Circuit Descriptions, Abbreviation List, and IC Data Sheets
9.
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
µ
A are generated to adjust the cut off voltage. During
the second frame, three pulses with a current of 20
µ
A 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 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 analogue output amplifiers on the CRT panel.
The R-signal is amplified by a circuit built around transistors
TS7311, 7312 and 7313, which drives the picture tube
cathodes. For the other two signals see the blockdiagram in
chapter 6.
The supply voltage for the amplifier is +160 V and is derived
from the line output stage.
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.
9.5
Deflection
9.5.1
Horizontal Drive
The horizontal drive signal is obtained from an internal VCO,
which is running at twice the line frequency. This frequency is
divided by two, to lock the first control loop to the incoming
signal.
When the IC is switched ‘on’, the ‘Hdrive’ signal is suppressed
until the frequency is correct.
The ‘Hdrive’ signal is available at pin 30. The ‘Hflybk’ signal is
fed to pin 31 to phase lock the horizontal oscillator, so that
TS7401 cannot switch ‘on’ during the flyback time.
When the set is switched on, the ‘+8V’ voltage goes to pin 9 of
IC7200. The horizontal drive starts up in a soft start mode. It
starts with a very short T
ON
time of the horizontal output
transistor. The T
OFF
of the transistor is identical to the time in
normal operation. The starting frequency during switch on is
therefore about 2 times higher than the normal value. The ‘on’
time is slowly increased to the nominal value in 1175 ms. When
the nominal value is reached, the PLL is closed in such a way
that only very small phase corrections are necessary.
The ‘EHTinformation’ line on pin 11 is intended to be used as a
‘X-ray’ protection. When this protection is activated (when the
voltage exceeds 6 V), the horizontal drive (pin 30) is switched
'off' immediately. If the ‘H-drive’ is stopped, pin 11 will become
low again. Now the horizontal drive is again switched on via the
slow start procedure.
The ‘EHTinformation’ line (Aquadag) is also fed back to the
UOC IC7200 pin 54, to adjust the picture level in order to
compensate for changes in the beam current.
The filament voltage is monitored for ‘no’ or ‘excessive’ voltage.
This voltage is rectified by diode 6413 and fed to the emitter of
transistor TS7405. If this voltage goes above 6.8 V, TS7405 will
conduct, making the ‘EHT0’ line ‘high’. This will immediately
switch off the horizontal drive (pin 30) via the slow stop
procedure.
The horizontal drive signal exits IC7200 at pin 30 and goes to
TS7401, the horizontal driver transistor. The signal is amplified
and coupled to the base circuit of TS7402, the horizontal output
transistor. This will drive the line output transformer (LOT) and
associated circuit. The LOT provides the extra high voltage
(EHT), the VG2 voltage and the focus and filament voltages for
the CRT, while the line output circuit drives the horizontal
deflection coil.
