Ricoh PRIPORT VT 2000 Series Service Manual page 53

28 February '91
2.5.5 Black Level Correction (Line mode only)
Amp.
The black level correction compensates for the image density. Black level
correction is used only when in line mode.
The lower threshold level (V
high density original or a low density original.
At first, the black level reference of the A/D converter is set to 35% of the
original peak hold white level (P
is set to the "0" position.
If while reading the original image the black level of the image is lower than
V (P x 35%), the A/D converter outputs the LUF signal.
L H
The LUF signal is applied to selector 1 as a clock pulse. The LUF signal
increments the counter of selector 1 from 0 to 1. This changes the V
30% of P . If the LUF signal is again applied to the counter of selector 1,
H
incrementing it from 1 to 2, the analog switch of selector 1 changes from 0
to 1.
The following table shows the number of clock pulses needed to increment
the analog switch of selector 1.
Counter (4 bit)
0.1
2.3
4.5
6.7
8.9
A.B
C.D
E.F
Selector 1
VT2100/2130/2150: 1.7V
Selector 2
3
) of the A/D converter is set according to a
L
Selector 1 Switching
SW 0
SW 1
SW 2
SW 3
SW 4
SW 5
SW 6
SW 7
0
1
2
3
4
5
6
VT2300/2500: 1.4V
VS
Y
) because the analog switch of selector-1
H
V
P x 35 %
H
P x 30 %
H
P x 25 %
H
P x 20 %
H
P x 15 %
H
P x 10 %
H
P x 5 %
H
P x 0 % (Black Level)
H
2-26
Counter
CK
LUF
4-bit A/D Conversion
VL
L
to
L