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SAA7824 Datasheet(PDF) 33 Page - NXP Semiconductors |
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SAA7824 Datasheet(HTML) 33 Page - NXP Semiconductors |
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33 / 89 page  2003 Oct 01 33 Philips Semiconductors Product specification CD audio decoder, digital servo and filterless DAC with integrated pre-amp and laser control SAA7824 In the sledge jump mode maximum power (user programmable) is applied to the sledge in the correct direction while the actuator becomes idle (the content of the actuator integrator leaks to zero just after the sledge jump mode is initiated). The actuator can be electronically damped during sledge jump. The gain of the damping loop is controlled via the hold_mult parameter. The fast track jumping circuitry can be enabled or disabled via the xtra_preset parameter. 7.16.4.5 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 optimum performance, the gain control should only be activated for a short time (for example, when starting a new disc). This gain control differs from the level initialization. The level initialization should be performed first. The disadvantage of using the level initialization without the gain control is that only tolerances from the front-end are reduced. 7.16.5 OFF-TRACK COUNTING The Track Position signal (TPI) is a flag which is used to indicate whether the radial spot is positioned on the track, with a margin of ±0.25 of the track pitch. In combination with the Radial Polarity flag (RP) the relative spot position over the tracks can be determined. These signals can have uncertainties caused by: • Disc defects such as scratches and fingerprints • The HF information on the disc, which is considered as noise by the detector signals. In order to determine the spot position with sufficient accuracy, extra conditions are necessary to generate a Track Loss signal (TL) and an off-track counter value. These extra conditions influence the maximum speed and this implies that, internally, one of the following three counting states is selected: 1. Protected state: used in normal play situations. A good protection against false detection caused by disc defects is important in this state. 2. Slow counting state: used in low velocity track jump situations. In this state a fast response is important rather than the protection against disc defects (if the phase relationship between TL and RP of 0.5 π radians is affected too much, the direction cannot then be determined accurately). 3. Fast counting state: used in high velocity track jump situations. Highest obtainable velocity is the most important feature in this state. 7.16.6 TRACK COUNTING MODES Fast counting mode is auto-selected for a track crossing speed above 1200 tracks/s. In this case the off-track counting decrements occur only for effect of the RP signal, and the direction of the jump is already known because the Slow counting mode occurs before going into Fast counting mode. When the Slow counting mode is selected, the maximum track crossing speed that can be reached is 12 kHz (providing that the maximum value for rad_pole_lead is used). In this case the direction of the jump is given by the phase shift between RP and TL (+90 degrees for outward jumps, −90 degrees for inward jumps). The number of pulses in the TL signal gives the number of tracks crossed. When the Fast counting mode is enabled, whenever the track crossing speed goes below 12 kHz, the counting mode is automatically changed to Slow. 7.16.7 DEFECT DETECTION A defect detection circuit is incorporated into the SAA7824. If a defect is detected, the radial and focus error signals may be zeroed, resulting in better playability. The defect detector can be switched off, applied only to focus control or applied to both focus and radial controls under software control (part of foc_parm1). The defect detector (see Fig 22) has programmable set points selectable by the parameter defect_parm. 7.16.8 OFF-TRACK DETECTION During active radial tracking, off-track detection has been realised by continuously monitoring the off-track counter value. The off-track flag becomes valid whenever the off-track counter value is not equal to zero. Depending on the type of extended S-curve, the off-track counter is reset after 0.75 extend or at the original track in the 2.25 track extend mode. |
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