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2. MICRO CONTROLLER AND SMALL SIGNAL FUNCTION WITH TDA 9351:-
This IC is a
combination of microcontroller and chroma (Small Signal function). This IC performs all important
functions in TV receiver. The details of IC are bifurcated in two parts. Small Signal function and
microcontroller
2A:-
SMALL SIGNAL FUNCTION
:
2A.1: VISION IF AMPLIFIER:
The vision IF amplifier can demodulate signals with positive and
negative modulation. The PLL demodulator is completely alignment-free. The VCO of the PLL
circuit is internal, and the frequency is fixed to the required value by using the clock frequency
of the Controller/ Teletext decoder as a reference. The setting of the various frequencies (38,
38.9, 45.75 and 58.75 MHz) can be made via the control bits IFA-IFC in sub address 27H.
Because of the internal VCO, the IF circuit has a high immunity to EMC interferences.
2A.2: VIDEO SWITCHES:
The video switch has one input for an external CVBS or Y/C signal.
The selected CVBS signal can be supplied to pin 38, the IF video output. The selection
between both signals is realized by means of the SVO bit in sub address 22H. The video ident
circuit can be connected to the incoming ‘internal’ video signal or to the selected signal. This
ident circuit is independent of the synchronisation and can be used to switch the time-constant
of the horizontal PLL depending on the presence of a video signal (via the VID bit). Because of
the availability of the Y/C input and the subcarrier output an external comb-filter can be applied.
In that case an external video switch (or comb-filter with integrated switch) must be used. The
subcarrier output is combined with a 3-level output switch (0 V, 4 V and 8 V). The output level
and the availability of the subcarrier signal is controlled by the CMB1 and CMB0 bits. The
output can be used to switch sound traps etc.
2A.3: SOUND CIRCUIT (QSS VERSION)
The sound IF amplifier is similar to the vision IF amplifier and has an external AGC decoupling
capacitor. The single reference QSS mixer is realised by a multiplier. In this multiplier the SIF
signal is converted to the intercarrier frequency by mixing it with the regenerated picture carrier
from the VCO. The mixer output signal is supplied to the output via a high-pass filter for
