PRIVATE LINE DATA TKANSMISSION 1473 



supplies are locked in a constant frequency network, frequency shifts 

 of some ±2 cycles may be expected between input and output. 



For paired cables, these are known as Kl andK2 systems. For coaxial, 

 they are LI and L3, for open-wire, J, and for microwave radio, TD-2. 



3.1.5 Other Broadband Carrier Systems 



An carrier system has been developed for open wire, and combina- 

 tions of it are used with N for open-wire and carrier. These are com- 

 pandored systems. 



3.2 Use of Bandwidth 



This section examines the more important factors which affect the 

 choice of how the available bandwidth of a facility is to be used, either 

 in one band or a subdivided band. 



3.2.1 Baseband Transmission 



This is the simplest type of transmission. It is used in telegraph loops 

 and other short distance telegraph transmission. A mark is indicated by 

 placing marking voltage across the wire line, and a space by placing 

 spacing voltage. In the simplest systems the latter is zero. In "polar" 

 systems it is the negative of marking voltage. 



The frequency spectrum of the signal runs down to and includes dc, 

 as illustrated by the solid lines in (a) of Fig. 5. 



With many transmission facilities it is difficult or impossible to trans- 

 mit the dc; i.e., the circuit cuts off as is illustrated by the dotted lines. 

 In such cases it is impossible to distinguish between a permanent mark 

 and a permanent space. 



Extra pulses can, however, be added to the signal to insure that marks 

 or spaces are not permanent, but are relieved by the opposite signal in 

 some maximum interval of time. In such cases the received signals can 

 be clamped on mark or space signals and the opposite condition can be 

 readily distinguished. This is sometimes called "dc restoration," and 

 strictly speaking the system ceases to use baseband transmission. It 

 may be designated as "modified baseband transmission." Methods other 

 than clamping have been suggested for dc restoration. 



Reverse pulses can be systematically inserted after each mark or space 

 pulse, according to various patterns.'^ Two suggested are "dipulse" and 

 "dicode" pulses. Such signals approach carrier signals, which are dis- 

 cussed below. 



The principal weakness of baseband transmission appears when it is 

 sent over C carrier or other single sideband telephone facilities, where the 

 recovered signal may vary in frequency from that sent. This causes a 

 distortion of the received pulse which makes it difficult to recognize. 



