A–20
5. S/N
S/N (Signal to Noise) is the ratio of the signal power level to the noise
power level at optional point on a telephone line.
Fig. A-13 S/N
Generally, S/N can be calculated by the following formula:
Fig. A-14 S/N Calculation Formula
As can be known from the formula, a “good S/N” is a large S/N. That is,
the signal power level is large compared to the noise power level. A “poor
S/N” is when the above condition is reversed.
It can be said that the performance of the receiving facsimile is high if it
can receive image signals at a small S/N. If signals can be received prop-
erly from large noise, this is a clear indication that performance is high.
The Fig. A-15 shows the relationship between the signal level and noise
level.
Fig. A-15 Relationship between Signal Level and Noise Level
Measured waveform
Telephone
Exchange
Telephone
Measurement
point
S/N = 10 log
10
Signal power[mW]
Noise power[mW]
[dB]
Example of
small S/N
Example of
large S/N
Noise level
Noise level
Signal level
Signal level
