Xerox Phaser 6180MFP Service Manual page 175

Theory of Operation
The difference between analog and digital signals can be easily understood
by comparing analog measuring instruments, such as clocks and scales with
their needles and gradations to digital gauges that display results as a value.
An analog instrument with a continuously moving needle, at least in theory,
can be read beyond the decimal point to infinitely small divisions
(12.47253...g, 35.1864...g, etc.). A digital instrument, however, can only
display results to the minimum necessary decimal place (12.5 g, 35.0 g, etc.).
Conversion of an analog signal to digital signal is called AD conversion. DA
conversion is the conversion of digital signal to analog signal. Image data read
by a Fax is a digital signal in which 0s and 1s are assigned according to
whether or not there is black in the squares of a paper surface divided into a
grid. Fax communications that use an analog telephone network perform DA
conversion before transmitting the scanned image from the phone, and AD
conversion before printing the received data.
Phaser 6180MFP Multifunction Printer Service Manual 2-93