TDA8841

Manufacturer Part NumberTDA8841
DescriptionI2C-bus controlled PAL/NTSC TV processor
ManufacturerPhilips Semiconductors
TDA8841 datasheet
 


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Page 48/68

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Philips Semiconductors
2
I
C-bus controlled PAL/NTSC/SECAM TV
processors
47. Because of the 2-point black current stabilisation circuit both the black level and the amplitude of the RGB output
signals depend on the drive characteristic of the picture tube. The system checks whether the returning measuring
currents meet the requirement and adapts the output level and gain of the circuit when necessary. Therefore the
typical value of the black level and amplitude at the output are just given as an indication for the design of the RGB
output stage.
The 2-point black level system adapts the drive voltage for each cathode in such a way that the 2 measuring currents
have the right value. This has the consequence that a change in the gain of the output stage will be compensated
by a gain change of the RGB control circuit. Because different picture tubes may require different drive voltage
amplitudes the ratio between the output signal amplitude and the inserted measuring pulses can be adapted via the
2
I
C-bus. This is indicated in the parameter “Adjustment range of the ratio between the amplitudes of the RGB drive
voltage and the measuring pulses”.
Because of the dependence of the output signal amplitude on the application the soft clipping limiting has been
related to the input signal amplitude.
48. For the alignment of the picture tube the vertical scan can be stopped by means of the VSD bit. In that condition a
certain black level is inserted at the RGB outputs. The value of this level can be adjusted by means of the brightness
control DAC.
49. When the reproduction of 4:3 pictures on a 16:9 picture tube is realised by means of a reduction of the horizontal
scan amplitude the edges of the picture may slightly be disturbed. This effect can be prevented by adding an
additional blanking to the RGB signals. The blanking pulse is derived form the horizontal oscillator and is directly
related to the incoming video signal (independent of the flyback pulse). The additional blanking overlaps the normal
blanking signal with about 1 s on both sides. This blanking is activated with the HBL bit (only in the
TDA8843/44/47/54/57).
50. Signal-to-noise ratio (S/N) is specified as peak-to-peak signal with respect to RMS noise (bandwidth 5 MHz).
51. This is a current input.
52. The beam current limiting and the vertical guard function have been combined on this pin. The beam current limiting
function is active during the vertical scan period.
53. Via the “blue stretch” (BLS bit) or “extended blue stretch” (EBS bit) function the colour temperature of the bright
scenes (amplitudes which exceed a value of 80% of the nominal amplitude) can be increased. This effect is obtained
by decreasing the small signal gain of the red and green channel signals which exceed the 80% level. The effect is
illustrated in Fig.22.
December 16, 1997
Tentative Device Specification
TDA884X/5X-N2 series
48