Daewoo DTF-2950-100D, Руководство по обслуживанию, Страница: 54 / 92

CP-850FX Service Manual
Europe R&D 53
5.4.2.10 Analog Back End
The digital RGB signals are converted to analogue RGB by three 10-bit digital to analogue
converters (DAC). Each RGB signal has two additional DACs with 9-bit resolution to adjust
analogue brightness (40% of the full RGB range) and cutoff / black level (60% of the full RGB
range). An additional fixed current is applied for the blanking level.
The back-end supports the insertion of two external analogue component signals, only one is
used for OSD/Text from the microcontroller. These signals are clamped, processed in an
analogue matrix (RGB2), converted by a voltage/current converter (VCC), and inserted into the
main RGB by the fast blank switch. The analogue RGB outputs are current outputs with current-
sink characteristics.
5.4.2.11 Analog RGB Insertion
Each component signal is clamped, converted to RGB if required, and inserted into the main
RGB by the fast blank switch. The external component signals are adjustable independently as
regards DC level (brightness) and magnitude (contrast).
Fast Blank selection logic Over-/underlay of the external component signal and the main RGB
signal depends on the fast blank input signal.
5.4.2.12 CRT Measurement and Control
In order to define accurate colour on different CRT displays, the cut-off and white drive settings
are adjusted in factory depending on the characteristic of CRT phosphor.
To guarantee correct colours during the lifetime of the display, a build in automatic tube control
loop measures and adjusts the black level on every field and white point every third field. The
display processor is equipped with an 9/12-bit PDM-ADC for all picture tube measuring purposes.
This MADC is connected to the SENSE input pin, the input range is 0 to 2.6 V. Cutoff and white
drive current measurement are carried out with 8-bit resolution during the vertical blanking
interval. The current range for cutoff measurement is set by connecting the sense resistor RC591
to the SENSE input. Due to the fact of a 1:10 relation between cutoff and white drive current the
range select 2 output (RSW2) becomes active for the white drive measurement and connects
R533 in parallel to RC591, thus determining the correct current range.
During the active picture, the MADC is used for the average beam current limiter with a 12-bit
resolution. Again a different measurement range is selected with active range select 1&2 outputs
(RSW1&RSW2) connecting R534 in parallel to R533 and RC591.
These measurements are typically done at the summation point of the picture tube cathode
currents.
The picture tube measurement returns results on
every field for:
– cutoff R
– cutoff G
– cutoff B
– white drive R or G or B (sequentially)
The average beam current limiter (BCL) works on both the digital YC r C b input from VSP and
the inserted analog RGB signals (OSD and Teletext) by using the sense input for the beam
current measurement. The BCL uses a different filter to average the beam current during the
active picture resulting in a 12-bit resolution. The filter bandwidth is approximately 4 kHz. The
beam current limiter allows the setting of a threshold current, a gain and an additional time
constant. To accommodate several CRT’s, beam current threshold and gain can be modified by
microcontroller option2. If the beam current is above the threshold, the excess current is low-
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