Ricoh VT3500, Руководство по обслуживанию

Страница: 137 / 330

[Peak Hold]
1. Standard White Peak Hold
Before scanning the document image, the white platen cover is scanned.
While the first five lines are scanned, the peak hold timing signal (PKHTM)
turns off and so the switch [A] on the A/D conversion PCB turns on. This
allows the V0 signal to reach the peak hold circuit. Since the peak hold set
signal (PKHST) also turns off in this period, the whitest datum of each pixel is
stored as a standard white peak voltage (VPH-SW). Since the shading
distortion memory reset signal (SHMRT) also turns on in this period, switches
in the selector are switched to "0". Therefore, white peak voltage (VPH-SW)
is sent to the A/D converter as a "+REF" voltage. The A/D converter changes
the analog image data of the white platen cover to a digital signal and this
digital signal is stored in memory as the shading distortion data.
2. Original Background Peak Hold
After the five lines of the platen cover have been scanned, the peak hold set
signal (PKHST) turns on to clear the standard white peak voltage (VPH-SW)
in the peak hold circuit.
While the 64 millimeter width (1,024 pixels) in the middle of the original is
scanned, the peak hold timing signal (PKHTM) turns off to send the image
data to the peak hold circuit. (This is done about 1 millimeter from the leading
edge of original.) The peak hold circuit holds the whitest (lightest) image
voltage in 1024 pixels as an original background peak voltage (VPH-OB).
This is because the peak hold set signal (PKHST) turns off while the original
image is scanned.
Since the scanning process starts before the light intensity of the fluorescent
lamp stabilizes, the light intensity tends to increase for a little while. The
image signal from the CCD increases until the light intensity stabilizes. As a
result, lighter image densities may not appear on prints after the light
stabilizes. To prevent this, the original background peak voltage (VPH-OB) is
changed when a higher (lighter) image signal is applied to the peak hold
circuit.
1 December 1993
2-101