NEC VT440 Service Manual page 79

CIRCUIT DESCRIPTION
4-3 RGB/Component signal processor and A/D converter block
The video signal input entered in Pin 15 of the Mini D-SUB is fed to the A/D converter IC (IC3401: CXA3506R)
through the buffer circuit. In the case of a component signal, only the chroma signals (Pb, Pr, Cb, Cr) are
entered after their levels have been lowered at the attenuator of -6B.
In the A/D converter IC, the video signal from the video decode IC or the video signal at Pin 15 of the Mini
D-SUB is selected. After the contrast has been adjusted at the built-in analog amplifier, this signal is
processed for A/D conversion into the 8-bit digital signal.
The 8-bit digital signal is entered in the signal processor IC (IC3502: Endeavor) for signal processing.
The signal to be used to clamp the video signal (clamp pulses) is the clamp pulse signal from the sync
signal processor IC (IC1006: M5234FP) or the one from the signal processor IC (IC3502: Endeavor)
chosen by the changeover action at the signal changeover block (IC3301: YAMAG). This signal is then
entered in the A/D converter IC (IC3401: CXA3506R) for signal clamping. Generally for the VIDEO and
HD signals, the clamp pulses of the signal processor IC are used. For the RGB signals, the clamp pulses
of the sync signal processor IC are generally used.
4-4 Audio signal processing
The audio signal passes through the buffer circuit of the MAIN PWB, and is then fed to the output AMP IC
(IC4100: AN7512SH) that is incorporated in the VOLUME control of the KEY PWB.
The VOLUME control is effected at the serial D/A (IC1003: CXA1315). The amplified audio signal is output
(MAX: 1W) to the speaker system (8W x 1).
4-5 Interface with I/O PWB and MAIN PWB
Pin No.
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
2
4-6 List of I C control ICs
Item name
CXA2139S
TC9090AF
CXA1315M
CXA3506R
6-6
Signal name
DGND
EXT3
EXT2
EXT1
DGND
RXD
DGND
TXD
DGND
VCCS+5V
VCCA+9R1
VCCA+9R1
AGND
AUDIO
AGND
Circuit symbol Function
IC1104 Video decode
IC1101 Y/C separation
IC1003 Serial D/A
IC3401 A/D converter
Pin No. Signal name
16 S-C
17 AGND
18 S-Y
19 AGND
20 CV
21 AGND
22 V
23 AGND
24 H
25 AGND
26 B
27 AGND
28 G
29 AGND
30 R
Slave address
8AH/8BH
B2H
40H/41H
9AH
Remarks