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7.2
Unusual Factors
(1) Impulse noise
"Impulse noise" is the irregular noise that occurs because of the contact
points at an exchange.
This noise is easily generated by step-by-step exchange that operate by
mechanical contacts.
Impulse noise is no problem in telephone conversations, except that it can
be a little offensive to the human ear when it occurs. In G3 facsimile trans-
mission, however, the same image defects that occur with white noise and
level variation occur.
(2) Short breaks
A "short break" is a sudden drop in the reception signal level of 6 dB or
more for 1 msec to 60 sec continuous. Short breaks are caused by fading in
microwave radio ralay and satellite communications system zones or by
faulty contacts on equipment and cables.
Short breaks do not cause any special problems in telephone conversations
as long as the break is not continuous. However, in G3 facsimile transmis-
sion, the same phenomenon as in level variation and impulse noise occur
significantly.
100mV
0
10
Fig. A-25 Impulse Noise Caused by a Step-By-Step Exchange
10
Impulse noise
or
6
level variation
5
0
1ms
Fig. A-26 Short Break Area
20
30
40
50
60
70
Time (msec.)
Short break
Level variation
2ms
60s
Continuous time
1
2
3
4
80
5
6
A
çi
Line trouble
or
level variation
120s
A–27
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