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5. FOCUS OK CIRCUIT
6.
FROM RF SUMMING AMPLIFIER
Vec
1C801 KA9201 (CXA1081)
0)
FOCUS OK
FOCUS OK AMP
COMPARATOR
The focus OK circuit generates a timing window to lock on the focus servo from a focus search status. Pin
® will get the HPF (High Pass Filter) output from an RF signal from Pin @-the LPF (Low Pass Filter)
output (opposit phase) for the focus OK amplifier output.
The focus OK amplifier output is inverted when Vari-Varo+-0.37V.
C803 is for determining the time constants of HPF in the EFM comparator and mirror circuits as well as
LPF in the focus OK amplifier. When 0.0022 F is selected for C803, fc (cut-off frequency)=1kHz, prevents
the block error rate from degenerating as a damaged RF envelope results from the scratched disc, etc.
EFM COMPARATOR
The EFM comparator changes
RF signal into a binary value. As the asymmetry
generated due to
variations in disc manufacturing cannot be eliminated by the AC coupling alone, the reference voltage of
EFM comparator is controlled utilizing the fact that the generation probability of 1, 0 is 50% each in the
binary EFM signals.
AUTO ASYMMETRY
VC
1801 KA9201 (CXA1081)
CONTROL AMP
ace
1 0.01
OGND=0V
'
t
4
t
AUTO
'
ASYMMETRY 4
BUFFER
RF 1
(a)
ste
ccteeeens
-. mW =
As this comparator is a current SW type, each of the H and L levels does not equal the power supply
voltage, requiring feedback through a CMOS buffer.
DEFECT CIRCUIT
The RFI signal bottom, after being inverted, is held with two time constants, one long and one short. The
short time-constant bottom hold is done for a disc mirror defect more than 0.1m sec. The long time-
constant bottom hold is done with the mirror level prior to the defect. By differentiating this with a capacitor
coupling and shifting the level, both signals are compared to generate the mirror defect detection signal.
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