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• Shading memory
A pre-scan is performed at the start of document reading to store the read-
ing brightness data for the base color of the white mylar sheet to shading
memory. Then, the data stored to shading memory is compared with the
reading brightness data when the document is read to generate uniform
image reading brightness data.
• A/D conversion circuit
This circuit converts the analog signals (voltage signals) that arrive from
the contact sensor to digital image signals.
In the A/D conversion circuit, A/D conversion processing is performed
based upon the reference voltage. As adjusting this reference voltage also
adjusts the reading density range (difference between minimum and maxi-
mum output levels) of each pixel, this influences the signal level after A/D
conversion.
• D/A conversion circuit
This circuit converts digital signals for shading compensation data, for
example, into analog signals.
It also generates the white side reference voltage for the A/D conversion
circuit.
• ABC circuit
The ABC (Auto Background Control) function converts the level of the
skin color density in a document so that the reading density of skin color in
a color background document is adjusted as white skin color. This function
is achieved by varying the white reference voltage of the A/D conversion
circuit according to the brightness level of the document to be input.
As shown in Fig. 3-18, the skin color in a document is eliminated by low-
ering the density range of a color background document to lower than that
of a white background document.
If there were no ABC circuit, color background documents would be read
as black documents.
<White background
document>
Document
surface
FFH (white)
Output level
00H (black)
(A): Reading density range of white background document
(B): Reading density range of color background document
<Color background
document>
A
B
Fig. 3-18 ABC Circuit Functions
READING SECTION
The ABC circuit changes
the reference voltage to
lower the FFH (white)
level (to reduce the
reading density range).
FFH (white)
00H (black)
3–15
1
2
3
4
5
6
ït
çi
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