Canon imageRUNNER 2520 Series, Service Manual, Page: 44 / 402

The reader relay PCB is used to process images on a line-by-line basis. Its major functions are as follows:
1. Analog image processing
• CMOS sensor drive
• CMOS sensor output gain correction and offset correction
• CMOS sensor output A/D conversion
2. Digital image processing
• Shading correction
Target
value
CMOS sensor (1 line)
Analog image signal
Analog image
processing
- Gain correction
- Offset correction
A/D
conversion
CMOS sensor
drive control
CMOS sensor
control signal
Gain
correction
data
Digital
image
signal
IC1
10
Digital image
signal
8
Reader relay PCB
Contact image sensor (CIS)
J554
RAM
Shading correction
IC2
Serial
communication
I/F
● CMOS Sensor Drive
The CMOS sensor used in this machine is a 1-line linear image sensor consisting of 7488 photocells. After completion of
photoelectric conversion in the lightreceiving block, the signals are output to the AP circuit in the reader relay PCB in parallel for
each channel (total four channels) of the CMOS sensor array.
CMOS sensor 1 CMOS sensor 4 CMOS sensor 3
H L L H H L H L H H L ………
output
Light receiving block
● CMOS Sensor Output Gain Correction and Offset Correction
The analog video signals output from the CMOS sensor are corrected so that they will have a specific gain level (gain correction),
and the output voltages generated in the absence of incident light are also corrected so that they it will have a specific offset level
(offset correction).
● CMOS Sensor Output A/D Conversion
After completion of the above corrections, the analog video signals are converted to digital signals corresponding to individual
pixel voltage levels by the A/D converter.
● Shading Correction (Outline)
The CMOS sensor outputs are necessary even for the following reasons even when the density of the original is uniform:
(1) Variation in sensitivity among CMOS sensor pixels
(2) Variation in light intensity of rod lens array
The machine performs shading correction to even out the CMOS sensor output.
2. Technical Explanation
34