65

Tracking Servo
The purpose of the tracking servo is to keep the laser beam positioned at
the center of the disc’s data (stream). This is achieved mechanically by
attaching a lightweight coil of wire to the laser’s objective (final) lens. This
coil faces a stationary magnet. Voltage applied to the tracking coil moves
the coil and its attached lens. The laser beam that passes through the
center of the lens is moved with the lens from side to side.
The sources of the tracking coil signals are four detectors in the optical
assembly base unit (E-H). These signals are matrixed by IC001 to make
a tracking error (TE) signal. When the TE signal is connected to the
tracking coil by IC701, the servo loop is complete and the servo is turned
on. The laser beam is now locked to the stream of disc data.

Tracking Servo

The tracking servo in IC701 is turned on by IC202 after the second focus
search while the disc is spinning. System Control IC202 must first com-
municate with IC601, which relays that command to Servo IC701.
System Control IC202 Uses HGA IC601
IC202 sends a tracking servo on command using data on the parallel bus.
The transfer of data must be supported by additional signals from IC202:
IC202 to IC601 Communications Signals
Signal Active Signal Present when
Chip Select (CS1 or CS4) Low IC202 and IC205 gets Vcc
Clock IC202/pin 5 Low IC202 gets B+
Write IC202/pin 22 Low IC202 and IC205 gets Vcc
Six Address lines HA 0-5 IC202 gets B+
16 Data lines HD 0-15 IC202 gets B+

HGA IC601 to Servo DSP IC701 Communications

IC601 controls many ICs. One of them is Servo DSP IC701. Communi-
cations between these ICs use four control lines to transfer the address
and data on the parallel bus:
IC601 and IC701 Communications
Name From IC When present
1. Chip
Select System Control IC202 Power on/off, playback, tray
open, close and start/stop.
2. Read System Control IC202 Always when powered on
3. Write System Control IC202 Completion of command
(PB, pause, pwr on/off)
4. Interrupt Servo IC701 Completion of command
(PB, pause, pwr on/off)
Address Bi-directional Always when powered on
Data Bi-directional Always when powered on
Servo IC701 turns on the tracking servo by completing the tracking servo
loop. IC701 amplifies the tracking error signal input pin 69 and outputs it
as a differential drive signal at pins 92 and 97.

Track Counting in Pause or Picture Jump

The new single DVD/CD laser still projects three light beams close to-
gether on to the disc. However, the early electronics inside the optical
assembly produce new tracking detector outputs (E-H) in addition to the
standard A-D RF outputs. IC001 matrixes the E-H tracking outputs to
produce:
TE = tracking error signal for the playback servo
TZC = tracking zero crossing signal from IC001/pin 32 to count tracks in
pause or when approaching a disc destination.
Both of these signals are applied to IC701. When the tracking servo is
turned on by IC202, IC701 is instructed to amplify (and LPF) the TE sig-
nals from IC701/pin 69 and produce complementary signals at pins 92
and 97. The analog TZC signal at IC701/pin 20 is judged and sent to
IC202 (via IC601) as digital track count information on the parallel bus.
Driver IC801 just amplifies the input signal to drive the tracking coil.