ng func- approach can be applied to the more advanced low rate encoding tion to take account of the sensitivity of the human ear to different systems e.g. 8kbit/s coders where the distortion is not quantising frequencies. Measurements made with a psophometer are generally distortion in its true sense. However it is important to have a expressed in dBmp where the 'p'indicates the use of the psophome- planning measure for the distortion introduced by these systems as tric weighting function. they can have a significant effect on the quality seen by customers Subjective tests have indicated that for telephones on average and the qdu approach is generally used. lines the noise values need to be less than -65dBmp to have a negligible effect on customer opinion. As the overall noise value 30.4.5 Propagation delay increases there is a worsening in customer opinion although the rate of worsening depends on the overall loudness rating of the connec- Propagation delay (sometimes called absolute delay) is introduced tion. Supplement 3 to the CCITT P series Recommendations by all the transmission components of an overall connection. The (CCITT, 1989b) shows typical results for combinations of noise and principal contributors in modern networks are long terrestrial CO- axial cable or fibre systems, digital switches, satellite and radio systems and low rate encoders. Delay has two principal effects on 30.4.3 Loss/frequency distortion communication; it increases the subjective disturbance due to the presence of an echo signal, and high values of delay can disrupt conversational fluency. Loss/frequency distortion is determined by the transmission loss at CCITT Recommendations G.114 covers recommended limits for frequencies other than the reference frequency of 1000Hz. In mod- end to end propagation delay where it is recommended that one-way ern networks the main contributors to loss/frequency distortion are delays should not exceed 400ms. Delays of less than 400ms may be the telephone set characteristics, 2-wire local lines and 