Xerox Phaser 6500 Service Manual page 126

Theory of Operation
Scanner
Control Circuit
Printer
2-72
The scanner splits the image into a fine grid and reads the brightness (white/
black) of each cell. This operation is called scanning. The white/black information
is converted to a digital signal: bright cells become 1, dark cells become 0.
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.
The digital signal from a scanned image is subjected to DA conversion
(modulation) by the control circuit to enable transmission over an analog
telephone line. After conversion, the data is sent as an analog signal. The sound
audible during transmission is image data that has become an analog audio
signal.
The analog signal arriving over the telephone line is then subjected to AD
conversion (demodulation) by the control circuit of the receiving Fax machine, and
restored to a digital signal. The digital signal is then sent to the printer.
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.
The black/white information obtained from the AD conversion is sent to the
printer, where black cells are reproduced on the paper at the positions where they
were on the original.
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Phaser 6500/WorkCentre 6505 Service Manual