NEC DTE-25G5THS Service Manual page 34

Service Manual CP-785A
the desired free-running frequency and the bandgap reference to obtain the correct absolute value of the
output signal. The VCO of the PLL is calibrated during each vertical blanking period, when the IC is in
search or SECAM mode.
The base-band delay line (TDA 4665 function) is integrated. This delay line is also active during NTSC
to obtain a good suppression of cross colour effects. The demodulated colour difference signals are
internally supplied to the delay line.
RGB output circuit and black-current stabilisation
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 Vg2 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.
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