Ccd Drive; Ccd Gain Correction, Offset Correction; Ccd Output A/D Conversion; Outline Of Shading Correction - Canon iR C6800 Series Service Manual

2. Digital Image Processing
CCD/AP
PCB

5.3.6.2 CCD Drive

The machine's CCD is a linear image sensor consisting of 3 lines (R, G, B, 1 line each), each line composed of 7350
photo cells. The signal that has been put through photo conversion in the light receiving segment is divided into 2
analog signals of 2 channels for output: even numbered pixels (EVEN) and odd numbered pixels (ODD).
Light-receiving segment

5.3.6.3 CCD Gain Correction, Offset Correction

The analog video signal from the CCD is processed so that the rate of amplification is even (gain correction); the
output in the absence of incident light is also processed for a specific level (offset correction).

5.3.6.4 CCD Output A/D Conversion

The odd numbered and even numbered pixel analog video signals after the foregoing correction are then converted
into 10 bit digital signals by the A/D converter according to their pixel voltage levels.

5.3.6.5 Outline of Shading Correction

The output of the CCD may not always be uniform because of the following reasons even when the density of the
original in question is uniform:
1. variation in the sensitivity among individual pixels of the CCD.
2. difference in the level of transmission between the center and the edge of the lens.
3. difference in the intensity of light between the middle and the edges of the scanning lamp.
4. deterioration of the scanning lamp
The machine executes shading correction to even out the output of the CCD.
The machine executes either of the following 2 shading mechanisms: shading correction it carries out for every copy
and shading adjustment for which the target value is set in service mode.
J101 J204
Gain correction
data
J205
J102
Digital image
signal
10
Shift register
6
(photodiode)
Shift register
Reader controller PCB
Target
CPU
value
Shading correction
F-5-33
Output buffer
Gate
5 4 3 2 1
Gate
Output buffer
F-5-34
EEP-ROM
SRAM
J203
Digital image
signal
8
Odd-numbered pixel data
Even-numbered pixel data
Chapter 5
0002-5168
0002-5170
0002-5171
0002-5172
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