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SAA7824 Datasheet(PDF) 32 Page - NXP Semiconductors |
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SAA7824 Datasheet(HTML) 32 Page - NXP Semiconductors |
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32 / 89 page  2003 Oct 01 32 Philips Semiconductors Product specification CD audio decoder, digital servo and filterless DAC with integrated pre-amp and laser control 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 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. ACCESS TYPE JUMP SIZE(1) ACCESS SPEED Actuator jump 1 − brake_distance decreasing velocity Sledge jump brake_distance −32768 maximum power to sledge(1) |
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