PRIVATE LINE DATA TRANSMISSION 1453 



signal-to-noise penalty comes from the method of multiplexing an aux- 

 iliary channel needed for word start indications. 



Where a lower bit rate is acceptable, a 750 bit per second amplitude- 

 modulated, double-sideband system which is now available, and has 

 been tested extensively, is somewhat more rugged. It permits operation 

 over the great majority of telephone facilities and will also be described 

 in more detail. 



The most rugged system considered uses frequency-shift transmission. 

 This form of transmission has been used extensively for some time on a 

 multiple-channel, voice-frequency telegraph basis in which each channel 

 is capable of 46 to 74 bits per second. When used in this form to handle 

 high speed data signals, it requires relatively complicated terminal 

 devices because the several channels have to be merged to provide one 

 high speed data system. However, when frequency shift is used as a 

 single channel over an untreated telephone message facility the system 

 promises to be relatively simple and to give a total possibility up to 

 1,200 bits per second according to the type of facility. 



1.2 Summary Table 



These findings are concisely grouped in a summary table, Table I. 

 These entries are based upon present knowledge and are believed to be 

 reasonably accurate, although estimates regarding impulse noise need 

 more extensive checking, particularly in the case of the broad-band, 

 frequency-shift system. 



The table compares relative estimated performances of three broad- 

 band systems among themselves, and ^^•ith a subdivided channel (or 

 telegraph) system. The performances considered cover the effects of 

 noise and delay distortion, and the bit rates considered achievable in a 

 1,000-cycle band and in a 300- to 2,800-cycle telephone band. Some 

 crude approximations covering speech are also given as a matter of 

 interest. 



The noise performance is gi\'en in terms of relati\'e total power capac- 

 ity required in the line for a given error rate in the presence of a gi^'en 

 noise. The double sideband system is taken as reference. Allowance is 

 made in the multiple channel or telegraph system for occasional peaking 

 caused by temporary unfavorable phasing. In this part of the comparison 

 a 12 channel system has been assumed. This is about as many channels 

 as can be used on a telephone facility that has heavy impulsive noise. 



The delay distortion figures represent some present ideas on good 

 engineering design in the allowable impairment of signal-to-noise ratio. 



