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ST92195 ST92T195 ST92E195 - SYNC EXTRACTOR
7.6 SYNC EXTRACTOR
The Sync Extractor extracts the Sync pulses from
the CVBS signal and determines the CVBS
HSYNC and VSYNC signals. The CVBS HSYNC
signal is fed to the VPS/WSS slicer and to the
Teletext Slicing Unit to re-synchronize them at
each TV line.
The Sync Extractor Unit provides the CVBS
VSYNC pulse to the Csync Controller unit which
might use it for some standalone modes when the
chassis time base is switched off (refer to the Sync
Controller chapter for more details).
7.6.1 Time Windowing For Slicers
A line counter counts the CVBS HSYNC pulses in
order to open and close the proper windows within
each field for slicing the Teletext (lines 6 to 22 for
the first field, lines 318 to 335 for the second field),
VPS (lines 15 to 17) or WSS (lines 22 to 24) data.
The line counter is able to count a complete field
before overflowing.
An extended Teletext Search window can be pro-
grammed by software through the TXTES bit of
the Teletext Slicer Control register (TXSCR) (refer
to the Teletext Slicing Unit chapter). The extended
window starts at line 2 (first field) or line 315 (sec-
ond field).
7.6.2 Field Detection
To secure the slicing units functionality, a field de-
tection is implemented. This field detection flag is
updated at each beginning of field. In case a field
start is missed, the field detection flag will keep
toggling according to its previous value and the
end of field predicted by the line counter (312 or
313 lines).
7.6.3 CVBS Amplitude Information
The Sync Extractor unit provides information
about the real time amplitude of the CVBS signal.
This information is obtained by storing the peak
value of the sync pulse referred to the black level.
This analog voltage is bufferized and forwarded to
the on-chip A/D Converter.
7.6.4 CVBS Signal Quality
The quality of the CVBS Sync pulses is also
checked by the Sync Extractor according to the
following criteria: a TV line is declared “bad” if two
or more HSYNC pulses are found within 64 s, or
if the HSYNC pulse is missing. Each time a “bad”
TV line is detected, a “Bad TV line” pulse is for-
warded to the Acquisition which is part of the Tele-
text Slicing Unit. These pulses are counted only
during the Vertical Blank Interval, providing a qual-
ity criterion of the CVBS signal at the time the slic-
ers treat it. For more details on the subject, refer to
the Teletext Slicing.
7.6.5 Sync Extractor Switch Off
When the VPS/WSS and Teletext slicers are
switched off, the Sync Extractor automatically en-
ters an idle state to prevent current consumption.
Figure 86. Time Windowing for Slicers
n
CVBS1
SYNC Extractor
Analog
Front-End
Digital
Filtering
Hcvbs (to Slicers, Sync. Controller, VPS/WSS PLL.)
Vcvbs (to Slicers, Sync. Controller)
Bad TV Line (to Acquisition Unit)
Teletext Extended Search Window
Teletext Search Window
VPS Search Window
VSS Search Window
VDETB/VBGP (to A/D Converter, Channel 0)