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(4) Level variation
"Level variation" refers to the signal level change according to time.
(5) White noise
"White noise" occurs in amplifier vacuum tubes, transistors and resistors,
and is uniformly distributed among all frequencies.
(6) Crosstalk
"Crosstalk" is the voice signals you can hear on other lines that is caused
by electromagnetic or inductive coupling. Crosstalk occurs when the level
at an EO is -15 dBm or more, and is the equivalent to white noise.
(7) Frequency offset
"Frequency offset" is the difference between the input signal frequency
and the output signal frequency.
As SSB (Single Side Band) transmission is used in FDM, the carrier wave
(fc1) is not transmitted from the transmitter to the receiver. Instead, the
receiver uses its own carrier (fc2) to regenerate the signal. Generally, a
slight frequency difference ( f) between fc1 and fc2. The Fig. A-23 shows
its generation procedure.
Frequency offset does not cause any special problems in telephone conver-
sations. In G3 facsimile transmission, however, waveforms may be dis-
torted and the S/N deteriorates.
fs
N
H
Exchange
LPF
fs
: Signal frequency
fc1, fc2
: Carrier frequency
MOD
: Modulator
DEM
: Demodulator
fc1, fc1 fs
MOD BPF
fc1+fs
CH1
CH2
CH3
Transmission
fc1
line
CH3
CH2
CH1
DEM
BPF
H
N
BPF
LPF
Fig. A-23 FDM and Frequency offset
fc2, fc1+fs fc2
BPF
DEM
LPF
CH1
fs+ f
CH2
( f=fc1-fc2)
CH3
fc2
N
H
CH3
CH2
Exchange
CH1
BPF MOD
: Hybrid circuit
: Balancing network
: Band pass filter
: Low pass filter
1
2
3
4
5
6
A
çi
A–25
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