Canon fax basic 2000 サービスマニュアル

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