oad requirement is +29.73dBmO or 20.11.2 Multichannel load increase 3.2dB higher. It will therefore normally be necessary to drop the transmission Companding gives an increase in the peak multichannel load. The level by 3.2dB (WdB) and to raise it again before expansion to avoid increase is mainly determined by the compression ratio, but the overloading the system (see Figure 20.17). unaffected level and both the attack and decay times all impact on This adjustment to the transmission level will reduce some of the the overall loading. companding advantage by increasing the thermal noise contribution Considering the main contribution from the compression ratio, the at the output of the expandor where the extra WdB gain is applied. power in an single average talker is given in Table 20.6 where Pvo = -12.9dBmO, g=5.8 = and t = 0.25 to derive L =-15dBmO. 20.11.3 Compandor noise advantage When compression is applied to the signal then the parameters above are modified by the compression ratio to give From Figure 20.16 it can be seen that while no speech is transmitted P vo = -12.9/2dBmO, g = 5.8/2 and t = 0.25 to derive Lo= = the noise power will be subject to an attenuation of 25dB at the -11.48dBmO. expandor output. Compressor Expandor Correcting Elements WIDE BAND Channel Channel (LINE SYSTEM) Inputs Outputs Frequency Translation Frequency Translation Modulators Demodulators Figure 20.17 Transmission level correction for companding load increase 20/18 Frequency division multiplexing The theoretical noise advantage will therefore be given by Equa- 20.12.1 Through connection filter tion 20.38. In this situation the band of interest has to be cleaned before passed Noise advantage = 25 - W dB (20.38) back into the modulation process. From Figure 20.1 and Figure 20.19 a typical demodulator filter Fb selects the required band of However, the circuit noise while speech power is transmitted is frequencies (band N) from the high order broadband of channels. virtually unaffected by the compression/expansion process and Becau