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BELL SYSTEM TECHNICAL JOURNAL 



similar to the problem just discussed except instead of uniform spacing 

 between possible transitions of 1/50 second, the transitions may be 

 spaced at intervals of either. 022 second, .031 second, or combinations 

 of these intervals, and the frequency of repetition is 6.13 instead of 

 5 per second. 



As indicated in Fig. 1, there are six teletypewriter characters (Blank, 

 T, 0, M, V, and Letters) which correspond to 6.13 cycle reversals 

 biased by certain amounts. It will now be shown that if these six 

 characters can be transmitted without distortion, the other 26 charac- 

 ters will also be transmitted without distortion. The method is the 

 same as that used in the preceding problem. 



START 

 •^0 



^1 



^2 



^3 



U 



I'S 



^6 



Fig. 29 — 7.42-Unit code start-stop teletypewriter signal. 



Figure 29 indicates any teletypewriter signal where io = 0, and 

 i'e = 1. t\, i-i, H, i\, ib, may have values of or 1 depending on the 

 particular character. The Fourier series for the received current over 

 a circuit with a transfer admittance Y having unit value at zero 

 frequency, is 



m = 



ii + ij + iz + ij + ib + 1-42 

 7.42 



+ - X — ~ \ii[_s\n n(o)t — a) — sin w(co/ — 2a)] 



+ z'zCsin n(o3t — 2a) — sin n(ut — 3a)^ 

 + 7'3[sin w(oo/ — 3a) — sin n(o:t — 4a)] 

 + ?"4[sin n{oot — 4a) — sin n(wt — 5a)] 

 + ibl^sin n{wt — 5a) — sin w(co/ — 6a)] 

 + 1 [sin n{wt — 6a) — sin n co/]}. 



Suppose that the transmitted frequency band is limited to 6 times the 

 fundamental, i.e. « = 1 to 6, and F„ adjusted so that the characters 

 "Letters" V, M, 0, T and "Blank" are transmitted perfectly. 



The transfer admittance will now be determined for this case. The 

 expression for /(/) may be written for the characters just mentioned, 

 and would have the value 1/2 at ojt = a, for "letters," at cot = 2a for 

 V, at co/ = 3a for M, etc. Accordingly there result six equations which 

 may be simplified as follows : 



