saa7825 NXP Semiconductors, saa7825 Datasheet - Page 30

• Manufacturer Part Number: saa7825
• Description: Cd Audio Decoder, Digital Servo And Filterless Dac With Integrated Pre-amp And Laser Control
• Manufacturer: NXP Semiconductors

Philips Semiconductors
7.16.3
7.16.3.1
Five initially loaded coefficients influence the start-up
behaviour of the focus controller. The automatically
generated triangular 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
7.16.3.2
The focus control loop contains a digital PID controller
which has 5 parameters that are available to the user.
2003 Aug 13
handbook, full pagewidth
CD audio decoder, digital servo and filterless
DAC with integrated pre-amp and laser control
F
OCUS SERVO SYSTEM
Focus start-up
Focus position control loop
single Foucault
SATELLITE
SATELLITE
DIODE R1
DIODE R2
D3
D1
D2
n
signal.
Fig.20 Detector arrangement.
n
n
signal
signal is
astigmatic focus
SATELLITE
SATELLITE
DIODE R1
DIODE R2
D2
D4
30
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.
7.16.3.3
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.
7.16.3.4
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.
D1
D3
Dropout detection
Focus loss detection and fast restart
double Foucault
SATELLITE
SATELLITE
DIODE R1
DIODE R2
D1
D2
D3
D4
Objective specification
MBG422
SAA7825