Daewoo 2898ST Service Manual page 47

RGB output and black current stabilization
The colour difference signals (R-Y, B-Y) are matrixed with the luminance signal (Y) to obtain the RGBout output
signals (pins 19, 20, 21).
In the TDA8375A the matrix type automatically adapts to the decoded standard (NTSC,PAL) .
Linear amplifiers are used to interface external RGB in signals (pins 23,24,25) from the SCART connector.
These signals overrule the internal RGB signals when the data insertion pin 26 (FBI) is switched to a level between
1.0V and 3.0V.
The contrast and brightness control and the peak white limiter operate on both internal and external RGB signals
R,G and B each have their own, independent gain control to compensate for the difference in phosphor efficiencies
of the picture tube: so called "white point" adjustment.
The nominal amplitude is about 2V black to white, at nominal input signals and control settings.
TDA8375A has a black current stabilization loop, that automatically adjust the black level to the cut-off voltage of the
picture tubes three gun cathodes.
Since no current is flowing when the voltage the cathode is equal to the cut-off voltage of the tube, the loop stabilizes
at a very small gun current.
This "black current" of the three guns is measured internally and compared with a reference current, to adjust the
black level of RGBout.
The black level loop is active during 4 lines at the end of the vertical blanking.
In the first line the leakage current is measured (max. acceptable 100 A).
In the next three lines the black levels of the three guns are adjusted.
The nominal value of the 'black current' is 10 A.
The ratio of the 'black currents' for the 3 guns tracks automatically with the white point adjustment, so the
back-ground colour is the same as the adjusted white point.
At switch-on of the TV receiver the black current stabilization circuit is not yet active and RGBout are blanked.
Before the first measurement pulses appear, 0.5 sec delay ensures that the vertical deflection is active, so the pulses
will not be visible on the screen.
During the measuring lines RGBout will supply 4V pulses to the video output stages.
The TDA8375A waits until the black current feedback input (pin 18) exceeds 200 A, which indicates that the picture tube
is warm-up.
Then the black current stabilization circuit is active.
After a waiting time of about 1.0 sec, the blanking of RGBout is released.
Tuning
The AFC information of the TDA8375A is not available as an analogue voltage.
Automatic following (=frequency tracking, AFC) can be done via the I2C-bus by software.
The TDA8375A AFC window is typically 80 kHz wide.
This value is made higher than the 62.5 kHz tuning step, to prevent an automatic following loop from continuously
adapting the tuning frequency..
With this AFC window ( 40 kHz) the maximum tuning error is less than 62.5 kHz.
For high speed search-tuning-algorithms, the AFC window can be widened to 240 kHz via bit AFW.
TDA8395 SECAM decoder
The TDA8395 is an alignment-free SECAM colour decoder, including a Cloche filter, demodulator and line
identification circuit.
The Cloche filter is a gyrator-capacitor type.
Its frequency is calibrated in the vertical retrace period.
The calibration reference( pin 1 ) is obtained from the TDA8375A color X-tal oscillator (pin 33).
Pin 7 is a decoupling for the Cloche reference.
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