Loewe Q2500B Serviceanleitung Seite 209

Technology of Q 2500 colour TV set
72
Chroma
73
CVBS/Y
74
AV3/Synch on Y
75
H-Synch/V GA
The chroma signal is clamped to its middle
value. The following automatic amplifier con-
trol in the FBAS/Y path has a control range of
-4.5 dB to +6 dB and ensures that even for
different input levels the FBAS/Y signal is al-
ways optimally adapted to the ADC. In the
chroma branch a fixed amplification is suffi-
cient.
The FBAS/Y signal, as also the chroma signal
for Y/C operation are digitised at a cycle rate
of 20.25 MHz. The resolution of the two ADCs
is 8-bit. Therefore, at the output of the input
interface there is an 8-bit wide FBAS or Y data
stream and for Y/C operation also an 8 bit
wide chroma data stream. Both ADCs need a
corresponding reference value. This is pro-
Document Q 2500
Block diagram, VPC 3230 input interface
Clamp
Input
multiplexer
Clamp
I 2271
VPC 3230
Reference
generation
78
AGC
+6/-4,5 dB
Amplification
Frequency
C 473/489
duced internally in the reference generation
and stored externally on pin 78 by C 488/489.
4.3.2 20.25 MHz clock generator
The frequency determining 20.25 MHz Quarz
X 2283 is connected to pin 62/63 of VPC
3230. The clock generator is controlled from
the synchronisation block. In normal operation
the clock generator is line frequency coupled.
In the VCR mode, the line frequency coupling
is switched off.
86
ADC
ADC
DVCO
150
ppm
63
59 62
20,25 MHz
U5SB
© Loewe ProCollege
Digital CVBS
or Y signal
digital Chroma
System Clock