one of a would be seen due to the random nature of the noise voltage. If a specified frequency. rectifying a.c. voltmeter is used then form factor correction should This weighting also must take account of the effects of attenuation be used for the measurements. This requires knowledge of the caused by the telephone handset. Thus standard line weighting character of the noise to be measured. networks have been produced which take account of these factors A voltmeter will indicate the total r.m.s. voltage of the noise if the and provide a frequency shaping effect which matches that of a voltmeter has a frequency response greater than the bandwidth of telephone user and telephone handset. the noise spectrum to be measured. If the voltmeter has a frequency response less than that of the bandwidth of the noise then the reading will be proportional to the noise within the bandwidth of the meter. 10.9.6 Psophometric When the noise to be measured is in a narrow frequency band within a broadband spectrum, then a filter of the required bandwidth Psophometric line weighting is the standard produced by the Inter- must be used between the voltmeter and the point to be measured. national Telegraph and Telephone Consultative Committee It is also common to use frequency shaping, in addition to band (CCITT). It is defined as the noise measured on a telephone line limiting. which is known as noise weighting. Signal to noise ratio 10/13 10.9.8.2 Noise meter The measurement of noise in a system or device is often performed Input noise using a noise meter. This is actually an a.c. voltmeter preceded by a Output noise filter which is tunable or switchable over a range of frequencies N OUT = N IN much greater than their noise bandwidths. In most cases these meters will have a fixed input impedance and are often calibrated in terms of power rather than voltage. Figure 10.16 Output noise in terms of input noise In the case of an average reading voltmeter form factor correction must be applied. This is caused because the noise is amplifi