SAA7824 Datasheet, PDF (32/89 Pages) NXP Semiconductors – CD audio decoder, digital servo and filterless DAC with integrated pre-amp and laser control (PhonIC)

Philips Semiconductors
CD audio decoder, digital servo and filterless
DAC with integrated pre-amp and laser control
Product specification
SAA7824
7.16.3.5 Focus loop gain switching
The gain of the focus control loop (foc_gain) can be
multiplied by a factor of 2 or divided by a factor of 2 during
normal operation. The integrator value of the PID is
corrected accordingly. The differentiating (foc_pole_lead)
action of the PID can be switched at the same time as the
gain switching is performed.
7.16.3.6 Focus automatic gain control loop
The loop gain of the focus control loop can be corrected
automatically to eliminate tolerances in the focus loop.
This gain control injects a signal into the loop which is used
to correct the loop gain. Since this decreases the optimum
performance, the gain control should only be activated for
a short time (for example, when starting a new disc).
7.16.4 RADIAL SERVO SYSTEM
7.16.4.1 Level initialization
During start-up an automatic adjustment procedure is
activated to set the values of the radial error gain (re_gain),
offset (re_offset) and satellite sum gain (sum_gain) for TPI
level generation. The initialization procedure runs in a
radial open loop situation and is ≤300 ms. This start-up
time period may coincide with the last part of the motor
start-up time period:
• Automatic gain adjustment: as a result of this
initialization the amplitude of the RE signal is adjusted to
within ±10% around the nominal RE amplitude
• Offset adjustment: the additional offset in RE due to the
limited accuracy of the start-up procedure is less than
±50 nm
• TPI level generation: the accuracy of the initialization
procedure is such that the duty factor range of TPI
becomes 0.4 < duty factor < 0.6 (default duty
factor = TPI HIGH/TPI period).
7.16.4.2 Sledge control
The microcontroller can move the sledge in both directions
via the steer sledge command.
7.16.4.3 Tracking control
The actuator is controlled using a PID loop filter with user
defined coefficients and gain. For stable operation
between the tracks, the S-curve is extended over 75% of
the track. On request from the microcontroller, S-curve
extension over 2.25 tracks is used, automatically changing
to access control when exceeding those 2.25 tracks.
Both modes of S-curve extension make use of a
track-count mechanism. In this mode, track counting
results in an ‘automatic return-to-zero track’, to avoid
major disturbances in the audio output and providing
improved shock resistance. The sledge is continuously
controlled, or provided with step pulses to reduce power
consumption using the filtered value of the radial PID
output. Alternatively, the microcontroller can read the
average voltage on the radial actuator and provide the
sledge with step pulses to reduce power consumption.
Filter coefficients of the continuous sledge control can be
preset by the user.
7.16.4.4 Access
The access procedure is divided into two different modes
(see Table 14), depending on the requested jump size.
Table 14 Access modes
ACCESS
TYPE
JUMP SIZE⁽¹⁾
Actuator jump 1 − brake_distance
Sledge jump brake_distance −32768
ACCESS
SPEED
decreasing
velocity
maximum
power to
sledge⁽¹⁾
Note
1. The microcontroller can be preset.
The access procedure makes use of a track counting
mechanism, a velocity signal based on a fixed number of
tracks passed within a fixed time interval, a velocity set
point calculated from the number of tracks to go and a user
programmable parameter indicating the maximum sledge
performance.
If the number of tracks remaining is greater than the
brake_distance then the sledge jump mode should be
activated or, the actuator jump should be performed. The
requested jump size together with the required sledge
breaking distance at maximum access speed defines the
brake_distance value.
During the actuator jump mode, velocity control with a PI
controller is used for the actuator. The sledge is then
continuously controlled using the filtered value of the radial
PID output. All filter parameters (for actuator and sledge)
are user programmable.
2003 Oct 01
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