Dell 2135CN, Service Manual

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Chapter 6 Principle of Operation (Fax)
Version 1 2008.02.01
5. Unit Mechanisms
Scanner
The scanner consists of a lamp (fluorescent lamp, etc.) that illuminates the original document with
uniform light and charged coupled device (CCD) that reads the light reflected from the image.
A CCD is a light-receiving element that produces an electrical signal in response to light. In the case
of a FAX, a number of CCDs (e.g. 2048) are arranged in a line.
The white areas of the original document reflect the light from the lamp. The black areas reflect no
light. The CCDs read the light reflected from the original, outputting sequentially to the control circuit
which areas are white and which black as binary data (1/0 digital data: 1 bit).
To scan the original, the CCD device must be shifted a distance of one line after
each line is scanned. When the original is scanned on the platen glass (as for a
flatbed scanner), the CCD unit is moved with respect to the original. In the case of a
FAX equipped with the ADF (Automatic Document Feeder), scanning via the ADF is
performed by moving the original with the CCD fixed at one position. This is known
as constant velocity transport (CVT).
During scanning, the finer the grid into which the original is divided, the greater the
scanning precision of the original image. For a G3 FAX (normal mode: G3 Normal),
scanning is performed at the resolution of 8 divisions per millimeter (200 dpi) in the
horizontal direction and 3.85 divisions per millimeter in the vertical direction. This
means that the 200 dpi in-line CCD unit is shifted approximately four times per
millimeter in the vertical direction. For an A4 original, the data amounts to
approximately two million pixels. In the high-quality mode (G3 Fine), scanning
resolution is 8 divisions per millimeter in the horizontal direction and 7.7 divisions
per millimeter in the vertical direction, where the data amounts to approximately
four million pixels. As resolution increases, the amount of data also increases,
lengthening the transmission time.
Control Circuit
The control circuit executes scanning of image data by controlling the image scanner. A line of
CCDs scans the original image one line at a time. When scanning of one horizontal line is
completed, the next line below is scanned. As this continues, the original is scanned from end to end
one line at a time and converted to digital data as black-and-white information.
Because this image data is a set of digital signals, it cannot be transmitted using an analog
telephone line. It must be subjected to DA conversion (modulation). On the other hand, the receiving
FAX machine must perform AD conversion to restore the incoming digital data to analog data.
DA conversion, analog signal transmission, analog signal reception, and AD conversion are all
performed by a modem (modulator/demodulator) in the control circuit. A modem consists of a
network control unit (NCU) for connecting to the telephone line and an A/D conversion unit for
performing DA and AD conversions.