OM8373 datasheet(85/108 Pages) PHILIPS

85 / 108 page

2004 Jan 15

Philips Semiconductors

Tentative Device Speciﬁcation

TV signal processor-Teletext decoder with

embedded

µ-Controller

OM8373; OM8378

44. The vertical blanking pulse in the RGB outputs has a width of 27 or 22 lines (50 or 60 Hz system). The vertical pulse

in the sandcastle pulse has a width of 14 or 9.5 lines (50 or 60 Hz system). This to prevent a phase distortion on top

of the picture due to a timing modulation of the incoming flyback pulse.

45. 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 25H.

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.

46. Conditions: frequency is 50 Hz; normal mode; VS = 1F.

47. The output range percentages mentioned for E-W control parameters are based on the assumption that 400

µA

variation in E-W output current is equivalent to 20% variation in picture width.

48. The ICs have a zoom adjustment possibility for the horizontal and vertical deflection. For this reason an extra DAC

has been added in the vertical amplitude control which controls the vertical scan amplitude between 0.75 and 1.38

of the nominal scan. At an amplitude of 1.06 of the nominal scan the output current is limited and the blanking of the

RGB outputs is activated. This is illustrated in Fig. 36.

When the vertical amplitude is compressed (zoom factor <1) it is still possible to display the black-current measuring

lines in the vertical overscan. The feature is activated by means of the OSVE-bit in sub-address 26H. Because the

vertical deflection output stage needs some time for the excursion from the top of the picture to the required position

on the screen the vertical blanking is increased when the OSVE-bit is activated. The shape of the vertical deflection

current for a zoom factor of 0.75 with OSVE activated is given in Fig. 38. The exact timing of the measuring pulses

and vertical blanking for the various conditions is given in Fig. 40.

The nominal scan height must be adjusted at a position of 19 HEX of the vertical ‘zoom’ DAC.

49. At a chrominance input voltage of 660 mV (p-p) (colour bar with 75% saturation i.e. burst signal amplitude

300 mV (p-p)) the dynamic range of the ACC is +6 and

−20 dB.

50. The ACL function can be activated by via the ACL bit in the subaddress 20H. The ACL circuit reduces the gain of the

chroma amplifier for input signals with a chroma-to-burst ratio which exceeds a value of 3.0.

OM8373 Datasheet(PDF) 85 Page - NXP Semiconductors

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