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CXA1372BQ/BS

Focus OK circuit

15k

92k

VG

54k

20k

0.625V

RFO

RFI

FOK

FOCUS OK AMP

FOCUS OK

COMPARATOR

RF signal

33

39

40

C5

0.01µ

The focus OK circuit creates the timing window okaying the focus servo from the focus search state.

The HPF output is obtained at Pin 39 from Pin 40 (RF signal), and the LPF output (opposite phase) of the

focus OK amplifier output is also obtained.

≈ –0.37V.

Note that, C5 determines the time constants of the HPF for the EFM comparator and mirror circuit and the LPF

of the focus OK amplifier. Ordinarily, with a C5 equal to 0.01µF selected, the fc is equal to 1kHz, and block

error rate degradation brought about by RF envelope defects caused by scratched discs can be prevented.

EFM comparator

EFM comparator changes RF signal to a binary value. The asymmetry generated due to variations in disc

manufacturing cannot be eliminated by the AC coupling alone. Therefore, the reference voltage of EFM

comparator is controlled through 1 and 0 that are in approximately equal numbers in the binary EFM signals.

As this comparator is a current SW type, each of the High and Low levels is not equal to the power supply

voltage. A feedback has to be applied through the CMOS buffer.

EFM low-frequency components leak badly, and the block error rate worsens.

31

C8

C9

ASY

R8

R9

CMOS

BUFFER

CXD2500

100k

20k

VC

Vcc

40k

40k

AUTO ASYMMETRY

BUFFER

AUTO ASYMMETRY

CONTROL AMP

39

RFI

EFM COMPARATOR

32

EFM

DGND = 0V

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