 noise temperature is not confined to noise sources. sufficient to assume that the psophometric weighting of 3kHz white For example the noise power measured at the output of an amplifier noise decreases the average power by about 2.5dB. or antenna can be expressed in terms of the equivalent noise tem- The reference frequency specified at the point of minimum atte- perature. In the case of an antenna the noise is due to radiation from nuation in the voice channel is 800 Hz. The full specification is objects on earth as well as in outer space and the antenna noise provided in the CCITT G series Recommendations temperature is used. It is common to refer to average noise power delivered to 600 Another concept which is used is excess noise temperature T ohms instead of r.m.s. noise voltage, this power is expressed in which is the difference between the noise temperature of the source picowatts. T and the noise temperature T of a thermal noise source at room The noise units used are picowatts psophometric (pWp) or deci- temperature (17° C or 290° K). (Equation 10.34.) bels psophometrically weighted (dBmp), as in Equations 10.35 and 10.36, for noise which is flat from OkHz to 3kHz. Tx = T - T (10.34) (psophometric mV) X 10 10.35) pWp = pwatts Many commercially available noise sources with resistive termi- nations are calibrated in these terms. In use care must be taken to correct for actual temperature in such devices. dBmp = 10 log pWp) (10.36) 10.9.4 Effects of noise on speech telephony 10.9.7 FIA C-message noise units Noise is a complex signal and the measurement of its effects in In North America there are two forms of line weighting that are telephone channels carrying speech is made more difficult by the encountered, FIA and C-message. fact that, to the telephone user, it is the annoyance that is caused by noise that is important rather than the absolute magnitude of the FIA line weighting is SO called because it is based upon the FIA noise power. The subjective interfering effect of noise is a function handset de