Video Amplifier System With On-Screen Display - NEC MultiSync V720 Service Manual

9.

Video amplifier system with on-screen display

LM1281 (U201)
Outstanding features:
* Three-channel video amplifier bandwidth 85 MHz @ -3 dB (4 Vp-p output)
* OSD TTL input, bandwidth 50 MHz
* High-speed video/OSD changeover
* High impedance DC contrast control above the 0-4V, 40dB range
* High impedance DC OSD contrast control above the 0-4V, 40dB range
Red OSD Input
Green OSD Input
Blue OSD Input
Video/OSD Switch
Red Video In
V
CC1
Ground
Green Video In
V
CC1
Ground
Blue Video In
V
REF
Video Contrast
OSD Contrast
Description of the functions
Figure 1 shows the block diagram of LM1281, in conformity with the pin layout of the IC.
Every channel accepts both the video signal and the OSD signal at the input anplifier (A1).
Also the video/OSD changeover signal passes either the video signal or the OSD signal through LM1281,
or is connected with the input amplifier for control purposes. The next contrast adjustment block is a drive
adjustment type one.
The reference level for the DC return circuit is set by means of the RGB cutoff adjustment pin (PIN 25).
Attention must be paid to the fact that the blank clamp gate is active when it is stuck at the LOW state.
Under ordinary circumstances, these pins are controlled by means of the standard TTL signal.
BLOCK AND CONNECTION DIAGRAM
1
2
Contrast
3 -A1
Attenuation
4
5
6
Contrast
7
-A1
Attenuation
8
9
10
Contrast
-A1
11
Attenuation
12
13
14
Top View
Drive
-A2
Attenuation
Contrast
-A2
Attenuation
Contrast
-A2
Attenuation
65
28 Red Drive Adjust
Green Drive Adjust
27
Blue Drive Adjust
26
25 RGB Cutoff Adjust
24 Red Clamp Cap
23 Red Video Out
22 V
CC2
21 Ground
20 Green Video Out
19
Green Clamp Cap
18 Blue Video Out
17 Blue Clamp Cap
16
Blank Gate
15
Clamp Gate