Daewoo DDT-21H9ZZF, Service Manual

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Service Manual CP-099F
5-3-12 Display Processing
In the display processing the conversion from digital YC r C b to analogue RGB is carried out.
In the luminance processing path, contrast and brightness adjustments and a variety of features, such as black-level
expansion, dynamic peaking and soft limiting, are provided. In the chrominance path, the C r C b signals are converted
to 4:4:4 format and filtered by a colour transient improvement circuit. The YC r C b signals are converted by a program-
mable matrix to RGB colour space. The digital OSD insertion circuit allows the insertion of a 5-bit OSD signal.
The OSD signals and the display clock are synchronised to the horizontal flyback.
5-3-13 Chroma Transient Improvement
The intention of this block is to enhance the chroma resolution. A correction signal is calculated by differentiation of
the colour difference signals. The differentiation can be selected according to the signal bandwidth, e.g. for PAL/NTSC/
SECAM or digital component signals, respectively. The amplitude of the correction signal is adjustable. Small noise
amplitudes in the correction signal are suppressed by an adjustable coring circuit. To eliminate ‘wrong colours’,
which
are caused by over and undershoots at the chroma transition, the sharpened chroma signals are limited to a proper value
automatically.
5-3-14 Video Back-end
The digital RGB signals are converted to analogue RGBs using three video digital-to-analogue converters (DAC) with
10-bit resolution. An analogue brightness value is provided by three additional DACs. The adjustment range is 40 % of
the full RGB range. Controlling the white-drive/analogue brightness and also the external contrast and brightness
adjustments is done via the Fast Processor, located in the front-end. Control of the cutoff DACs is done via I 2 C bus
registers. Finally cutoff and blanking values are added to the RGB signals. Cutoff (dark current) is provided by three 9-
bit DACs. The adjustment range is 60 % of full scale RGB range. The analogue RGB-outputs are current outputs with
current-sink characteristics. The maximum current drawn by the output stage is obtained with peak white RGB. An
external half contrast signal can be used to reduce the output current of the RGB outputs to 50% . Cutoff and white-
drive current measurement are carried out during the vertical blanking interval. They always use the small bandwidth
setting.
5-3-15 CRT Measurement and Control
The display processor is equipped with an 8-bit ADC for all measuring purposes. The ADC is connected to the
SENSE input pin. Cutoff and white-drive current measurement are carried out during the vertical blanking interval.
5-3-16 Average Beam Current Limiter
The average beam current limiter (BCL) uses the SENSE input for the beam current measurement. The BCL uses a
different filter to average the beam current during the active picture. The filter bandwidth is approx. 2 kHz. The beam
current limiter has an automatic offset adjustment that is active two lines before the first cutoff measurement line. The
beam current limiter function is located in the front-end. The data exchange between the front-end and the back-end is
done via a single-wire serial interface. The beam current limiter allows the setting of a thresh-old current. If the beam
current is above the threshold, the excess current is low-pass filtered and used to attenuate the RGB outputs by
adjusting the white-drive multipliers for the internal (digital) RGB signals, and the analogue contrast multipliers for the
analogue RGB inputs, respectively. The lower limit of the attenuator is programmable, thus a minimum contrast can
always be set. During the tube measurement, the ABL attenuation is switched off. After the white-drive measurement
line it takes 3 lines to switch back to BCL limited drives and brightness.
5-3-17 Analogue RGB Insertion
The VCT 38xxA allows insertion of external analogue RGB signals. The RGB signal is key-clamped and inserted into
the main RGB by the Fast-Blank switch. The external RGB input can be overlaid or underlaid to the digital picture.
The external RGB signals can be adjusted independently as regards DC level (brightness) and magnitude (contrast). All
signals for analogue RGB insertion (RIN, GIN, BIN, FBLIN) must be synchronised to the horizontal flyback, other-
wise a horizontal jitter will be visible. The VCT 38xxA has no means for timing correction of the analogue RGB input
signals. RGB signals are not digitalised and therefore cannot be processed by the picture scaler.
5-3-18 Fast-Blank Monitor
The presence of external analog RGB sources can be detected by means of a Fast-Blank monitor. With a special
monitor logic it is possible to detect if there is an external RGB source active and if it is a full screen insertion or only
a box. The monitor logic is connected directly to the FBLIN pin. The controlling software uses this information to
disable all picture format using display scaler.