Auto Shading - Ricoh A193 Service Manual

IMAGE PROCESSING

2.4.3 Auto Shading

A193D517.wmf
There are two different auto shading methods, black level correction and
white level correction. Auto shading corrects errors in the signal level for each
pixel.
1) Black Level Correction
The CPU reads the black level from the dummy pixels located at one end of
the CCD signal (64 pixels at the end are blacked off) and takes the average
value of the black dummy data. Then, the CPU deletes the black level value
from each image pixel.
2) White Level Correction
Before scanning the original, the machine reads a reference waveform from
the white plate. The average of the white video level for each individual pixel
pixel is stored as the white shading data in the FIFO memory in the IPU chip.
The video signal information for each pixel obtained during image scanning is
corrected by the IPU chip.
In book mode, auto shading is done at the beginning of each scan.
In ADF mode, auto shading is done at a specific time interval. This interval
can be set with SP4-913 (the default setting is 30 seconds). The machine
waits until the end of the page before performing the auto shading.
(A193) 2-10 SM