PRELIMINARY DATA SHEET
VDP 31xxB
2.8. Display Part
2.8.2. Black Level Expander
The black level expander enhances the contrast of the
picture. Therefore the luminance signal is modified with
an adjustable, non-linear function. Dark areas of the pic-
ture are changed to black, while bright areas remain un-
changed. The advantage of this black level expander is
that the black expansion is performed only if it will be
most noticeable to the viewer.
In the display part the conversion from digital YC C to
r
b
analog RGB is carried out. A block diagram is shown in
Figure 2–20. In the luminance processing path, contrast
and brightness adjustments and a variety of features,
such as black level expansion, dynamic peaking and
soft limiting, are provided. In the chrominance path, the
C C signals are converted to 20.25 MHz sampling rate
r
b
and filtered by a color transient improvement circuit. The
YC C signals are converted by a programmable matrix
to RGB color space.
The black level expander works adaptively. Depending
r
b
on the measured amplitudes ‘L ’ and ‘L
’ of the low-
min
max
pass-filtered luminance and an adjustable coefficient
BTLT, a tilt point ‘L ’ is established by
t
The display processor provides separate control set-
tings for two pictures, i.e. different coefficients for a
‘main’ and a ‘side’ picture.
L = L
+ BTLT (L
– L ).
max min
t
min
Above this value there is no expansion, while all lumi-
nance values below this point are expanded according
to:
The digital OSD insertion circuit allows the insertion of
a 5-bit OSD signal. The color space for this signal is con-
trolled by a partially programmable color look-up table
(CLUT) and contrast adjustment.
L
out
= L + BAM (L – L )
in in t
TheOSDsignalsandthedisplayclockaresynchronized
to the horizontal flyback. For the display clock, a gate
delay phase shifter is used. In the analog backend, three
10-bit digital-to-analog converters provide the analog
output signals.
A second threshold, L , can be programmed, above
tr
which there is no expansion. The characteristics of the
black level expander are shown in Fig. 2–13 and Fig.
2–14.
The tilt point L is a function of the dynamic range of the
t
video signal. Thus, the black level expansion is only per-
formed when the video signal has a large dynamic
range. Otherwise, theexpansiontoblackiszero. Thisal-
lows the correction of the characteristics of the picture
tube.
2.8.1. Luma Contrast Adjustment
The contrast of the luminance signal can be adjusted by
multiplication with a 6-bit contrast value. The contrast
value corresponds to a gain factor from 0 to 2, where the
value 32 is equivalent to a gain of 1. The contrast can be
adjusted separately for main picture and side picture.
L
max
a)
L
t
L
min
Lmax
Lout
Ltr
b)
Lt
BAM
BTLT
L
t
Lmin
Ltr
BTHR
Lin
Fig. 2–14: Black-level-expansion
a) luminance input
b) luminance input and output
Fig. 2–13: Characteristics of the black level expander
Micronas
15
MICRONAS [ MICRONAS ]
