tion products falling into the tion can be made for unlinearity contributions using Equations channel of interest for each product (A + B, 2A - B, etc.). 20.20 to 20.22, where W'(x) is the approximate noise power in a The total intermodulation noise in any particular channel is the selected channel, Pm(x) is the power of the (x) product in dBmO for summated power contribution from each of these products. 0dBmO fundamentals, U (x) is the factor obtained from Figure Bennett's formula, rearranged to give the weighted noise con- 20.13 for a channel at frequency f. tribution W(x) within a specified channel is as in Equation 20.19. W(A+B)=P_(A+B) +U*(A+B)-33.5 dBmOp (20.20) (20.19) The suffix (x) refers to the type of intermodulation under con- W (A-B) = P_(A-B) sideration i.e. H(A+B) etc. (Table 20.7) In Equation Pm(x) is +U*(A-B)-33.5 dBMOp (20.21) Table 20.7 Types of intermodulation (A+B-C)P(A+B-C) +U*(A+B-C)-47 dBmOp (20.22) Type(X) 20.7.4.6 Weighted noise power in pWOp The total weighted intermodulation noise power can be determined from Equation 20.23, where X = all products, (A+B), (A-B), etc. (20.23) A+B+C A+B-C 20.7.4.7 Determination of unlinearity noise using spectral A-B-C densities The problem with Bennett's method is the difficulty of dealing with the power of the intermodulation product (x) in dBmO at the output shaped frequency spectra (pre-emphasis at the output of Line re- of the system for 0dBmO fundamentals; k(x) is the speech tone peaters for instance.) modulation factor (a factor in dB to convert the sinusoid to A method using spectral densities (Bell, 1971) overcomes this the equivalent intermodulation product power for bands of 4kHz difficulty and can be extended to include the shape of the H2a and gaussian noise); P is the power in a single average talker in dBmO H3a across the band. (see Table 20.6); g is the standard deviation of the distribution of all From Equation 20.10 the ratio of the total second order distortion talkers from loud to soft (see Table 20.6); e(x) and d(x) are factors volta