Canon GP160, Service Manual

Page: 77 / 797

COPYRIGHT © 1999 CANON INC. CANON GP160 REV.0 FEB. 1999 PRINTED IN JAPAN (IMPRIME AU JAPON)
CHAPTER 4 IMAGE PROCESSING SYSTEM
4-3
C. A-D conversion circuit / ABC circuit
The CCD’s PCB A-D conversion circuit converts the analog image signal (voltage
signal), which has been sent from the CCD, into an 8 bit digital image signal.
The A-D conversion circuit performs A-D conversion processing based on the refer-
ence voltage. By increasing or decreasing the reference voltage, the density range (the
difference between the max. output level and min. output level) of each pixel will also
increase or decrease, therefore it has an affect on the signal level after the A-D conver-
sion.
The ABC (Auto Background Control) function applies this characteristic, and
changes the reference voltage in accordance with the output signal from the A-D con-
version circuit, and by altering the density range, converts the documents background
density. This technique is used for adjusting the density when copying from a coloured
background document to one with a white background.
As show in figure 4-202, in the case of a document with a coloured background, by
lowering the density range compared to a white background, the coloured background is
erased.
When the unit’s copy function density adjustment is set at “Auto”, the ABC function
comes into operation automatically. After A-D conversion, the image signal is returned
by the ABC circuit, and judgement of the A-D conversion circuit output signal level takes
place, the reference voltage is converted in accordance with the judgement results. ABC
control takes place by the voltage being input into the A-D conversion circuit,.
Figure 4-202
FFH (white)
00H (black)
Document
surface
<White background documen>
<Coloured background document>
A
B
: White background document density range
: Coloured background document density range
A
B
FFH (white)
00H (black)
By means of the ABC
circuit, the reference
voltage changes and
the FFH (white) level
is lowered.
(The density level is
decreased)
Output level