Servo control
Figure 20.22 illustrates the main components of the servo control system of a DVD player. The RF amplifier receives the RF signal which it passes to the RF processor via a low pass filter (LPF). It also receives photo-detectors signals: A, B, C, D and so on from the optical head for infra-red, red and blue-laser discs formats. The photodiode signals are processed by the dig- ital servo RF amplifier to produce three error signals: focus error (FE), tracking error (TE) and pull-in (PI) which are fed into the servo DSP for fur- ther processing. In addition, some players employ a fourth error signal, namely tilt (or skew) to detect and compensate for warped DVD discs.
The error signals
The servo DSP also receives a spindle control signal from the RF processor. The spindle control signal represents the data flow rate. A fast flow rate indi- cates a fast speed and vice versa. The servo DSP processes the servo input sig- nals and produce focus, tracking, spindle speed, sled and depending of the manufacturer, tilt (skew) control signals which are fed to the appropriate driv- ers on their way to the relevant actuators and motors. The servo DSP is fully programmed and controlled by the sys con microprocessor chip as shown.