TDA8841

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


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Philips Semiconductors
2
I
C-bus controlled PAL/NTSC/SECAM TV
processors
33. During switch-on the horizontal output starts with the double frequency and with a duty cycle of 75% (VOUT = high).
After about 50 ms the frequency is changed to the normal value. Because of the high frequency the peak currents in
the horizontal output transistor are limited. Also during switch-off the frequency is switched to the double value and
the RGB drive is set to maximum so that the EHT capacitor is discharged. The switching to maximum drive occurs
only when RBL=0, for RBL=1 the drive voltage remains minimum during switch-off. After about 100 ms the RGB drive
is set to minimum and 50 ms later the horizontal drive is switched-off.
It is possible to discharge the EHT capacitor in the vertical overscan so that the screen stays black during switch-off.
This feature is activated by the OSO bit. In this condition the vertical scan is stopped and the current is set to the
maximum scan value. It should be checked with the picture tube supplier whether this mode of switch-off is allowed
for the given picture tube.
The horizontal output is gated with the flyback pulse so that the horizontal output transistor cannot be switched-on
during the flyback time.
34. The vertical blanking pulse in the RGB outputs has a width of 26 or 21 lines (50 or 60 Hz system). The vertical pulse
in the sandcastle pulse has a width of 14 lines. This to prevent a phase distortion on top of the picture due to a timing
modulation of the incoming flyback pulse.
35. The timing pulses for the vertical ramp generator are obtained from the horizontal oscillator via a divider circuit.
During TV reception this divider circuit has 3 modes of operation:
a) Search mode ‘large window’.
This mode is switched on when the circuit is not synchronized or when a non-standard signal (number of lines
per frame outside the range between 311 and 314(50 Hz mode) or between 261 and 264 (60 Hz mode) is
received). In the search mode the divider can be triggered between line 244 and line 361 (approximately
45 to 64.5 Hz).
b) Standard mode ‘narrow window’.
This mode is switched on when more than 15 succeeding vertical sync pulses are detected in the narrow window.
When the circuit is in the standard mode and a vertical sync pulse is missing the retrace of the vertical ramp
generator is started at the end of the window. Consequently, the disturbance of the picture is very small. The
circuit will switch back to the search window when, for 6 successive vertical periods, no sync pulses are found
within the window.
c) Standard TV-norm (divider ratio 525 (60 Hz) or 625 (50 Hz).
When the system is switched to the narrow window it is checked whether the incoming vertical sync pulses are
in accordance with the TV-norm. When 15 standard TV-norm pulses are counted the divider system is switched
to the standard divider ratio mode. In this mode the divider is always reset at the standard value even if the vertical
sync pulse is missing.
When 3 vertical sync pulses are missed the system switches back to the narrow window and when also in this
window no sync pulses are found (condition 3 missing pulses) the system switches over to the search window.
The vertical divider needs some waiting time during channel-switching of the tuner. When a fast reaction of the
divider is required during channel-switching the system can be forced to the search window by means of the NCIN bit
in subaddress 08.
When RGB signals are inserted the maximum vertical frequency is increased to 72 Hz. This has the consequence
that the circuit can also be synchronised by signals with a higher vertical frequency like VGA.
36. Conditions: frequency is 50 Hz; normal mode; VS = 1F.
37. During switch-off the RGB drive outputs are shortly set to maximum so that the EHT capacitor of the picture tube can
be discharged. The switch-off behaviour depends on the switch-off mode. When the vertical scan is set in the
overscan during switch-off the RGB outputs are first set to minimum during 20 ms, then the drive is set to maximum
during 80 ms followed by a period of 60 ms with the drive on minimum. When the vertical scan is active during
switch-off the RGB drive voltages are directly set to maximum when the switch-off command is given. The output
remain high during a period of 100 ms followed by a period of 60 ms with the drive on minimum.
38. The output range percentages mentioned for E-W control parameters are based on the assumption that 400 A
variation in E-W output current is equivalent to 20% variation in picture width.
December 16, 1997
Tentative Device Specification
TDA884X/5X-N2 series
46