Canon fax basic 2000 서비스 매뉴얼

G3 FACSIMILE COMMUNICATIONS

5–8

4. CODING SCHEMES

4.1

Why is Coding Necessary?

One of the features of a G3 facsimile is that image data is coded before it is

transmit and received. The reason for coding is to reduce the size of the

data to be actually transmitted to reduce the transmission time.

The following example shows by how much the transmission time can be

reduced through coding.

Here, let’s consider an example where an A4 document is transmitted at

standard resolution.

Fig. 5-5   Size of Information in One Page of an A4 Document

In this way, the size of information of a single A4-size page becomes

roughly 2,000,000 bits. Let’s try considering the time required to send

2,000,000 bits of information without coding.

When the document is transmitted using a 9600 bps MODEM, it takes

three minutes 26 seconds, and when it is transmitted using a 14400 bps

MODEM, it takes two minutes 17 seconds. Without coding, transmission

of a standard A4 document will take longer than one minute. Why, then,

can the transmission time be reduced with coding?

Normally, a document transmitted by a facsimile has lots of white areas.

Text that is written in these white areas take on meaningful information.

The white areas have no meaning. On a facsimile, white areas without text

is “unwanted space,” and black areas with text is “information.” If an area

is black, this does not necessarily mean that this is information; a com-

pletely black document, for example, has little “information” and can be

termed a document having many “unwanted areas.” The extent that these

ISO
Standard A4
document

V

e

rtical-scanning

direction

1728 bits

Information (number of pixels) in one page
of an A4 document
1728 x 1145 = 1,978,560 (bits)

Horizontal scanning
direction

1145 lines