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GB 76
9.
(but less than 3 V), the RGB signals at pins 51, 52 and 53 are
inserted into the picture by using the internal switches.
Also some picture improvement features are implemented in
this part:
•
Black stretch This function corrects the black level of
incoming signals, which have a difference between the
black level and the blanking level. The amount of
extension depends upon the difference between actual
black level and the darkest part of the incoming video
signal level. It is detected by means of an internal
capacitor.
•
White stretch This function adapts the transfer
characteristic of the luminance amplifier in a non-linear
way depending on the average picture content of the
luminance signal. It operates in such a way that
maximum stretching is obtained when signals with a low
video level are received. For bright pictures, stretching is
not active.
•
Dynamic skin tone correction This circuit corrects
(instantaneously and locally) the hue of those colours
which are located in the area in the UV plane that
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 colour 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).
9.3.6
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 stabilisation is achieved in this functional block to
obtain an accurate biasing of the picture tube. Therefor 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 stabilisation 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 stabilisation 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 mA are generated to adjust the
cut off voltage. During the second frame, three pulses
with a current of 20 mA 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 colour
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
being overdriven, which could otherwise cause serious
damage in the line output stage. The reference used for
L01.1A
Circuit Description
this purpose is the DC voltage on pin 54 (BLCIN) of the
TV processor. Contrast and brightness reduction of the
RGB output signals is therefore proportional to the
voltage present on this pin. Contrast reduction starts
when the voltage on pin 54 is lower than 2.8 V.
Brightness reduction starts when the voltage on pin 54 is
less than 1.7 V. The voltage on pin 54 is normally 3.3 V
(limiter not active). During set switch-off, the black
current control circuit generates a fixed beam current of
1 mA. This current ensures that the picture tube
capacitance is discharged. During the switch-off period,
the vertical deflection is placed in an over-scan position,
so that the discharge is not visible on the screen.
9.3.7
RGB Amplifier
From outputs 56, 57 and 58 of IC7200, the RGB signals are
applied to the integrated output amplifier (7330) on the CRT
panel. Via the outputs 7, 8 and 9 the picture tube cathodes
are driven.
The supply voltage for the amplifier is +200 V and is derived
from the line output stage.
9.3.8
SCAVEM (if present)
The SCAn VElocity Modulation (SCAVEM) circuitry is
implemented in the layout of the picture tube panel. It is thus
not an extra module. This circuit influences the horizontal
deflection as a function of the picture content. In an ideal
square wave, the sides are limited in slope due to a limited
bandwidth (5 MHz).
SCAVEM will improve the slope as follows:
At a positive slope, a SCAVEM current is generated which
supports the deflection current. At the first half of the slope,
the spot is accelerated and the picture is darker. At the
second half of the slope, the spot is delayed and the slope
becomes steeper.
At the end of the slope, the SCAVEM-current decays to zero
and the spot is at the original position. An overshoot occurs
which improves the impression of sharpness.
At the negative slope, the SCAVEM-current counteracts the
deflection. During the first half of the slope, the spot is
delayed and the slope becomes steeper. During the second
half the spot accelerates, the SCAVEM-current is zero at the
end of the slope.
Via the three resistors R3371, R3379 and R3386, Red,
Green and Blue are added together, buffered and offered to
the emitter of TS7363. On the collector of this transistor,
configured in a common base, the sum of these 3 signals is
obtained. Via the emitter follower formed with TS7360, this
signal is conveyed to the differentiator C2376 and R3392.
Only the high frequencies are differentiated (small RC-time).
The positive and negative pulses of this signal drive
respectively TS7365 and TS7362 into conductivity. The DC
setting of the output stage is set by R3363, R3374, R3378
and R3384. The working voltage of the transistors is settled
at half the supply voltage.
At the positive section of the pulse, the current flows through
TS7365 and the SCAVEM coil. At the negative section of the
pulse, the current flows through TS7362 and the SCAVEM
coil.
9.4
Synchronisation
Inside IC7200 (part D), the vertical and horizontal sync-
pulses are separated. These 'H' and 'V' signals are
synchronised with the incoming CVBS signal. They are then
fed to the H- and V-drive circuits and to the OSD/TXT circuit
for synchronisation of the On Screen Display and Teletext
(or Closed Caption) information.
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