Canon fax basic 2000 서비스 매뉴얼

A–22

7. FACTORS WHICH CAUSE DETERIO-

RATION IN QUALITY OF FACSIMILE

TRANSMISSION

7.1

Usual Factors

(1) Transmission loss

“Transmission loss” is the amount that a signal transmitted from the trans-

mitting terminal attenuates until it arrives at the receiving terminal. Trans-

mission loss is expressed in dB.

Transmission loss has the greatest influence on the quality of telephone

conversations. So, a loaded cable is added to subscriber lines, and for long-

distance lines a digital transmission line comprising coaxial cable and a

microwave radio relay system are used. As a result, most of the transmis-

sion loss occurs on the subscriber line.

Fig. A-17 Example of Loss Distribution on a Toll Line

Loaded Line

In the past, when there were no suitable amplifiers, excessive attenuation

prevented telephone conversations from being conducted when long-dis-

tance calls were made. However, as you can tell from the Fig. A-18, the

amount of attenuation is reduced by self-induction action if coils are

inserted in the line at fixed intervals. These coils are called “Loading coils”

and insertion of these coils is called “load”. A line equipped with loading

coils is called a “loading cable” and a line without loading coils a “Non-

Loading cable (NL)”. Loading cables, however, cause group delay distor-

tion.

Currently, all newly installed subscriber lines are non-loading cables.

7.5

0.5

Telephone

Subscriber line

Units in figure: dB

Telephone

Subscriber line