Pioneer PD-6050 Service Manual - Page 10

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The focus signal would then be positive because (A + C)

is greater than (B + D). However, after the peak (vertical

line) is reached at point 6, it begins to return to zero. If it

becomes zero at point 4, the beam becomes an ellipse that

is wider than it is high because (A + C) is less than (B

+ D) and the focus signal becomes negative. After peak-

ing at point 2, the focus signal returns to zero just as when

the objective lens is too far from the disc. Focusing signals

produced in the above manner are shown in fig. 2-5. Due

to its shape, this is called an S-curve, an important graph that

expresses the properties of the focus signal.

Fig, 2-6 Detection of tracking error

Fig* 2-5 S-curve

Since the real purpose of the focus servo is to maintain

the focus signal at zero, only a tiny section at the center of

the S-curve appears as residual error.

Fig. 2-6 shows how the tracking signal is detected. The

beam from the laser diode is divided into three beams. The

± 1 order beams on either side of the 0 order beam are used

to produce the tracking signal. These two beams are, like the

0 order beam, are directed at the disc in a tiny spot. In prin-

ciple, the spots of the two side beams are an equal distance

from the center spot as shown in fig. 2-6. (The actual dis-

tance is much greater than that shown in the figure.) These

two side beams are reflected and diffracted and returned to

their respective detection elements in the photo diode. If these

two elements detect the same intensity from both beams, it

can be assumed that the primary (0 order) beam is correctly

following the line of pits on the disc. Fig. 2-7 shows the rela-

tionship between the track and the output of each photo di-

ode element (A, B and C).

Fig. 2-7 Tracking error and the RF signal

1 0