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2-3-2. AGC Circuit
This circuit controls the gain so that the output level of the
AGC amplifier becomes constant. The AGC circuit amplifies
the gain by approximately 1 8 dB under the minimum
illumination condition. The AGC level, determined by the
reference voltage of Pin @ , is adjusted by the voltage at
Pin @ of IC91 1 (MSM6524GS}, and is then AE corrected by
the PWM output of Pin @ of IC932 (S87C51 FC).
2-3-3. High Luminance Detection Circuit
In the high luminance detection circuit, a signal in which the
sample and hold output by XSHD has been sliced is added
to a signal in which the maximum value of F1 , F2 and F3
(mentioned later) has been sliced for the light amount at
which the most sensitive colour filter of CCD is saturated ,
and then the signal is fed from Pin ®> to IC91 4 (CXA 1 391 R)
as the CS signal. This signal eliminates the green colouring
phenomenon which occurs when the signals are saturated
and the linearity in the chroma modulation becomes
deficient.
2-3-4. Colour Separation Sample and Hold Circuit
The AGC amplifier output is input to the colour separation
sample and hold circuit. Since XSP1 and XSP2 are 7.2 MHz
sampling pulses with a phase difference of 1 80 ° , contin
uous F1 and F2 signals in a cycle of 2 picture elements can
be extracted. Fig. 2 - 7. shows this relationship. Fl and F2
are chroma signals with G+Cy (Mg+Cy), Mg+Ye (G+Ye)
components respectively. ( ) shows the 1 H delayed signal.
After passing through the LPF to eliminate the excess high
frequency components, F1 and F2 are output from Pins @
and @ via clamp and blanking circuits.
On the other hand, the XSP3 (Pin @ ) of IC91 7
(CXA 1 390AR) is earthed and the AGC output is passed
through the LPF, clamp and pre-blanking circuits to Pin @ .
The excessive high frequency component and the 7.2 MHz
component are eliminated by T91 90 and T91 01 as the F3
signal ,
and
the output is fed to
(CXA 1 391 AR) as the high frequency luminance signal.
G + Cy
AGC
output
(Mg + Cy)
1
XSP
XSP 2
Fig. 2-7. Separation Sample and Hold
Pin @ of IC91 4
Mg + Ye
G + Cy
Mg + Ye
(G + Ye)
(Mg + Cy)
(G + Ye)
2-4. IC91 3, IC91 4 AND IC91 9 (Fig. 2-8)
2-4-1 .Chroma Synchronisation
The chroma separated outputs F1 and F2 are fed through
Pins @ and ® of IC91 4 (CXA1 391 R). As mentioned in
2-1 -3, the difference between F1 and F2 becomes a signal
(Co) in which (2R - G) and (G - 2B} appear alternately every
two lines. Co is fed from Pin @ to Pin @> of IC91 3
(CXL1 51 7N). IC91 3 (CXL1 51 7N) contains three channels of
the narrow band 1 H delay line driven by two clocks (XDL 1
and XDL2 of 7.2 MHz) which are phase shifted by 1 80 ° .
Signal C1 created by the 1 H delayed Co is again input to
Pin @ of IC91 4 (CXA 1 391 R}. Co and C, are synchronised
by the M PX after passing through the LPF to generate
continuous 2R-G and G - 2B signals. (See Fig. 2-9.)
The level of the C signal is adjusted by the voltage of IC91 1
(MSM6524GS) (Pin ® ) so that the continuous 2R - G and
G - 2B signals have no level difference for every 1 H .
O n the other hand, after F1 and F2 are added, the signal
passes through the clamping circuit and LPF and is used as
the narrow band luminance signal Yo. Yo is used for the
RGB matrix and V aperture circuits.
2-4-2. V Aperture Circuit
In order to correlate the three lines by delaying Yo by a total
of 2H, Yo is passed through Pin ® (Yo), IC91 3 (Pin ® to Pin
@ } , Pin @ (Y1 } , GC and clamping circuit, Pin @ , IC91 3
(Pin ® to Pin ® }, Pin ® , GC, clamping circuit and LPF.
Yo and Y1 signals pass through LPF, KNEE (white
compression}, 3H aperture control, LPF, clamping circuit,
slicer (noise reduction) and GC, resulting in the generation
of a V aperture signal (V AP) .
The 3H aperture control circuit calculates Y1 - (Yo+Y2}/2 as
shown in Fig . 2-1 0. The VAP is fed from Pin ® of IC91 4
(CXA 1 391 R) to Pin @ of IC91 2 (CXA 1 392R}.
In order to eliminate the quasi signal generated at the
vertical edge section, the VAP is added to the high
luminance
detect
(CXA1 390AR) after passing through the absolute value
circuit (ABS), and is then fed from Pin @ to IC91 2
(CXA 1 392R) as the chroma suppression signal (CSY).
- 4-1 4 -
signal
(CS)
created
by
IC91 7
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