Ccd Drive; Ccd Gain Correction, Offset Correction; Ccd Output A/D Conversion; Outline Of Shading Correction - Canon imagerunner C5180 Service Manual

Chapter 6
(2) Digital Image Processing
(a) Shading Correction
J101
Gain correction
CCD/AP
PCB
J102
Digital image

6.3.6.2 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).
Shift register
Light-receiving segment
(photodiode)
Shift register

6.3.6.3 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).

6.3.6.4 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.

6.3.6.5 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:
- variation in the sensitivity among individual pixels of the CCD.
- difference in the level of transmission between the center and the edge of the lens.
- difference in the intensity of light between the middle and the edges of the scanning lamp.
- deterioration of the scanning lamp
The machine executes shading correction to even out the output of the CCD.
6-22
Reader controller PCB
J204
data
J205
signal
10
Gate
6 5 4
Target
CPU
value
Shading correction
F-6-33
Output buffer
3 2 1
Gate
Output buffer
F-6-34
EEP-ROM
SRAM
Digital image
signal
8
Odd-numbered pixel data
Even-numbered pixel data
J203