a signal power is -80dBm at a -30dBm trans- of frequency stability. The pilots are placed at the low end of the mission level point, then the signal is defined as -50dBmO. cable frequency spectrum for minimum phase error shifts. These phase shifts occur from the cable and equalisers. Some commonly 20.7.2 Psophometric weighting used frequency comparison pilots are given in Table 20.5. When noise, either thermal or unlinear noise, is added to a telephone Table 20.5 Commonly used frequency comparison pilots conversation, the degree of annoyance or the effect on the intelligi- bility of the conversation is not the same for all frequencies of added noise. A weighting curve has been constructed to characterise this effect. Coaxial system Pilot frequency Various curves exist (CCITT Bell System C message etc.) all 1.3MHz, 300 channel 60kHz giving weighting factors of 2 to 2.5dB. The UK adopt the CCITT curve in Figure 20.9 providing a weighting factor of 2.5dB. This 4MHz, 960 channel 60kHz 4MHz, 900 channel 300kHz Frequency kHz 12MHz, 2700 channel 308kHz 18MHz, 3600 channel 564kHz 60MHz, 10800 channel 564kHz Noise contributions Noise is the largest source of degradation to analogue systems. Two contributions have to be considered, thermal and unlinearity -10dB noise. To evaluate the noise performance of an equipment both noise source contributions are calculated in pWOp and summated. 20.7.1 Definitions -20dB There are three power levels in common use, dBr, dBm and dBmO. Figure 20.9 Psophometric weighting curve (CCITT) 20.7.1.1 The OdBr point is the level at a reference point within the system factor allows that for a 4kHz channel, considered over the speech that all the transmission levels refer. This used to be the 2 wire audio bandwidth of 3.1kHz, 2.5dB more Gaussian type noise can be point or the 2 wire point of origin, as it was sometimes called. tolerated from the system when weighting is applied. 20/10 Frequency division multiplexing When this correction is applied the suffix 'p' is added to the noise more complex i