Philips 7629 Service Manual page 69

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-pointblack 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
change in the gain of the output stage will be compensated by a gain change of
the RGB control circuit. Pin 55 (BLKIN) of the UOC is used as the feedback input
from the CRT base panel.
Blue stretch This function increases the color temperature of the bright scenes
(amplitudes which exceed a value of 80% of the nominal amplitude). This effect
is obtained by decreasing the small signal gain of the red and green channel
signals, which exceed this 80% level.
Beam current limiting A beam current limiting circuit inside the UOC handles the
contrast and brightness control for the RGB signals. This prevents the CRT from