1078 THE BELL SYSTEM TECHNICAL JOURNAL, SEPTEMBER 1956 



4.3 Production of Impulse Errors — Nomenclature and Discussion 



To expedite the discussion of impulse errors, the following system of 

 nomenclature is used. Any impulse having the same polarity as the 

 signal pulse is designated as "plus." Those having the opposite polarity 

 are "minus." Two types of errors are produced. First, a spurious pulse 

 may be added to the regenerated signal; this is called an "insertion" 

 error. Second, a signal pulse may be removed, which is called an "omis- 

 sion" error. A "plus insertion" error is a spurious pulse introduced bj^ 

 an impulse having the same polarity as the signal. A "plus omission" 

 error on the other hand is pulse omitted because of a pulse of same 

 polarity as the signal. A "minus omission" error is a pulse omitted be- 

 cause of an impulse having a polarity opposite to that of the signal. 



A positive pulse, if large enough, can produce a spurious pulse at 

 any instant of time not already occupied by a pulse. The only require- 

 ment for the production of such a pulse is that the sum of the impulse 

 and timing wave exceed the trigger level.* On the other hand, a nega- 

 tive impulse cannot produce a spurious pulse but can only cause a 

 signal pulse to be omitted. If a pulse is to be omitted the sum of its 

 amplitude, the timing wave and the impulse must not exceed the trigger 

 level. It would be expected that the number of plus insertion errors will 

 exceed the minus omission errors. This follows from the fact that a 

 spurious pulse may be produced at any point not already occupied by a 

 pulse. On the other hand if a signal pulse is to be omitted the negative 

 impulse must occur in the time interval occupied by the signal pulse. 

 A positive impulse is indirectly responsible for the positive omission 

 error. When a spurious pulse is produced a short interval of time ahead 

 of a signal pulse, the latter may be removed by the inhibiting reaction 

 of the spurious pulse. There is no apparent way in which a minus insertion 

 error can be produced. This is confirmed by the fact that no error of this 

 type was observed in this investigation. Thus we have three types of 

 errors produced: plus insertion, minus omission and plus omission. 



4.4 Results of Impulse Interference Measurements 



Preliminary measurements of errors as functions of impulse amplitude 

 were made using random code. These measured values, shown on Fig. 8 

 exhibit many of the expected characteristics. For example the insertion 

 errors are more numerous than the omission and the threshold of the 

 plus omission errors is considerably higher than those of the other two. 



* The trigger level is normally considered to be the negative dc bias applied to 

 the emitter of the blocking oscillator. There are however other components of the 

 bias that will be discussed later. 



