1462 THE BELL SYSTEM TECHNICAL JOURNAL, NOVEMBER 1957 



signal amplitude. The receiver used AVC both for amplitude control 

 and space-to-mark slicing level adjustment. 



The timing of the receiver sampling instants to determine mark and 

 space indications was determined by a flywheel circuit which operated 

 from space-to-mark and mark-to-space transitions in the signal. This 

 is similar in principle to the old Baudot quadruplex arrangements in 

 telegraphy. It was a synchronous system and required a dummy signal 

 for a lining-up period pre\'ious to the actual transmission of information. 

 The lining-up was automatic but required some 15 to 50 milliseconds 

 (25 to 80 signal elements). 



A number of experiments with the system were made on actual lines. 

 Most of these showed successful transmission, though the error rates 

 were not measured quantitatively. The test showed the signal margin 

 against error on a cathode ray oscilloscope. The wave trace indicated 

 displacements both in the signal amplitude and in the timing of the 

 sampling instant. This margin has been found to correlate reasonably 

 well (in an inverse relationship) with the calculated delay distortions 

 of the circuits used. It also corresponds reasonably well to theoretical 

 expectations.'^ 



The system reported on by the Lincoln Laboratory of MIT^ shows 

 much general similarit}^ to the above. Perhaps the most distinctive fea- 

 ture of difference is in the use of a word start indicating or synchronizing 

 signal, in the form of the high le\-el pulse discussed above and somewhat 

 similar to that used in television for scanning-line synchronization. 



2.3 Double Sideband Systems 



A prototype of these systems has been described by Horton and 

 Vaughan.^ Several models have been derived from the prototype which 

 differ from it, somewhat, in several respects. Large portions of the sys- 

 tems are not germane to the present discussion, and only a brief de- 

 scription will be given of the signals. 



An outline of the signal spectrum for the most recent of these derived 

 models is illustrated in Fig. 2. The main signal is handled on a carrier at 

 1,500 cycles. The bit rate is 750 per second, and the nominal effectiA'e 

 band is shown as ±375 cycles. A schematic roll-off is indicated. The 

 words in this system are of about 100-signal element length, of which 8 

 are used for synchronization. The synchronization pattern involves a 

 .3-signal element ready pulse on the 600-cycle carrier, simultaneous with 

 the first 3 of the 6-signal element marking pulse on the 1,500-cycle 

 carrier. Following this comes a spacing bit, then another marking bit 

 on the 1,500 cycles only. The next bit is the first information bit. 



