Xerox Phaser 6128 MFP, Service Manual

Page: 112 / 606

2-58 Phaser 6128MFP Service Manual
Theory of Operation
Fax
Fax Overview
A Fax is a device that sends and receives image data using either an analog or a
digital telephone line.
The three basic units of a Fax are the Scanner (for reading the image), the Control
Circuit, and the Printer.
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.
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 signal, that
is, an 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 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.
Fax Scans
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.
s6128-058
Fax System (Overview)
Image Scanner Control Circuit Control Circuit Printer
Scanning
Printing
DA
Conversion AD
Conversion
Telephone
Line
Original
Digital Signal Analog Signal Digital Signal
Printing