L in dBm) then the thermal noise contribution to the noise power has been adopted for International use. (Most of the figures have in a 4kHz channel can be calculated in dBmOp (or pWattOp) as in been derived from Holbrook and Dixon, 1939.) expressions 20.5 and 20.6, where 3.6dB is the weighting factor for a 4kHz bandwidth. Definitions of the noise factor F are found in Connor, 1973. Loud and soft talker factor (0.115g2) is the average power in an individual talker. However, analys- -137.5G+F-T-3.6dBmOp (20.5) ing a large number of talkers will show that they all have different volumes compared to the standard talker i.e. there are loud and soft (20.6) talkers. Antilog The distribution of the talker volumes (in dB) has been found to be Normal, with a standard deviation of g. (Figure 20.10(a).) In order to find the average power per talker from a distribution 20.7.4 Noise due to unlinearity of talker volumes the average or expected value is derived from the log normal distribution (see Figure 20.10(b)). This derives that the When carrying a traffic channel load the unlinear transfer charac- average power per talker over a large number of talkers is 0.115g2 teristic of the active devices in the system generate harmonics and greater than the average power of a single talker. (Bennett, 1940) Standard Deviation mWatt Mean Value M dBm value = 0.115g Antilog(M/10) Figure 20.10 Log normal distribution Noise contributions 20/11 Activity factor (10log t) Third Harmonic Not all the channels are fully occupied with continuous talkers. This H3a -105dBmO gives rise to an activity factor t. Conversation between two parties consist of 50% listening and 50% talking. The activity in any one direction is therefore 0.5. Second Harmonic In addition not all of the channels are in use. Even in the peak period only 70% of the circuits are occupied at any one time. The H2a -75dBmO activity factor t is therefore reduced to 0.35. The figure normally used for t is 0.25. Linear Region Overload Region Values for and t are given in Table 20.6. 