Handbook for Sound Engineers

1138 Chapter 30

glide laterally beneath the disc, moving in response to
tracking information and user access demands. Further-
more, the pickup must maintain focusing and tracking
even under adverse playing conditions such as a dirty
disc or impact and vibration.
To achieve sharp focus on the data surface and inten-
sity modulation, a laser is used as the light source. CD
pickups use an AlGaAs semiconductor laser irradiating
a coherent-phase laser beam with a 780 nm wavelength
(some manufacturers use 790 nm).
CD players can employ either single-beam or
three-beam pickups; three-beam designs are more prev-
alent. A three-beam pickup uses a center beam for
reading data and focusing, and two secondary beams for
tracking. The design of a three-beam pickup is shown in
Fig. 30-6. To generate additional beams, the laser light
passes through a diffraction grating, a screen with slits
spaced only a few laser wavelengths apart. As the beam
passes through the grating, the light diffracts; when the
resulting collection is again focused, it will appear as a

single bright centered beam with a series of succes-

sively less intense beams on either side. Three beams

from this diffraction pattern usefully strike the disc. As

discussed, when a laser spot strikes land, the smooth

interval between two pits, the light is almost totally

reflected; when it strikes a pit (seen as a bump by the

laser), destructive interference and diffraction causes

less light to be reflected into the pickup. The inten-

sity-modulated light is collected by the objective lens

and passes through the reading portion of the pickup.

In many three-beam designs, the property of astig-

matism is used to achieve auto-focusing. A cylindrical

lens is used to detect an out-of-focus condition. As the

distance between the objective lens and disc reflective

surface varies, the focal point of the optical system also

changes, and the image projected by the cylindrical lens

changes its shape, as shown in Fig. 30-7. That change in

the image on a four-quadrant photodiode generates the

focus correction signal. For example, if the disc were

too near to the pickup’s objective lens, the focal length

Figure 30-5. CD player block diagram showing optical processing and output signal processing.

Crystal

EFM

demodulator

Demodulates

User EFM codes

bits

Amplifies signals

and reproduces

clock pulse

Keeps the

laser output

the same

Rotates the disc

at the regular

line speed

APC

Motor Laser

pickup

Beams

Compact disc

Pit

Laser beam

Focuses the beam on the disc

Sub-

beams Focus servo

Controls the beams

on the radial direction

Tracking servo

Pickup

servo

RF data

Moves the laser pickup

RF amplifier

& PLL

Channel bit data Bit clock

Frame

synchronization

Frame

clock

Audio data ++

redundant

bits

Data memory

16 kbits

De-interleaving and

circ decoding

for error correction

or compensation

Absorbs wow

and flutter and

arranges data in order

D/A

converter

Digital

filters

Clock

pulses

D/A

converter

Motor drive

servo Controls

Micro

computer

Clock pulse

oscillation circuit

Converts digital signal

into analog signals

LPF

LPF

Removes clock

pulse components

Operation

commands

Operation

Display

Display

commands

Program

memory

CLV

servo

Basic structure of a CD player

Arranged

audio data

Separated

audio signals

Analog

audio signals

Right

output

Left

output