Signal Path - Loewe Q2500B Serviceanleitung

Technology of Q 2500 colour TV set

4.2.14 Signal path

All FBAS or Y/C signals delivered by the re-
ceiving units, the three Euro/AV sockets of the
front interface, are directed via the video ma-
trix switch to the video processor VPC 3230.
The FBAS or Y signal is led from pin 17 of
I 1711, via the impedance converter Q 1814 to
pin 73 of the video processor I 2271. The filter
in the Y/FBAS path consisting of L 2259/57
and the associated capacitors has low pass
characteristics. Frequency components over 7
MHz are efficiently suppressed by the filter.
This prevents floating effects in the video sig-
nal, that may arise from the cycle rate of the
ADC and residual signals from neighbouring
channel. For Y/C operation the chroma signal
moves from pin 14 of I 1721, via the emitter
follower of Q 1824 to filter L/C 2266, which
suppresses noise components, and then to
chroma input pin 72 of 2271.
VPC 3230 is also equipped with two RGB in-
put interfaces.
Pins 1-3 of I 2271 are connected directly to
the Euro/AV socket 2 via the corresponding
input filter. The associated fast-blank signal is
applied to I 2271 on pin 79.
Q 2226 prevents the FB voltage exceeding
2.2 V and thereby damaging the videoproces-
sor. RGB operation via the Euro/Avc socket 2
is therefore also fast blank compatible. Y/U/V
operation is also possible from the second
RGB input from pin 4 of I 2271 , 5 and 6. The
RGB signals are fed from the optimum DVB
module or from the Euro/A socket 3 via this
input. The same goes for the Y/U/V signals of
the Euro/AV socket 3. The inputs 4, 5 and 6
from I 2271 are not fast blank compatible.
The video processor VPC 3230 digitises and
fully processes the FBAS analogue input sig-
Document Q 2500
nals or Y/C signal. On the output there is a
digital 8-bit wide luminance and an 8-bit wide
demodulated chroma signal with its U/V com-
ponents. The luminance and chroma signals
are transferred in multiplex procedures via
eight LM0-7 lines. The amplitude on the lines
is 3.3 Vss. The associated cycle frequency is
27 MHz and is output from pin 27 on I 2271.
In SAA 4979 the luminance and chroma sig-
nals are led to a demultiplexer and read into
the internal half picture memory. Selection
results with the 100 Hz cycle rate of 32 MHz.
With 3 Mbit, slightly more than a half picture
can be stored in intermediate memory, which
permits continuous single and dual selection.
The next IC in the signal path, SAA 4993, is
responsible for noise suppression and various
interpolations. Noise suppression is imple-
mented by signal from the output, which has
already been processed, being delayed for a
half picture by the second half picture memory
and then mixed with the currently applied sig-
nal.
In addition, this IC suppresses the intermedi-
ate line shimmer. Line interpolation also oc-
curs, so that 576 lines can be displayed in all
zoom functions. Movements between the pic-
tures are also interpolated, to ensure a con-
tinuous movement process over all half pic-
tures.
SAA 4979, which is the next IC in the signal
path, is used mainly as a DAC and for the
control of the digital 100 Hz video processing
in the circuit.
In the chroma branch there is a circuit for im-
proving colour transitions. It is used to in-
crease the slope of the colour transition flanks
in the differentiation and adder stages.
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