically
300 ms. This start-up
Product specification
SAA7372