ST92195D datasheet(148/249 Pages) STMICROELECTRONICS

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ST92195C/D - TELETEXT SLICER AND ACQUISITION UNIT

TELETEXT SLICER AND ACQUISITION UNIT (Cont’d)

7.7.3 Teletext Slicer

The CVBS1 input voltage is 1Vpp +/- 6dB (i.e.

from 0.5Vpp to 2.0Vpp). The CVBS1 pin is AC

driven through a 470 nF capacitor.

Note: An indicator of Teletext Slicer quality criteri-

on is to have data error-free (or less than 3 errors

per page) for an eye height reduction down to

30%.

7.7.3.1 CVBS Amplitude Control

A video pre-amplifier treats the CVBS internal sig-

nal. Software should adjust the signal amplitude

for the most effective working range of the Data

Slicer while keeping an optimized signal/noise ra-

tio. The pre-amplifier gain is programmed through

two bits in the TXSCR register.

The composite video signal enters the Sync Ex-

tractor through the CVBS1 pin. The sync pulse’s

peak value (with reference to black level) is stored.

This signal's pulse amplitude (VDETB) is then fed

to channel 0 of the A/D converter. The conversion

result can then be used for programming the best

gain value of the video pre-amplifier. See Figure

87.

The theoretical gain calculation is the following:

– Assume A

text is the effective peak amplitude of

the Teletext data within the video signal (this

peak amplitude being referred to the black level

clamp),

– The effective working range of the Data Slicer is

equal to 1.2V

– Theoretical gain= 1.2/ A

text

Figure 88. CVBS1 Functional Description

VR02109D

TXTGC(1:0)

VDETB

CVBS1

G= -5

ADCSEL

PREAMP

VBGP

(digital part)

TO SYNC EXTRACTOR

TO TELETEXT SLICER

TO ADC

TO VPS SLICER

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