Canon fax basic 2000 서비스 매뉴얼

A–28

(3) Phase hit

“Phase hit” is the sudden phase fluctuation caused by fading on carrier

equipment and on microwave transmission zones, or by the switching of

the transmission line on a multiplex transmission line.

Phase hit does not cause any special problems in telephone conversations.

However, in G3 facsimile transmission, the same phenomenon as in phase

jitter occur significantly.

7.3

Echo

“Echo” is the phenomenon where one’s own voice is reflected at the other

party and returns after a certain time to be heard.

Echo mainly occurs in international communications.

7.3.1 Causes of echo

Generally, some of the power in electrical signals is reflected between the

line and load (such as the exchange), between lines of differing character-

istics impedance, and in hybrid circuits (see figure Fig. A-29 for details of

a 2-wire/4-wire conversion circuit). It is very difficult to eliminate this

reflection. This difference in characteristic impedances is sometimes

called “impedance mismatching.”

Fig. A-27   Reflection of Signal Energy

Communication satellites and submarine cables are used as the transmis-

sion lines in international communications.

Since communications satellites are in stationary orbit at a height of

35,800 km over the equator, the one-way distance extends about 100,000

km. Submarine cables are shorter than that, though the distance still

reaches several thousands of kilometers.

Due to these distances, the propagation delay time will be 0.3 sec and 0.1

sec, respectively, even at the speed of electric current or radio waves

(approx. 300,000 km per second). Because of this signal propagation delay

Incidence wave

Transmission

wave

Line A

Reflection

wave

Line B