Gestetner A193 Service Manual page 75

IMAGE PROCESSING
1. Grayscale Processing
As stated on the previous page, the gray scale process generates up to 64
image density levels for each pixel. To realize this, this machine uses a form
of pulse width modulation. Pulse width modulation consists of the following
processes:
•
Laser diode pulse positioning (Horizontal)
•
Laser diode power/pulse width modulation (Vertical)
Laser diode power and pulse width modulation is done by the laser diode
drive board (LDDR), and will be explained in the Laser Exposure section.
Briefly, the width of the laser pulse for a pixel depends on the output level
(from 0 to 63) of the pixel.
For each pixel, the location of the active (laser on) part of the pixel can be either
at the left side of the pixel, at the center, or at the right side (see fig 2). The
machine determines which method to use depending on the settings of the
service program mode, SP 2-903. There are different settings for pixels at the left
edge, at the right edge, and in the middle of a series of black/grey pixels across
the main scan, and for single black pixels with white pixels at the left and right.
The edges of characters and lines become clearer and sharper with this
processing.
Fig. 1
2. Binary Picture Processing
Each video signal level is converted from 8-bit to 1-bit (black and white image
data) in accordance with a threshold value.
The threshold value can be adjusted using service program mode, SP 4-418.
The printout density of the black pixel depends on the pixel type (left, center,
or right of a series, or isolated, in the same way as for grayscale processing).
These values can be adjusted using service program mode, SP2-904.
(A193)
A193D518.wmf
2-14
Center
Fig. 2
A193D507.wmf
SM