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8-4.
Focus Servo Circuit
As shown in Fig. 8-5., the photoelectric current output from the optical block A to D detectors is input to the RF amplifier
(IC501), converted to voltage by the I-V amplifier inside, passed through the ABCD amplifier and F.E. (focus error) amplifier,
and the ABCD signal is output from Pin #∞ (ABCD) while the focus error signal is output from Pin #§ (FE) to the DSP/digital
servo (IC601). The A to D I-V amplifiers each have a gain switch for adjusting level. Using the serial data input from the
system controller (IC801) via the serial bus (SBUS), initial settings of the gain is performed together with ABCD level
adjustment according to the disc type each time PLAY, etc. to obtain the optimum output. To perform these, the system
controller reads the ABCD level data A/D converted from the DSP/digital servo (IC601) and control the gain switch so that the
data becomes the prescribed level.
The input of the DSP/digital servo (IC601) is incorporated with an A/D converter, which converts the ABCD signal and the
FE signal to 7-bit data at sampling frequencies of 22.05 kHz (fs/2) and 88.2 kHz (2 fs) respectively. The focus error signal is
then subject to servo calculation in the DSSP (digital servo signal processor) block, converted to the 88.2 kHz PWM wave, and
used to control the focus coil via the servo driver (IC551) and LPF (low pass filter).
In addition to the ABCD level adjustment mentioned earlier, the A/D converted ABCD signal data is also compared with the
reference level preset with the serial data from the system controller, and the FOK (focus OK) detection signal is output to the
system controller via the serial bus (SBUS).
The focus gain and focus bias adjustments are performed in the adjustment mode, the adjustment values are stored in the non-
volatile memory, the data is sent to the servo circuit via the serial bus (SBUS) at starting to set the focus gain and focus bias
adjustment data. In the case of focus bias, data on the bias amount during adjustment is sent to the DSSP block, added to the
focus error signal, and used to drive the focus coil.
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