Canon H1 Service Manual page 25

2
Technology > Controls (Reader)
● CCD Drive
The machine's CCD sensor is a 4-line linear image sensor consisting of 7500 pixels. After
completion of photoelectric conversion in the light-receiving block, the signals are output to
the analog front end PCB unit on CCD PCB in parallel for each channel (Color scan: R, G, B
B/W scan: B/W) of the CCD array.
Red(R)line
Green(G)line
Blue(B)line
Black & white(B/W)line
Light-receiving block
L
● Gain Correction and Offset Correction of CCD Output
The analog video signal generated by the CCD is corrected so that it will have a specific level
(gain correction); moreover, the output voltage occurring in the absence of incident light is
also corrected so that it will have a specific level (offset correction).
● A/D Conversion of CCD Output
The corrected analog video signal is converted into a digital signal that is suited to the voltage
level of individual pixels by the A/D converter.
2 Technology > Controls (Reader)
Expanse
Output
L H H L H L H H H L
.........
F-2-21
● Shading Correction (Overview)
The output of the CCD is not necessarily even for the following factors even when the density
of the original is uniform:
1) Variation in the sensitivity of the CCD among pixels
2) Variation in the intensity of the rod lens array
3) Variation in the intensity of light that goes through the center and surroundings of lens
4) Variation in the intensity of light at the center and surroundings of LED
5) LED deterioration
The machine executes shading correction to even out the output of the CCD.
Shading correction may be the shading correction executed for each job.
● Shading Correction
The machine executes the shading correction for every scan made.
In this correction, the analog image process unit on CCD PCB digitalizes the LED light
reflected by the standard white board. After the reflected light is digitalized, it is held in the
shading correction circuit on main controller PCB as a shading coefficient.
Shading correction circuit compares the stored target value with the shading coefficient. The
difference between the two will be held as the shading correction value for use in correcting
variation among CCD pixels when scanning the original, thus evening out the density levels
of the image.
CCD output
Target value
Measurement
value
Standard white plate
2-12
Features after correction
Features before correction
White
Original density
F-2-22
2-12