1476 THE BELL SYSTEM TECHNICAL JOURNAL, NOVEMBER 1957 



used over wire circuits. Hence its exact performance, particularly with 

 impulse noise, is only estimated. 



On occasion not all of the band illustrated in Fig. 5(d) is allowed for. 

 This leads to an increase in distortion of the signal, which has some simi- 

 larity to a very close-in echo. It uses up some of the additional noise 

 immunity provided by the FM, as an engineering compromise. 



Another direction along which the FM system may be practically 

 extended is to use four instead of two (marking and spacing) frequencies. 

 This would double the bit capacity at the expense of only a moderate 

 widening of the frecjuency band and somewhat tighter requirements on 

 noise and delay distortion (but not of level regulation, which would be 

 required for a similar extension of the AM signal) . 



3.2.4 Multichannel Systems 



It is possible to divide up the entire frequency band available into a 

 number of separate channels and use any one of the various carrier sys- 

 tems which have been described, in each individual channel. This may be 

 done because the nature of the information transmitted may be better 

 adapted to the narrow channel, as in conventional telegraph. It permits 

 certain elements of flexibility in layout, and offers certain noise advan- 

 tages (and also disadvantages) as discussed in the next section. 



In an idealized way one can proportion the various allowances for 

 nominal effective band, roll-off, guard band, and swing (FS) in the same 

 proportion in which they would occur in a single broad channel over the 

 whole facility. Thus no frecjuency space would be lost by the subdivision. 

 In practice, however, subdivision usually does lead to some actual loss 

 in the frequency space. 



A significant limitation to frequency subdivision lies in nonlinearity of 

 the facility. This leads to modulation products between the various 

 channels, which interfere with other channels. 



In the case of voice frequency telegraph and other multiple channel 

 systems the modulation effects are mitigated by allocating the carriers 

 at odd multiples of a basic frequency. That is, any given carrier / is set 

 at / = n/v, where n is odd and A; is a basic figure. Then the three second 

 order modulation products are 



2/ = 2nk, 



/i + f- = {fh + n-i) k, 



/i - fi = (wi - no) k. 



