Band Width Doubling; Colour Flank Sharpening; Y Signal Path - Loewe Q2500B Serviceanleitung

Technology of Q 2500 colour TV set
Owing to the narrow bandwidth the rise times
for colour change are very long, which ap-
pears on the screen as "washed out" transi-
tions.
a)
Uin,
Vin
b)
Ampl.
c)
Ampl.
d)
Uout,
Vout
a) U, V input of SAA
b) U, V Correction signal
c) U, V input + Correction signal
d) sharpend and limited U, V
To compensate for this deficiency, two circuit
components are incorporated into the SAA
4979, with which short rise times can be
achieved artificially, thereby improving the
colour transitions.
In the circuit before these components is a
demultiplexer, with which 8-bit wide parallel R-
Y- and B-Y signals respectively are derived
from the supplied signals.
Document Q 2500

4.6.2 Band width doubling

In order to increase the effectiveness of the
colour flank increase on the supply side, a
doubling of the bandwidth from 4:1:1 to 4:2:2
is implemented.
Using a linear phase interpolation filter addi-
tional sampling values are calculated from the
available chroma sampling values.
These calculated sampling values are inserted
respectively between two available values, by
which means the bandwidth is doubled. In this
t
way, steep flanks can be transmitted more
easily.
t

4.6.3 Colour flank sharpening

An improvement of the colour transition is
achieved by a dual differentiation of the origi-
nal chroma signal and the ensuing subtraction
of the derived correction signal of the chroma
signal.
The changes in these signals are evaluated in
separate circuits for U and V, and if a thresh-
t
old value specified by the software is ex-
ceeded a correction value is calculated. After
the two derived correction signals have been
cleaned of noise components, they are sub-
tracted from the available chroma signals U
and V. The subtraction is implemented by in-
version of the correction signals and subse-
t
quent addition with the original signal.
To avoid colour phase errors by the over-
shooter, it must be blocked by limiter stages.
Here analogue signals are produced from the
respective 8-bit wide digital R-Y and B-Y sig-
nals, which are then output at 1.8 Vss on pins
46 and 48.

4.6.4 Y signal path

The peaking circuit is not used. The signal is
led directly to the sampling stage. This oper-
ates in the same way as the sampling stage in
the chroma branch.
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