1460 THE BELL SYSTEM TECHNICAL JOURNAL, NOVEMBER 1957 



circuits, and the same problem is appearing with the proposed data 

 signal. The problem is discussed in more detail in subsequent sections. 



6. The specific signal suggested has the disadvantage that multiple 

 levels must be discriminated by the receiver. This increases vulnerability 

 to noise and level changes, as will now be discussed in some more detail. 



The specific signal which has been suggested, as noted before, is 

 outlined in Fig. 1. This signal comprises several levels. In the first place 

 it includes a word start indication, on a separate level from the mark 

 and space indications. In the second place the lowest amplitude level 

 (normally a space, but a mark in the "upset" signal, as has been noted) 

 is not made zero, but 0.25 the amplitude of the word start indication 

 or "synchronizing" pulse. The need for this is explained in the discussion 

 on vestigial sideband, below. 



Consider for the moment a signal of only two levels; these can be 

 taken as 1 volt and volts respectively. The discrimination between 

 them without error requires that an instantaneous noise pulse at this 

 time be kept to less than | volt. In these terms this represents an S/N 

 ratio of 6 db. 



The use of 0.25 volt minimum signal means that the amplitude range 

 between maximum and minimum is reduced from 1 volt to 1 — 0.25 = 

 0.75 volts. The maximum allowable noise pulse must be 0.75/2 = 0.375 

 volts for this signal. This is an 8.8-db S/N ratio, as compared with the 

 previous 6 db. It represents a handicap of approximately 3 db which 

 must be accepted as part of the price of the increased bit transmission 

 rate permitted by the use of vestigial sideband as compared with double 

 sideband transmission. 



An additional penalty comes from the use of three as against two 

 signal levels. The spacing signal level is set at 0.625 volt, midway be- 

 tween 1 and 0.25 volt. Discrimination between synchronization and 

 spacing signals can tolerate noise pulses of (1 — 0.625)/2 = 0.1875 

 volt. This amplitude is 15 db below synchronization level. Discrimina- 

 tion between spacing and marking signals tolerates maximum noise 

 pulses of (0.625 - 0.25)/2 = 0.1875 volt. This is again 15 db below 

 synchronization \eye\. Thus the signal tolerates a 15-db S/N ratio 

 between synchronization level and the level of maximum noise pulses. 



The difference between the approximate 9-db S/N for the two level 

 vestigial sideband signal and the 15 db represents the 6-db handicap 

 caused by the multiple level discrimination in the signal. This is the 

 price paid for a distinctive word start indication. 



The price also applies to sudden level changes. In a two level signal 



