PRIVATE LINE DATA TKANSMISSIOX 1475 



enough to prevent such an overlap. This is indicated in Fig. 5(c). It does 

 not prevent third order modulation effects. 



It has been found necessary to allocate frecjuency bands thus to avoid 

 the overlap discussed in the transmission of high grade telephotography 

 over telephone type facilities. It has also been noted that allocation ^vith 

 such overlap was undesirable in some data transmission experience. 

 However, the question has not been resolved in complete detail. For the 

 data service under consideration, which is expected to show a very low 

 error rate, it is deemed conservative to allocate bands without the over- 

 lap. 



This conclusion then leads to wasting a certain part of the lower fre- 

 quency range. It is still possible, however, to use this range for an auxil- 

 iary signal, as in the system illustrated in Fig. 2, if the auxiliary signal 

 occurs only during word starts. 



The double sideband frequency range, as indicated in Fig. 5(b) or 

 5(c), is about twice the baseband range of Fig. 5(a). It is possible to re- 

 duce this extension by cutting down one of the sidebands to a "vestige" 

 of itself and sending carrier at a reduced level, as indicated by the diag- 

 onal dotted line about the carrier in Fig. 5(c). This was proposed by 

 Nyquist.'^ It is done at the expense of an increased vulnerability to 

 noise, which in total amounts to some 5 or 6 db in certain typical cases."' ^' 

 In Section 2.1.3 discussion was given to account for 3 db of this. In the 

 references cited herewith it is noted that vestigial sideband transmission 

 is accompanied by an interfering component (called a "quadrature com- 

 ponent") which accounts for the other 2 or 3 db. 



3.2.3 FM Carrie?- Transmission 



Certain additional immunity to noise is gained by the use of frec{uency 

 modulation (or "frequency shift") of the carrier. The immunity which 

 can be obtained against impulse noise can be even greater than that 

 against random noise, provided that the receiver is precisely tuned. This 

 was noted in Section 2.4 in connection with voice frequency telegraph. 



The noise immunity obtained from FS is in part due to the use of a 

 higher average power and is at the expense of a wider frequency band 

 as illustrated in Fig. 5(d). In addition to all the band that is used for a 

 double sideband system, a space must be allowed for the swing. FS is 

 also much less vulnerable to sudden level changes than DSB and thus 

 may well be preferable to DSB for medium bit -rate service. As shown in 

 Table I, these advantages can be obtained with only a small sacrifice of 

 bit rate compared to DSB for equal bandwidths. 



So far the single channel broadband FS system has not been generally 



