Daewoo DTR-14D3VG, Service Manual

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38
• EHT tr overvoltage protection
Both the vertical and the E-W drive can be modulated for EHT compensation.
This tracking makes the picture size independent of EHT variations due to the beam current. The compensation
range is -5 to +5%. The horizontal tracking can be switched off by I
2 C bus function HCO(Horiz. Compensation).
A second function of this pin is for overvoltage protection, XPR(X-ray PRotection). XPR is set to “one” when the
voltage on the pin exceeds 3.9 Volts and can be read by I
2 C bus. It is possible to switch the horizontal output
automatically off via slow stop for XPR = 1 when PRD (PRotection Detection mode) is set to 1.
Filters and CVBS/Y/C switches
See also the related block diagram as well as the diagrams at the end of the report.
The main functions are :
- CVBS
INT
, CVBS
EXT
, CVBS/Y and Chroma signal selection.
- Filter calibration.
- Chrominance signal processing.
- Luminance signal processing.
• CVBS
INT
, CVBS
EXT
, CVBS/Y and Chroma signal selection
The input selector has CVBS
INT
, CVBS
EXT
, CVBS/Y and Chroma as inputs which can be selected via the I 2 C
bus(INA, INB, INC)
For the TDA884X/5X devices, the selected video signal, CVBS1 out(2Vpp), is present at pin 38.
When Y/C is selected then the Y and C signals are added to form a CVBS signal and then supplied to the CVBS1
output.
• Filter calibration
The filter calibration loop is an auto-tuning loop which calibrates every field retrace. The loop is stabilised when the
resonant frequency of the cloche filter is Fsc(Fsc = VCXO reference signal which is at 4.4MHz or 3.6MHz
depending upon which Xtal is selected).
The chroma bandpass and chroma trap filters are also controlled to Fsc. The chroma bandpass centre frequency
can be set to 1.1Fsc via I
2 C bus command CB.
For SECAM reception the cloche resonant frequency is set to 4.28MHz and the chroma trap is shifted to 4.3MHz to
ensure optimal subcarrier rejection.
• Chrominance signal processing
For chroma signal processing, the selected signal is supplied to both the PAL/NTSC chroma bandpass filter and the
SECAM colche filter via a variable gain amplifier which is controlled by ACC and ACL detection circuits.
The dynamic range of the ACC is 26dB and detects only the burst amplitude; consequently the burst signal at the
bandpass/cloche filter input is constant for a burst signal range +6dB–> -20dB where 0dB = 300mV
PP
burst.
The ACL is a chroma amplitude detector and is active when the chroma/burst ratio exceeds approximately 3. It
ensures that CVBS signal to chroma bandpass & cloche filter is limited for large chroma/burst ratios(>3). which
results in a constant saturation for such non-standard transmissions.
The ACL is independent of the ACC; it controls only the chroma amplitude and does not influence the colour burst
sensitivity. The ACL function can be switched on/off via bus command ACL.
The output signal of the chroma bandpass circuit is supplied to the PAL/NTSC decoder and the output signal of the
cloche filter is supplied to the SECAM decoder for further chroma processing.