In the mono inter -carrier sound versions the Automatic Volume Leveling (AVL) function can be
activated. The pin to which the external capacitor has to be connected depends on the IC version.
2.4 Video switching
The video processor (STV2248C) has three CVBS inputs and two RGB inputs. The first CVBS
input is used for external CVBS from SCART 1, the second is used for either CVBS or Y/C from
either SCART2 or BAV/FAV or SVHS, and the third one is used for internal video. The selection
between both external video inputs signals is realized by means of software and hardware switches.
2.5 Synchronization circuit
The video processor (STV224X) performs the horizontal and vertical processing. The external
horizontal deflection circuit is controlled via the Horizontal output pulse (HOUT). The vertical
scanning is performed through an external ramp generator and a vertical power amplifier IC
controlled by the Vertical output pulse (VOUT).
The main components of the deflection c ircuit are:
? PLL1: the first phase locked loop that locks the internal line frequency reference on the
CVBS input signal. It is composed of an integrated VCO (12 MHz) that requires the chroma
reference frequency (4.43MHz or 3.58MHz crystal oscillator refe rence signal), a divider by
768, a line decoder, and a phase comparator.
? PLL2: The second phase locked loop that controls the phase of the horizontal output
(Compensation of horizontal deflection transistor storage time variation). Also the horizontal
position adjustment is also performed in PLL2.
? A vertical pulse extractor.
? A vertical countdown system to generate all vertical windows (vertical synchronization
window, frame blanking pulses, 50/60Hz identification window...).
? Automatic identification of 50/60Hz scanning.
? PLL1 time constant control.
? Noise detector, video identification circuits, and horizontal coincidence detector.
? Vertical output stage including de-interlace function, vertical position control.
? Vertical amplitude control voltage output (combined with chroma reference output and
Xtal 1 indication).
2.6 Chroma and luminance processing:
The chroma decoder is able to demodulate PAL, NTSC and SECAM signals.
The decoder dedicated to P AL and NTSC sub-carrier is based on a synchronous demodulator,
and an Xtal PLL locked on the phase reference signal (burst).
The SECAM demodulation is based on a PLL with automatic calibration loop.
The color standard identification is based on the burst recognition.
2
Automatic and forced modes can be selected through the I C bus.
2
NTSC tint, and auto flesh are controlled through I C bus.
Xtal PLL can handle up to 3 crystals to work in PAL M, PAL N and NTSC M for South America.
ACC an ACC overload control the chroma sub-carrier amplitude within 26dB range. Both
ACC s are based on digital systems and do not need external capacitor.
All chroma filters are fully integrated and tuned via a PLL locked on Xtal VCO signal.
A second PLL is used for accurate fine -tuning of the SECAM bell filter. This tuning is achieved
during the frame blanking. An external capacitor memorizes the bell filter tuning voltage.
A base-band chroma delay-line rebuilds the missing color line in SECAM and removes transmission
phase errors in PAL.
The base-band chroma delay line is clocked with 6MHz signal provided by the horizontal scanning
VCO.
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