Marantz DV4100 User Manual download pdf (Page 103)

IC Descriptions.

GB 47ASD-1 9.

In the sledge jump mode maximum power (user

programmable) is applied to the sledge in the correct

direction, while the actuator becomes idle.

Radial Automatic Gain Control loop:

The loop gain of the radial control loop can be corrected

automatically to eliminate tolerances in the radial loop. This

gain control injects a signal into the loop which is used to

correct the loop gain. Since this decreases the optimal

performance the gain control should only be activated shortly

(for instance when starting a new disc). This gain control

differs from the earlier mentioned level initialisation. This

level initialisation should be done first. The level initialisation

without the gain control reduces tolerances from the front-

end only.

9.8 The radial PID.

Since we are dealing with a big variety of applications and

drives, the servo controllers in MACE2 should be adjustable

within a large frequency range.

In order to read out the track properly -a track consists of

sequential ordered data pit's which hold audio, video or ROM

data - the focus and radial position controls must follow the

moving track within some tenths of a micrometer, despite of

disc imperfections and external disturbances.

For instance, a rotating disc causes, due to track eccentricity,

track unroundness, or disc skew, track movements up to

some millimetres. The control loop reduces this to about one

tenth of a micrometer.

9.9 Initialisation control.

Due to optical, electrical and mechanical tolerances in CD

players, properties of the servo signals such as offset and

gain can vary. In general, without proper signal processing, a

simple PID controller function cannot cope with this relatively

large offset and gain spreads. Therefore, gain and offset

adjustments during manufacture or active control, to

compensate for these signal imperfections, become

inevitable.

Adjustment procedures in the factory are expensive. So,

automatic adjustment procedures have been implemented in

order to avoid most of the potentiometer adjustments. In the

MACE2 servo automatic adjustments are applied to the

radial error signal only.

9.10 The AGC.

The Automatic Gain Control is used in the MACE2 digital

servo to adjust the radial and focus bandwidths to a nominal

value. Injecting a signal in the loop and measuring the phase

of its resulting signal (wobble method) does this.

Principle of the Automatic Gain Control.

The principle of the Automatic Gain Control (AGC) circuit, as

used in MACE2, is drawn in the next figure.

Figure 9-3 Principle of the automatic gain control.

A sine wave signal with a certain frequency is injected in the

control loop at the summation point. The resulting signal is

measured before the injection point. The injected signal is

shaped by H/1+H . This measured signal is multiplied by a

phase shifted version of the injected signal. The result of the

multiplication is low pass filtered and integrated. (The

integrator is started at the nominal gain of the control loop).

The result of the integration is fed to the adjustable loop gain.

This principle, synchronous detection, is also known as

'wobble method'.

9.11 The Fast brake.

The fast brake is an aid to speed up radial capture after a

high-speed jump. It is a separate radial control with a much

higher bandwidth. The radial control output can be switched

between fast brake mode and original radial control mode.

The fast brake helps the radial actuator at the end of a jump

to "stick" to the right track. In fast brake mode, the actuator

starts to follow the track movements. It's a bit like jumping on

a moving train. If you run as fast as the train, you can just step

in. After a radial open loop jump the tracks are moving (as a

result of eccentricity) at a very high speed underneath the

radial actuator. This speed is too high for a normal radial

control loop to do radial capture. When the radial control is

switched over to fast brake mode for a short term, this moving

of the tracks underneath the actuator becomes much slower,

(because the actuator follows the track movement), so when

you switch back to original radial control, it's much easier to

do radial capture.

9.12 The Defect Detector.

Because of the possible earlier mentioned defects

(fingerprints, etc) a defect detection circuit is incorporated

into the MACE2 servo. If a defect is detected, the radial and

focus error signals may be zeroed, resulting in better

playability.

9.12.1 Operation:

The defect detector prevents the light spot from going out of

focus and going off track due to disc dropout excitations. The

defect detector can be switched on and off under software

control and can be applied to the focus control only, or both

to the focus and radial control.

Whenever this circuit detects a defect, it will hold all radial

and focus controls.

The hold signal is generated whenever the reflected light

intensity drops rapidly (< 1:5 ms) down to roughly 75% of the

actual intensity level. In that case the output of the

comparator becomes active and controls the focus and radial

signal switch.

This circuit improves the playability of the application (black

dot performance, etc) and is programmable to optimise it for

specific disc defects. The actions of this circuit can be

monitored on the DEFO pin (active high).

An external defect detection circuit can be added by

removing the connection between DEFO and DEFI (normal

operation) and inserting the external circuitry.

These signals are afflicted with some uncertainties caused

by:

– Disc defects like scratches and fingerprints

– The HF information on the disc, which is considered as

noise by the detector signals.

9.13 Laser Drive On.

The LDON pin is used to switch the laser drive off and on. It

is an open drain output. In case the laser is on, the output has

a high impedance. The pin will be automatically driven if the

focus control loop is switched on.

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