Daewoo DTC-14D9T Service Manual page 29

FUNCTIONAL DESCRIPTION
APPENDIX
RGB output circuit and black-current stabilization
In the RGB control circuit the signal is controlled on contrast, brightness and saturation. The ICs have a linear input for
external RGB signals. The signals for OSD and text are internally supplied to the control circuit. The output signal has an
amplitude of about 2 Volts black-to-white at nominal input signals and nominal settings of the various controls.
To obtain an accurate biasing of the picture tube the 'Continuous Cathode Calibration' system has been included in these
ICs. A black level off set can be made with respect to the level which is generated by the black current stabilisation
system. In this way different colour temperatures can be obtained for the bright and the dark part of the picture.
The black current stabilisation system checks the output level of the 3 channels and indicates whether the black level of
the highest output is in a certain window or below or above this window. This indication is read from the status byte 01
and is used for automatic adjustment of the Vg2d voltage during the production of the TV receiver.
During switch-off of the TV receiver a fixed beam current is generated by the black current control circuit. This current
ensures that the picture tube capacitance is discharged. During the switch-off period the vertical deflection is placed in an
overscan position so that the discharge is not visible on the screen.
2. IF
The TDA9346/TDA9341 has an alignment free IF PLL demodulator. The fully integrated oscillator is automatically
calibrated, using the 12 MHz crystal as a frequency reference. The IF frequency is simply set in TV-Processor by I2C bus.
The AFC information is available via I2C bus from the TV-Processor status bytes. The controlling software uses this
information for tuner frequency tracking ( automatic following ). The AFC window is typically 125Khz wide. The minimum
frequency step of the tuner is 62.5 kHz.
This AFC function is disabled when a program is tuned using the direct frequency entry or after fine tuning adjustment.
Therefore it is recommended to tune a channel with the TV search function ( manual or ATSS ) or by using the direct
channel entry to enable the Automatic Frequency Control.
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