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RGB Video Processor TDA4780, ICt1
General
The TDA4780 circuit is an RGB Video Processor with an
automatic cut-off control, gamma adjustment, dynamic
black control, and blue stretch.
The IC contains a linear matrix to convert the luminance
and colour difference signals into RGB signals. The proc-
essor is also able to process two external RGB signals.
All parameters and functions are controlled using the IIC-
bus.
2 7
S D A
I C-bus
2 8
interface
S C L
Sand-
1 4
castle
Control
registers
0.45V/
8
Y
1.4V
V B S
7
PAL/SECAM
V
6
N TS C
U
Matrix
1 0
R
1 1
G
1 2
B
Fast
1 3
F s w
signal
2
switch,
R
3
blanking
G
4
B
1
F s w
Signal switches
Fast signal source switches select RGB signals from one
of three input signal alternatives: the matrix stage, the tele-
text circuit or the PIP module. This selection is controlled
using the IIC-bus or by using fast blanking signals FB on
pins 1 and 13. During the vertical and horizontal blanking
time, an artificial black level is inserted in order to clip off
the sync pulse of the luminance signal, to suppress hum
during the cut-off measurement time, and to eliminate
noise during these intervals.
Y-matrix and adaptive black level, gamma and satu-
ration controls
Saturation control is performed by varying the amplitude
of the RGB signals relative to the luminance signal ampli-
tude. For this reason the luminance signal has to be re-
generated in the Y-matrix from the RGB signals.
After the Y-matrix, the luminance signal is fed to the adap-
tive black level control stage. This detects the lowest volt-
Nom white
level
Nom black
level
2
1st and 2nd
TDA4780
switch on
delays
Sandcastle
detector
Timing
generator
Timing pulses
Y-Matrix
Adaptive
black
control
G a m m a
adjust
R
Saturation
G
adjust
B
Supply
1 8
Peak dark
IIC bus data
storage
and control
Input signal
Principle of the adaptive black control and gamma adjust
Functional description
Signal input stages
The luminance and colour difference signals from the Fea-
ture box module are input to pins 8 (Y), 6 (U) and 7 (V). The
luminance signal input level is selectable (0.45 Vpp or 1.4
Vpp) using the IIC-bus (pins 27 and 28). 1.4 Vpp is used in
this application. The signals are fed to a linear matrix stage
to convert them into RGB signals.
RGB signals from the teletext circuit are fed to the input
pins 2 (R), 3 (G) and 4 ((B). RGB signals from the PIP mod-
ule (or from the VGA connector) are fed to the input pins
10 (R), 11 (G) and 12 (B).
All input signals are clamped in order to have the same
black levels at the signal switch input.
Y
2 6
Peak drive
limiter,
6-bit D/A
converter
Y
Average
beam and
peak drive
3x6-bit
(Hue)
limiting
D A C
D/A
converters
Blanking,
measure-
R
ment pulse
Contrast
Brightness
G
insertion,
adjust
adjust
Blue
white point
stretch
adjust
9
5
Grou nd
+ 8 V
age of the luminance component of the internal RGB sig-
nal during the scanning time and moves it to the nominal
black level. In order to keep the nominal white level con-
stant, the contrast is increased simultaneously. The proc-
ess expands the dynamic range of the contrast.
Next, the luminance signal is processed in the gamma ad-
justment stage. This has a non-linear transmission charac-
teristic. The gamma adjustment stage amplifies the lower
levels of the signal using a higher gain. This process im-
proves the separation of the dark parts of the picture from
each other. Finally the luminance signal is fed to the col-
our saturation control stage.
The Y-signal is fed in addition to the adaptive black level
control stage through an internal switch to the output pin
26 and onwards to the SVM module.
The gamma and saturation adjustments are controlled
using the IIC-bus.
Adaptive Black level Control
3x6-bit
reference
registers,
MUX, DAC
1 9
Cut-off
comparators
1 7
P D L
1 6
1 5
Average
beam current
P eak
detector
Cut-off
2 4
adjust,
R
2 2
output
G
2 0
stages
B
2 1 2 3 2 5
B
R
G
Cut-off storage
Gamma Adjust
13
Leakage
and cut-off
current
input
Leakage
storage
Peak drive
limiting storage
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