n.WP Wlirill AM) 77vM.V.SM//.S-.S70.V PERIORM .\ \'CE >27 



assunu'S tluil llic peak intcrlVrencc is iiiar,L!;iiuil lor all hamiwiillhs (<^ dh 

 down). 'riiel)an(l\vi(lthrati()istlicii2( 1 + ' ). It was i)rcvii)usly found 



thai in PCM-KM, with peak interference M db down, llie radio bandwidth 

 nuist be {b + 1 )/''/,. Since llie radio bandwidth of PC'M-AM is 2F„, the 

 bandwidth ratio is {b + l)/2. C^omparing these bandwidth ratios we see 

 that the PPM bandwidth required to operate in the presence of marginal 

 interference is nearly two times that required in PC^M-FM. Furthermore, 

 this PPM bandwidth ratio applies to marginal fluctuation noise wdiereas in 

 PCM-FM a more favorable result was obtained. 



Fig. 19— FDM-FM, Fluctu.atioiN Noise 



When a group of channels in frequency division is transmitted by fre- 

 quency modulation, the addition of channel voltages is translated to an 

 addition of instantaneous frequency shift. The non-simultaneous load 

 advantage applicable to a multichannel amplifier for frequency divided 

 channels thus becomes an advantage in reduction of total frequency swing 

 as compared to the sum of the individual peak frequency swings of the 

 channels. The numerical db increments versus number of channels listed 

 in Table I should, how^ever, be modified for the following reason: The 

 tluctuation noise spectrum in the output of an FM detector is not uniform 

 with frequency, and hence the noise is unequally distributed among the 

 channels. In order to obtain the same noise in all channels it is necessary to 

 taper the signal levels in such a way that the full load frequency swing pro- 

 duced by one channel is proportional to the frequency of the channel. The 

 frequency swing corresponding to full load in the top channel is therefore a 

 larger part of the maximum instantaneous swing required for the group than 

 the swings corresponding to lower channels. The result is, in effect, phase 

 modulation. The multiplex addition factors for tapered level channels 

 have not been determined experimentally. We have assumed here a 3 db 

 reduction in the power capacity values listed in Table I. These reduced 

 values then give the incremental capacity referred to full load on the top 

 channel. Curves are shown for 100, 500 and 1(X)0 channels. On account of 

 the multiplex addition factor, it is not possible to obtain results for other 

 numbers of channels from one curve by simply changing the frequency 

 scale. 



The derivation of these curves is straightforward but leads to an expres- 

 sion for the required bandwidth as a root of a cubic equation. As in the 

 case of Fig. 16 we shall discuss the FDM-FM curves by checking them 

 numerically. We have assumed that the channels are tapered in level and 

 that we have, in fact, phase modulation with its consequent flat base- 

 band noise distribution. To check the 60-db point on the 1000-channel 



