792 THE BELL SYSTEM TECHNICAL JOURNAL, JULY 1953 



produced by the television carrier and adjacent line scan multiples are 

 by far the strongest. 



The energy in a disturbing telephone channel tends to be concentrated 

 near the 1,000-cycle point in the voice frequency band. By careful 

 choice of the television carrier frequency, the difference products pro- 

 duced by cross modulation between telephone signals and the high 

 magnitude television signal components can be made to fall at fre- 

 quencies such that the high energy portions of these products are greatly 

 attenuated by the cut-off characteristics of channel filters. 



The message channels are spaced at 4-kc intervals controlled by car- 

 rier frequencies which are multiples of 4 kc. To obtain the maximum 

 advantage from the channel filter cut-off characteristic as described 

 above, it was found desirable to set the television carrier frequency 1 

 kc below a 4-kc multiple. A direct result of this allocation is a gain of 

 12 db in television signal-to-noise performance over what could be re- 

 aUzed if the carrier had been set at a 4-kc multiple. Such an allocation 

 would have required a 12 db lower magnitude of television signal in 

 order to meet the message channel objectives. 



2.153 Addition of Modulation Products Along the Line 



It has been established by analysis and experiment, that in a multi- 

 repeater system second order modulation products tend to accumulate 

 on a power basis while certain third order products tend to add on a 

 direct or voltage basis. This direct addition of third order products 

 depends on the slope of the phase curve being the same over small fre- 

 quency intervals from repeater to repeater. In multi-channel telephone 

 systems, the locations of channels in the frequency band are shifted at 

 intervals along the line to avoid this direct addition of third order prod- 

 ucts. In the combined telephone-television application of the L3 system 

 the A+B — C product illustrated in Fig. 8(g) is formed. Since the B 

 and C components are television line scan multiples which cannot be 

 shifted in location, certain components of this type product would add 

 directly in a 4,000-mile system. If this were allowed to take place the 

 requirements would be exceeded by many db. However, by placing the 

 delay distortion equalization only in the television band at approxi- 

 mately 200-mile intervals the phase of these products can be shifted 

 so that rms addition of products accumulated over several 200-mile 

 links of the system may be assumed. 



2.154 Wave Form of the Transmitted Television Signal 



Early studies of L3 led to the conclusion that the most economical 

 method of transmitting the television signal would be by amplitude 



