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Model No.: 14LAANZACDC
Version: 1.0
To correctly process chroma signals, firstly identify the input chroma signals. The
I
2
C bus controls LA76818A to identify chroma signals. To identify chroma signals,
identify signal systems and subcarrier frequency.
For NTSC signals, the subcarrier modulated by two color difference signals will not
be progressively out-phased while the subcarrier modulated by R-Y color different
signal PAL will be progressively out-phased. So whether the subcarrier phase
modulated by R-Y signal or phase of color sync signal is out-phased or not decides
that the signal is in NTSC or PAL system.
Both NTSC and PAL signal have a dual-frequency subcarrier, I.E. 4.43MHz and
3.58MHz. After detecting out two subcarrier frequencies at the same time of
detecting subcarrier phase, the corresponding 4.43MHz or 3.58MHz can be
identified (PAL4.43 and NTSC3.58 are standard systems while PAL3.58 and
NTSC4.43 are nonstandard).
The subcarrier restorer in LA76818A uses a 4.43MHz crystal (G201) externally
connected to N101’s Pin38 as an oscillating source to generate a 3.58MHz or
4.43MHz dual-frequency subcarrier.
VC01 is a crystal oscillator with high Q value and narrow oscillating range. VC02
is an inner oscillator with wide oscillating range. Firstly, oscillating signals
generated from VC02 and VC01 are discriminated in APC2, then the generated
error signal is filtered by the filtered by the filtering circuit externally connected to
LA76818A’s Pin36 to get consistent oscillating frequencies of VC02 and VC01.
Secondly, the oscillating signals are sent to APC1 by the tint adjuster to be
discriminated together with the chroma signal from ACC. The error signal
generated from the phase discriminator is filtered by the circuit externally
connected to Pin39 to control oscillating frequency of VC02 so that it is consistent
with chroma subcarrier frequency from ACC. Taking the calibrated subcarrier
regeneration signal as reference, further calibrate oscillating frequency of VC02 in
the APC2 circuit. Only after the APC2 circuit discriminates twice, can it correctly
calibrate subcarrier regeneration signal of one frequency (firstly calibrate subcarrier
regeneration signal on oscillating frequency of VC01; secondly recalibrate on
subcarrier frequency of received chroma signal through VC01).
The calibrated sub-carrier regeneration signal is sent into the decoder through the
PAL switch to demodulate out R-Y and B-Y color difference signals together with
the modulated signal from ACC.
