ADVANCE INFORMATION
VCT 38xxA
2.10.4.Digital Brightness Adjustment
Part 1 includes adjustable tilt point and gain. The gain
before the tilt value is 1. Above the tilt value, a part
(0...15/16) of the input signal is subtracted from the
input signal itself. Therefore, the gain is adjustable
from 16/16 to 1/16, when the slope value varies from
0 to 15. The tilt value can be adjusted from 0 to 511.
The DC-level of the luminance signal can be adjusted
by adding an 8-bit number in the luminance signal path
in front of the softlimiter.
With a contrast adjustment of 32 (gain+1) the signal
can be shifted by 100 %. After the brightness addition,
the negative going signals are limited to zero. It is
desirable to keep a small positive offset with the signal
to prevent undershoots produced by the peaking from
being cut. The digital brightness adjustment works
separately for main and side picture.
Part 2 has the same characteristics as part 1. The sub-
tracting part is also relative to the input signal, so the
total differential gain will become negative if the sum of
slope 1 and slope 2 is greater than 16 and the input
signal is above the both tilt values (see characteris-
tics).
Finally, the output signal of the soft limiter will be
clipped by a hard limiter adjustable from 256 to 511.
2.10.5.Soft Limiter
The dynamic range of the processed luma signal must
be limited to prevent the CRT from overload. An appro-
priate headroom for contrast, peaking and brightness
can be adjusted by the TV manufacturer according to
the CRT characteristics. All signals above this limit will
be ‘soft’-clipped. A characteristic diagram of the soft
limiter is shown in Fig. 2–15. The total limiter consists
of three parts:
2.10.6.Chroma Interpolation
A linear phase interpolator is used to convert the
chroma sampling rate from 10.125 MHz (4:2:2) to
20.25 MHz (4:4:4). All further processing is carried out
at the full sampling rate.
Output
511
Part 1
Part 2
Hard limiter
Calculation Example for the
0
Softlimiter Input Amplitude.
(The real signal processing in
the limiter is 2 bit more than
described here)
slope 1
[0...15]
2
4
6
0
2
4
400
300
200
100
0
8
10
12
Y Input
16...235 (ITUR)
range=
256...511
Black Level
Contrast
Dig. Brightness
BLE
16 (constant)
6
8
10
63
20
off
off
14
slope 2
[0...15]
12
14
Peaking
Limiter input signal:
(Yin-Black Level) Contr./32 + Brightn.
(235-16) 63/32 + 20 = 451
tilt 1 [ 0...511]
tilt 2 [ 0...511]
Limiter Input
0
100
200
300
400
500
600
700
800
900
1023
Fig. 2–15: Characteristic of soft limiter A and B and hard limiter
2.10.7.Chroma Transient Improvement
sharpened chroma signals are limited to a proper
value automatically.
The intention of this block is to enhance the chroma
resolution. A correction signal is calculated by differen-
tiation of the color difference signals. The differentia-
tion can be selected according to the signal bandwidth,
e.g. for PAL/NTSC/SECAM or digital component sig-
nals, respectively. The amplitude of the correction sig-
nal is adjustable. Small noise amplitudes in the correc-
tion signal are suppressed by an adjustable coring
circuit. To eliminate ‘wrong colors’, which are caused
by over and undershoots at the chroma transition, the
Micronas
21
MICRONAS [ MICRONAS ]
