Philips 20PT643F Service Manual page 90

matches the skin tone. The correction is dependent on the luminance, saturation,
and distance to the preferred axis.
The YUV signal is then fed to the color matrix circuit, which converts it to R, G, and B
signals.
The OSD/TXT signal from the microprocessor is mixed with the main signal at this point,
before being output to the CRT board (pins 56, 57, and 58).
RGB Control
The RGB control circuit enables the picture parameters contrast, brightness, and
saturation to be adjusted, by using a combination of the user menus and the remote
control. Additionally automatic gain control for the RGB signals via cut-off stabilization is
achieved in this functional block to obtain an accurate biasing of the picture tube.
Therefore, this block inserts the cut-off point measuring pulses into the RGB signals
during the vertical retrace period.
The following additional controls are used:
Black current calibration loop. Because of the 2-point black current stabilization
circuit, both the black level and the amplitude of the RGB output signals depend
on the drive characteristics of the picture tube. The system checks whether the
returning measuring currents meet the requirements, and adapt the output level
and gain of the circuit when necessary. After stabilization of the loop, the RGB
drive signals are switched on. The 2-point black level system adapts the drive
voltage for each cathode in such a way that the two measuring currents have the
right value. This is done with the measurement pulses during the frame flyback.
During the first frame, three pulses with a current of 8 µA are generated to adjust
the cut off voltage. During the second frame, three pulses with a current of 20 µA
are generated to adjust the 'white drive'. This has as a consequence, that a