EN 669.L01.1E ABCircuit Description
Also some picture improvement features are implemented in voltage on pin 54 is normally 3.3 V (limiter not active).
this part:During set switch ?off?, the black current control circuit
Black stretch This function corrects the black level of generates a fixed beam current of 1 mA. This current
incoming signals, which have a difference between the ensures that the picture tube capacitance is discharged.
black level and the blanking level. The amount of extension During the switch-off period, the vertical deflection is
depends upon the difference between actual black level placed in an over-scan position, so that the discharge is not
and the darkest part of the incoming video signal level. It is visible on the screen.
detected by means of an internal capacitor.
White stretch This function adapts the transfer 9.3.7 RGB Amplifier
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 From outputs 56, 57 and 58 of IC7200, the RGB signals are
applied to the integrated output amplifier (7330) on the CRT
is obtained when signals with a low video level are panel. Via the outputs 7, 8 and 9, the picture tube cathodes are
received. For bright pictures, stretching is not active.driven.
Dynamic skin tone correction This circuit corrects
(instantaneously and locally) the hue of those colours The supply voltage for the amplifier is +200 V and is derived
from the line output stage.
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.9.3.8 SCAVEM (if present)
The YUV signal is then fed to the colour matrix circuit, which The SCAn VElocity Modulation (SCAVEM) circuitry is
converts it to R, G and B signals. implemented in the layout of the picture tube panel. It is thus
The OSD/TXT signal from the microprocessor is mixed with the not an extra module. This circuit influences the horizontal
main signal at this point, before being output to the CRT board deflection as a function of the picture content. In an ideal
(pins 56, 57 and 58). square wave, the sides are limited in slope due to a limited
bandwidth (5 MHz).
9.3.6 RGB Control
SCAVEM will improve the slope as follows:
At a positive slope, a SCAVEM current is generated which
The RGB control circuit enables the picture parameters
supports the deflection current. At the first half of the slope, the
contrast, brightness and saturation to be adjusted, by using a spot is accelerated and the picture is darker. At the second half
combination of the user menus and the remote control. of the slope, the spot is delayed and the slope becomes
Additionally automatic gain control for the RGB signals via cut-
off stabilisation is achieved in this functional block to obtain an steeper.
At the end of the slope, the SCAVEM-current decays to zero
accurate biasing of the picture tube. Therefor this block inserts and the spot is at the original position. An overshoot occurs
the cut-off point measuring pulses into the RGB signals during which improves the impression of sharpness.
the vertical retrace period.
At the negative slope, the SCAVEM-current counteracts the
deflection. During the first half of the slope, the spot is delayed
The following additional controls are used:and the slope becomes steeper. During the second half the
Black current calibration loop Because of the 2-point spot accelerates, the SCAVEM-current is zero at the end of the
black current stabilisation circuit, both the black level and
the amplitude of the RGB output signals depend on the slope.
drive characteristics of the picture tube. The system checks Via the three resistors R3371, R3379 and R3386, Red, Green
whether the returning measuring currents meet the and Blue are added together, buffered and offered to the
requirements, and adapt the output level and gain of the
circuit when necessary. After stabilisation of the loop, the emitter of TS7363. On the collector of this transistor, configured
in a common base, the sum of these 3 signals is obtained. Via
RGB drive signals are switched on. The 2-point black level the emitter follower formed with TS7360, this signal is
system adapts the drive voltage for each cathode in such a conveyed to the differentiator C2376 and R3392. Only the high
way that the two measuring currents have the right value.
frequencies are differentiated (small RC-time).
This is done with the measurement pulses during the frame The positive and negative pulses of this signal drive
flyback. During the first frame, three pulses with a current respectively TS7365 and TS7362 into conductivity. The DC
of 8 ?A are generated to adjust the cut off voltage. During
the second frame, three pulses with a current of 20 ?A are setting of the output stage is set by R3363, R3374, R3378 and
R3384. The working voltage of the transistors is settled at half
generated to adjust the ?white drive?. This has as a the supply voltage.
consequence, that a change in the gain of the output stage At the positive section of the pulse, the current flows through
will be compensated by a gain change of the RGB control
circuit. Pin 55 (BLKIN) of the UOC is used as the feedback TS7365 and the SCAVEM coil. At the negative section of the
pulse, the current flows through TS7362 and the SCAVEM coil.
input from the CRT base panel.
Blue stretch This function increases the colour
temperature of the bright scenes (amplitudes which 9.4Synchronisation
exceed a value of 80% of the nominal amplitude). This
effect is obtained by decreasing the small signal gain of the Inside IC7200 (part D), the vertical and horizontal sync-pulses
red and green channel signals, which exceed this 80% are separated. These ?H? and ?V? signals are synchronised with
level.
Beam current limiting A beam current limiting circuit the incoming CVBS signal. They are then fed to the H- and V-
drive circuits and to the OSD/TXT circuit for synchronisation of
inside the UOC handles the contrast and brightness control the On Screen Display and Teletext (or Closed Caption)
for the RGB signals. This prevents the CRT from being information.
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 9.5Deflection
processor. Contrast and brightness reduction of the RGB
output signals is therefore proportional to the voltage 9.5.1 Horizontal Drive
present on this pin. Contrast reduction starts when the
voltage on pin 54 is lower than 2.8 V. Brightness reduction The horizontal drive signal is obtained from an internal VCO,
starts when the voltage on pin 54 is less than 1.7 V. The
which is running at twice the line frequency. This frequency is |
ACEC
PHILIPS
