1458 THE BELL SYSTEM TECHNICAL JOURNAL, NOVEMBER 1957 



extends down to 1,200 cycles, and roll-off band down to about 1,000 

 cycles. The frequency space above the vestigial band and below the 

 roll-off band is essentially "dead" space, free of signal energy. 

 2. The signal comprises three components: 



(a) synchronization, (or start), to indicate word separations, 



(b) data, or the actual information, and 



(c) timing, to mark out the successive bit intervals in the data. 

 The signal is represented in Fig. 1 as the envelope of a carrier. The 



synchronizing signal has an amplitude of one and a duration of one 

 signal element. Data spacing signals have an amplitude of about 0.63 

 and a duration each of one signal element. Data marking signals have 

 an amplitude of about 0.25 and also one signal element duration. It 

 will be noted that this is an "upset" signal, in which spaces have more 

 power than marks. The two signal elements immediately before, and 

 again the two signal elements immediately after, the synchronization 

 signal are always to be spacing. A timing signal is not actually sent, 

 but instead the synchronizing signal is used to pull the phase of a highly 



1.00 



0.63 



0.25 



SYNCHRONIZATION 



SPACE 



•MARK 



I 

 I 

 I 

 I 

 I 

 I 

 I 

 I 

 I 

 I 



ZERO CARRIER 



MAXIMUM 

 CARRIER 



_A 



Fig. 1 — Multiple level signal. 



