Loewe Q2500B Serviceanleitung Seite 211

Technology of Q 2500 colour TV set
rier frequency the less pronounced the Moiré
becomes.
In order to keep interference with the picture
as low as possible, the phase position of the
colour carrier frequency from line to line is
shifted forward by 90°. In this way, bright and
dark pixels only overlap after four lines. At
normal distances the eye cannot detect this
interference.
This compensation is known as "Fourth Line
Offset Procedure".
This fourth line offset now permits the filtering
out of colour components from brightness in-
formation, and the reverse. The phase shift
from line to line causes a counter phasing of
the colour carrier every two lines.
If a two-line delay line is inserted into the cir-
cuit and the direct and delayed signals are
added together, the colour information is re-
moved and the brightness information is pre-
sent on the output at double the amplitude.
Document Q 2500
If one subtracts the two signals from one an-
other, the brightness information is eliminated
and the colour information is present at double
the amplitude on the output of the circuit.
CVBS
(PAL
signal)
1. Zeile
1
2. Zeile
1
3. Zeile
4. Zeile
Theoretically this circuit supplies error free
results. A condition for this, however, is the
phase and amplitude equality of the chroma
signal over 3 lines, which is only the case for
the same and equally saturated colours. In
practical terms picture joining can occur, in
which cross colour and cross luminance inter-
ference cannot be removed by the comb filter.
Therefore, another circuit is integrated, in
which three lines in sequence, from the point
of view of both phase and amplitude, are
compared.
If a deviation is detected that goes beyond a
specific difference, the comb filter function is
automatically switched off and the separation
of the two components is implemented tradi-
tionally with chroma trap and band pass.
88
2 H 2 H
delay delay
1 1
4 2
+
Chrominance
bandpass
Y
1 2 3
Chroma
1 2 3
© Loewe ProCollege
Y
+
-
1
4
C
f
4 5
f
MHz 5