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1998 Feb 26
25
Philips Semiconductors
Product specification
Digital servo processor and
Compact Disc decoder (CD7)
SAA7370
μ
A
μ
A
where f
sys
= 4.2336 MHz.
V
RH
is generated internally, and there are 32 levels which
can be selected under software control via the preset latch
command. With this command the V
RH
voltage can be set
to 2.5 V then modified, decremented one level or
incremented, by resending the command the required
number of times. In the application V
RL
is connected to
V
SSA1
.
7.14.2
S
IGNAL CONDITIONING
The digital codes retrieved from the ADCs are applied to
logic circuitry to obtain the various control signals.
The signals from the central aperture diodes are
processed to obtain a normalised focus error signal.
where the detector set-up is assumed as shown in Fig.19.
In the event of single Foucault focusing method, the signal
conditioning can be switched under software control such
that the signal processing is as follows:
I
in max central
)
f
sys
V
RH
V
RL
–
(
)
1.0
10
6
–
×
×
×
=
I
in max satellite
)
f
sys
V
RH
V
RL
–
(
)
0.5
10
6
–
×
×
×
=
FE
n
D2
D2
–
+
D1
D4
D4
–
+
D3
–
=
FE
n
2
D2
D2
–
+
D1
×
=
The error signal, FE
n
, is further processed by a
proportional integral and differential (PID) filter section.
A Focus OK (FOK) flag is generated by means of the
central aperture signal and an adjustable reference level.
This signal is used to provide extra protection for the
track-loss (TL) generation, the focus start-up procedure
and the drop out detection.
The radial or tracking error signal is generated by the
satellite detector signals R1 and R2. The radial error signal
can be formulated as follows:
where the index ‘s’ indicates the automatic scaling
operation which is performed on the radial error signal.
This scaling is necessary to avoid non-optimal dynamic
range usage in the digital representation and reduces the
radial bandwidth spread. Furthermore, the radial error
signal will be made free from offset during start up of the
disc.
The four signals from the central aperture detectors,
together with the satellite detector signals generate a track
position signal (TPI) which can be formulated as follows:
TPI = sign [(D1 + D2 + D3 + D4)
(R1 + R2)
×
sum_gain]
where the weighting factor sum_gain is generated
internally by the SAA7370 during initialization.
RE
s
R1
R2
–
(
)
re_gain
R1
R2
–
(
)
re_offset
×
+
×
=
Fig.19 Detector arrangement.
handbook, full pagewidth
D3
D1
D2
SATELLITE
DIODE R1
SATELLITE
DIODE R2
D1
D3
D2
D4
SATELLITE
DIODE R1
SATELLITE
DIODE R2
D1
D2
D3
D4
SATELLITE
DIODE R1
SATELLITE
DIODE R2
single Foucault
astigmatic focus
double Foucault
MBG422