ge to the fundamental signal at the output of the amplifier or to account for the relationship between the power in the talker (the system is given by Equation 20.24, where V. is the system input fundamental signal) and the resulting intermodulation product (e(x) signal (assumed Gaussian). is a factor to modify the talker volume P and d(x) to modify the standard deviation of talker volumes g); t is the transmission activity (20.24) a2 vi factor or the probability that a particular channel is active (see Table 20.6); u(x) is the number of channels involved in forming the particular intermodulation product and therefore tu(x) is the prob- ability that the particular intermodulation product from a particular The terms in Equation 20.24 can be expressed as a power in volts set of channels is present; C is the psophometric weighting correc- squared where S (f) and S,(f) are the power spectral densities in tion factor for 4kHz and is 3.6dB. U(x) is the number of intermodu- volts squared per Hertz. S (f) is the figure for the multichannel input Noise contributions 20/13 Number of channels in system - N n,M and N are all intergers of 4kHz. Number of channel spaces - n Channel of interest - M System start System top frequency frequency 2nd Order (M+n)/N (M-n)/N 0.25N 3rd Order 0.4N A+B-C 0.20N A-B-C 0.3N2 0.15N 0.10N 0.1A2 0.05N (M+2n)/N 0.125N 0.100N A+ B+C 0.075N2 0.050N 0.025N2 (M-2n)/N (M+2n)/N 2A- B (M-2n)/N * U - 0 outside range. Figure 20.12 U(x) values for large sysems 20/14 Frequency division multiplexing U* (x) Values N=number of 4kHz channels. A+B-C 10logN 10log 10log 10log 10log Frequency Figure 20.13 Approximate intermodulation factors U*(x) signal voltage, Vi, and S,(f) for the input signal voltage squared, vi Measurement of noise contributions This is shown in Equation 20.25. This is also referred to as white noise measurement. Both the thermal noise contributions and the intermodulation noise contribu- aS2(f) (20.25) tions can be measured directly with one test. This allows rapid field evaluation of instal