Ricoh VT3500, Service Manual

Page: 129 / 330

The light reflected from the original exposes the CCD (Charge Coupled
Device). The CCD is a solid-state device similar to a photodiode array, but
unlike a photodiode array, a CCD can read one complete scanning line at a
time.
The principle circuit of each pixel
(picture element) in the CCD is shown
on the right. The light reflected from the
original is sensed by photodiode [A].
Capacitor [B] stores the electrical
charge corresponding to the light
intensity. The CCD used in this model
has 5036 sets of such photodiodes and
capacitors in series (5036 pixels). To
increase scanning speed, the odd and
even pixel data are handled separately.
The line synchronizing signal (LSYNC) generated in the image processing
PCB turns off to do one line of main scanning. When main scanning for a line
starts, the shift signal (SH) is sent from the A/D conversion PCB. The one line
image data are stored in the capacitors of each pixel. At the same time, the
previous line data stored in the capacitors are transmitted in parallel to two
shift registers: one for odd pixels and one for even pixels. Then, the odd and
even pixel data are serially output. An output transistor source (OS)
synchronizes the odd and even clock pulses. The odd and even clock pulses
are activated or de-activated by the dropping edge of the reset signal (RS).
The CCD consists of 5,036 pixels. The first 32 pixels are dummy pixels. The
14 dummy pixels from the 16th are covered with aluminum film. These pixels
are used for black limit level data. The following 5,000 pixels are effective
pixels. All the data for one line of the original image are converted as
electrical charges and stored in a capacitor of effective pixels individually.
After the effective pixels, there are 4 more dummy pixels.
Since one line scanning period (3 milliseconds) counts 5,120 clock pulses (a
clock pulse corresponds to a pixel data output), there remains time for 84
clock pulses. The scanning position moves to the next line after this period (a
sub-scan is completed), and main scanning for the next line starts.
To Shift
Register
[A]
[B]
1 December 1993
2-93