520 - 44 - 00
8
7
6
5
7
3
14
18
13
20
11
10
8
VIDEO
SOURCE No. 1
OUT
GT4122
+IN B
S1
1
17
GB4551
V
REF
R
EXT
S2
+IN A
GB4551
GB4551
GB4551
GT4122
V
REF
R
EXT
+IN B
S1
S2
+IN A
GB4551
8
7
5
3
1
8
7
5
3
1
8
7
5
3
1
9
6
7
14
18
13
20
11
10
8
17
9
G
-IN A
OUT
-IN B
G
VIDEO
SOURCE No. 2
No. 18, No. 2
MIX CONTROL
VIDEO
SOURCE No. 3
(1+2) & No. 3
MIX CONTROL
IN OUT
IN OUT
IN OUT
IN OUT
IN OUT
Back Porch
Pulse
Back Porch
Pulse
Back Porch
Pulse
0.5V
SPAN
1k
Back Porch
Pulse
VIDEO 1 & 2
OUTPUT
VIDEO 1, 2 & 3
OUTPUT
0.5V
SPAN
0.1
1k
Back Porch
Pulse
-IN A
-IN B
10
10
10
10
10
8
7
5
3
1
V
C
V
C
Fig. 5 Three Level Mixer using Two GT4122 Devices
NOTES: 1.
All non-marked capacitors connected to GROUND are 470pF.
2.
All resistors are in ohms, all capacitors in
μ
F unless otherwise shown.
3.
For clarity, power supply and offset adjustments are not shown.
Figure 5 shows an implementation of the GT4122 as a 3-level
mixer incorporating external clamping circuits made up of
GB4551 high performance, back-porch clamps. These devices
are available from GENNUM Corporation. In this circuit, three
video signals are combined by cascading two GT4122 devices.
The control signal circuitry is not included in this circuit and
would depend on each individual requirement.
The input signals could be BORDER video, BACKGROUND
video or even a PREVIOUS video signal. In any case, full
control is achieved with a high degree of accuracy
by providing the appropriate KEY signals to the CONTROL
inputs of the two GT4122 devices.
THREE LEVEL VIDEO MIXER - GT4122