ied throughout the seasonal changes of cable loss and thus the traffic most commonly at group and supergroup level. power level at the repeater output can be maintained close to the Thermistor Controlled Attenuator (nominally -AdB) Pilot Level at Traffic out Traffic in Traffic out (dB rel. nominal) Pilot extract Pilot detector and filter DC amplifier Pilot level at Traffic In (dB rel. nominal) Figure 20.8 Automatic gain control (AGC) Noise contributions 20/9 thermal and intermodulation noise design optimum. The regulating This physical entity within a network has all but disappeared and pilot is transmitted at -10dBmO. (See also Section 20.14.3.) a point of reference is now normally taken as the virtual outgoing Commonly used regulating pilot frequencies are given in Table switch point and is set at -4dBr. From the transmissions point of 20.4. view the audio output from the channel translating equipment is adjusted to suit the required stated dBr level at that point and this Table 20.4 Commonly used regulating pilot frequencies becomes the level to which all the transmission levels within the system refer. Transmission levels are referred to in dBr. Coaxial system Pilot frequency 20.7.1.2 1.3MHz, 300 channel 1364kHz 4MHz, 960 channel 4092kHz The power at a various points in the transmission are referred to in dBm. If a test signal is injected into a 0dBr point at 0dBm then the 4MHz, 900 channel 4287kHz test signal level throughout the transmission can be referred to in 12MHz, 2700 channel 12435kHz It is sometimes the case that for noise calculations where the 18MHz, 3600 channel 18480kHz transmission level is required in dBm rather than dBr it is assumed 60MHz, 10800 channel 61160kHz that the power at the 0dBr point is 0dBm. 20.7.1.3 20.6.2.2 Frequency comparison pilots This is the power of a signal in dBm referred to a point where the The frequency comparison pilot is used at each translation point to transmission level is 0dBm. maintain the carrier generation master oscillators to a high degree For instance if 