Preliminary Processing
1. A/D conversion
•
The multiplexer selects the data from the image sensor, then the DIP
converts this to an 8- bit digital signal.
2. Video signal correction
•
The image sensor corrects for the gound level and amplification ratio by
itself.
•
The machine reads all-black and all-white waveforms before the scan-
ner moves to scan the document.
First, the image sensor reads the white plate that is behind the
exposure glass, with the lamp off. The machine stores the output data in
the VRAM (this is the all-black waveform). The image sensor then reads
the white plate with the lamp on, and the machine stores this data (this
is the all-white waveform).
Then, the machine starts scanning the document. The data output from
each element on the image sensor is compared with the above stored
values, and corrected (this is Auto shading).
•
The all-white waveform is also used as a peak level for the peak hold
function.
If Halftone was not selected
1. Background detection
•
Each pixel is tested in relation to its neighbours to determine if it is back-
ground noise and should be eliminated.
2. Auto density control
•
The most suitable threshold value is calculated from the density that is
detected from a fixed area on each original.
3. MTF
MTF (Modulation Transfer Function) can reproduce details such as thin lines,
points, and complex characters.
4. Thresholding
•
Each video data element is compared against the threshold level, which
depends on the contrast selected by the user. If an element is lighter
than the threshold level, it is converted to a white (0). Otherwise, it be-
comes a black (1).
5. Erasure of irregular dots
DETAILED SECTION DESCRIPTIONS
May 22nd, 1995
SCANNER
2-20