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THE BELL SYSTEM TECHNICAL JOURNAL, MAY 1954 



tinction between the presence and absence of pulses in a random pulse 

 train. In the particular case when pulses are transmitted at the minimum 

 interval ri = l/2/i possible without intersymbol interference in the ab- 

 sence of a low-frequency cut-off, the pulse train will ultimately vanish 

 when an infinite seciuence of pulses of one polarity is transmitted, as 

 illustrated for the last case in Fig. 33. 



The number of pulses of one polarity, or nearly all of the same po- 

 larity, which can be transmitted before the limiting condition illustrated 

 in Fig. 33 is approached depends on the extent of the low-frequency 

 cut-off. If the low-frequency cut-off is inappreciable, this number may 

 be sufficiently great so that the probability of encountering such a 

 sequence in a random pulse train and resultant errors in reception may 

 be so small that it can be disregarded. The reciuirement of the low- 

 frequency cut-off which is necessary to this end is evaluated below for 

 pulses transmitted at intervals n = l/2/i . 



TRANSMISSION FREQUENCY 

 CHARACTERISTIC WITHOUT 

 LOW-FREQUENCY CUTOFF 



IMPULSE CHARACTERISTIC 



WITHOUT LOW-FREQUENCY 



CUTOFF 



P-(^ 



LOW-FREQUENCY CUTOFF 

 COMPONENT 



Fig. 32 — Separation of low-frequency cut-off componente A-Ao and P-Po in 

 transmission frequency and impulse characteristics. 



