The Fax Function
The public service telephone network (PSTN) has a frequency bandwidth of only about 3 kHz. Therefore, to transmit fax signals over it, considerable modification of the analog signals generated by an image scanner is required.
Standards for such transmission and reception have been established by an international body, the Consultative Committee for International Telephone and Telegraph (CCITT). Their recommendations have evolved over the years in groups. Today most fax operations conform to Group 3 standards. Their details are given later in this chapter.
CCD or CIS scanners derive analog signals from the documents which they scan. These have to be converted to digital form before being subjected to compression. Some ...
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