Sharp QD-101MM, Service Manual

Page: 11 / 66

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2.2.Computer analog signal input circuit
IBM computer and APPLE computer output the following analog signals at the load of 75
Ω
.
The signals are converted into 8-bit digital signals by the A/D converter(IC315~317).
The computer analog signals are given to Pin 21 of the A/D converter, are divided by 256
between the voltages given to Pin 18(VRT) and Pin 24(VRB), and are digitally converted.
The following table shows the reference voltage which is given to the A/D converter.
2.3.Threshold level generation circuit
In order to convert the computer and composite analog signal into digital, it is necessary to set
the threshold level on the A/D converter. As shown in Fig. 3, VRT and VRB voltages are set
on the basis of the reference power of ZD1. VRT can be varied by key operation.
As the adjusting method from the main body, the MENU button and SELECT button are used
to display the contrast adjustment in the screen and to allow the set value to be changed with
UP and DOWN selector buttons.
Fig. 3 Threshold Level Generation Circuit
Table 2
Computer Analog Signal
R G B
IBM 0.7Vp-p 0.7Vp-p 0.7Vp-p
APPLE 0.7Vp-p
1.0Vp-p
(Synchronization signal
overlapped)
0.7Vp-p
Table 3
VRT VRB
Threshold
Level 3.8V(2.8~4.8V)
Adjustment are possible
1.0V