R^OX
B225
SECTION 3/2
3.1.3 Focus control loop
The lens can be moved vertically and is designed similar
to the mechanical system of a speaker. The lens is
controlled magnetically. The coil is stationary and
the magnet moves. The focus error signal FOC-ERR is
first amplified (lead amplifier IC2a, R3, R12, R16,
R17,
C3, C4) and subsequently taken to the controlling
amplifier
("lag"
amplifier Rl, R2, R5, Rll, R15, R29,
R30,
C2, C12, IC2b, Q6, Q7 and the impedance of the
lens
coil).
In STOP mode, the control loop is interrupted
via Ql by the signal FOC-0 {= H) which means that no
unnecessary power is consumed by the output-amplifier
(Q6 and Ql).
The start-up circuit (IC1, R6, R7
)
adjusts the focussing
to the reflective layer of the compact disc. The signals
FC-1 and FC-2 are alternatingiy switched to "H", causing
the lens to move alternatingiy up or down. Once the
correct focus has been found, the signal FOC-NEG briefly
drops to "L". The microprocessor then enables the focus
control loop
(FOC-0,
FC-1, and FC-2 -
"L"
)..
Up to 12,000
tracks per second are skipped in fast search mode.
Because the focussing lens readjusts for each track, a
whistling sound of approximately 12 kHz would develop as
is the case with a speaker. This whistling sound is
eliminated by R60, R61, C31, Q13 in the control loop.
This filter is only active in search mode.
3.1.4 Disc motor control loop
The data from the compact disc must be supplied as
steadily as possible to the digital signal processing
circuit.
The speed with which these data (bits) arrive
depends on the position of the laser pick-up (constant
peripheral speed between 1.2 and 1.4 m/s).
If the laser pick-up reads in the middle of the CD,
the^speed is-much higher than near the edge. The speed
correction signal MCES is generated in the error
correction Ic"(SAA 7020 on PCB
1.769.300).
The signal MCES is switched off or on via the signal
FOC-0 (R15, R16, IC3b on PCB
1.769.330).
The signal
is first filtered via a 2nd order low pass (R22, R23,
R25
r
R26, R28, C10, Cll,
IC3b).
The "lead network"
is formed by C9,R19, R21, R24, and R28, the
,r
lag network"
by IC3a, C6, C7
,
R8
,
Q2, and Q3.
As soon as the focal point has been found, the signal
FOC-0 changes to "LOW
1
' and the signal MCES is enabled.
A negative pulse via C5 is simultaneously triggered on
IC3a,
pin 3, thereby fully driving transistor Q3 in order
to rapidly accelerate the compact disc. The speed is
subsequently controlled by the MCES signal.
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