A/D Conversion Processing - Ricoh C226 Service Manual

OPTICS/IMAGE PROCESSING

2.1.2 A/D Conversion Processing

The analog signal from the contact image sensor is converted into a digital
signal that represents 64 grayscale steps. This process is identical to other
models, however it is carried out in the halftone processing chip while the
other models use a separate A/D conversion board.
Shading Distortion Correction
The image data from one main scan line does not exactly represent the line
from the original image, because of the following reasons:
1) Loss of brightness towards the ends of the exposure lamp.
2) Variations in sensitivity among elements of the contact image sensor
3) Distortions of the light path
Such distortions in the image data are corrected when they are converted
into digital data.
Before scanning the document, the scanner reads the white plate on the
back of the original scale. The output of each contact image sensor element
is changed into a 6-bit digital value and is stored in the shading distortion
memory.
To change the analog shading distortion signals to digital data, a scale of 64
steps is made between the whitest level when the white plate is scanned and
50% of the whitest level. Using this scale, the analog signal is changed into
6-bit digital data.
While an original is scanned, the 6-bit shading distortion value for each pixel
is sent from memory to the D/A converter, which is synchronized with the
image signal being sent to the A/D converter. The D/A converter changes the
distortion value to an electrical current. The current is then converted to the
voltage that is to be used as the high reference data for A/D conversion. In
this way, the high reference voltage that is for A/D conversion is changed
sequentially for each pixel depending on the shading distortion data for that
pixel.
C226 2-10 SM