Canon GP215 Service Manual page 86
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- Installation instructions (4 pages) ,
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- Service manual (458 pages)
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3. OPERATIONS AND TIMING
3. A-D Conversion Circuit/ABC Circuit
a. A-D Conversion Circuit
The A-D conversion circuit converts analog signals from the CCD to 8-bit digital image signals.
In general, the A-D conversion signal performs A-D conversion based on the reference voltage. As
such, increasing/decreasing the reference voltage will increase/decrease the dynamic range (difference
between maximum output level and minimum output level), affecting the signal level after A-D conversion.
(This means faithful digital conversion of analog inputs is not possible.)
The ABC circuit to be explained next takes advantage of this fact; i.e., the reference voltage is varied
according to the A-D conversion circuit output signal level to vary the dynamic range, thereby canceling
the background density of originals. (This processing is executed for every main scanning line.)
Outline of A-D conversion
<White original>
Surface of
an original
FFH (white)
Output level
00H (black)
b. ABC (auto background control) Circuit
The ABC circuit operates if 'priority to speed' is selected as part of AE (auto density mechanism) set-
tings; image signals after A-D conversion are returned to the ABC circuit for the following processing.
• Identifies the A-D conversion circuit output signal level.
• Varies the reference voltage based on the results of identification.
The reference voltage obtained here is sent to the A-D conversion circuit. The "priority on speed"
function may be adjusted in service mode ( * 3 * ); see p. 3-204
3–32
COPYRIGHT © 1996 CANON INC.
A
A
: Dynamic range of a white original.
: Dynamic range of a colored background.
B
Figure 3-303
<Colored original>
B
CANON GP215/200 REV.0 JULY 1996 PRINTED IN JAPAN (IMPRIME AU JAPON)
The ABC circuit
varies the reference
voltage to lower the
FFH (white) level,
thereby decreasing
the dynamic range.
FFH (white)
00H (black)
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