798 THE BELL SYSTEM TECHNICAL JOURNAL, JULY 1953 



between the two signals, the limitations on achievable maximum trans- 

 mission levels or magnitudes arise from certain types of second order 

 modulation products rather than from optimizing signal-to-noise per- 

 formance as in the all -telephone application. One of these types consists 

 of sum products of cross modulation between telephone and television 

 signal components. These form bar patterns and, in so far as signal-to- 

 interference ratio is concerned, are independent of the television signal 

 magnitude. Thus, adjusting such products to equal the appropriate 

 requirement has the effect of setting the maximum permissible magnitude 

 or level of the telephone signal. The second type of limiting product is 

 due to difference frequencies formed by cross modulation among the 

 television signal components. These fall into the telephone channels 

 and, after the telephone level has been set as described above, permit 

 calculation of the maximum permissible television signal magnitude. 



With signals set at —21 db level for telephone and +6 dbm unmodu- 

 lated carrier for television, all of the critical products discussed in sec- 

 tion 2.15 above and illustrated on Fig. 8 have adequate margin. The 

 40 db signal-to-noise objective for 4,000-mile television transmission is 

 met with about 2 db margin and long haul (4,000 mile) message channels 

 meet the -f 29 dba at the — 9 db level objective with about 5 db margin. 

 A margin of about 5 db is also realized with respect to repeater overload 

 performance. 



The single frequency pilots are adjusted to have the following values 

 of power at the output of a transmitting amplifier: 



7266 kc -16 dbm 



8320 kc -26 dbm 



All others -36 dbm 



With these values, modulation products produced by cross modulation 

 among the pilots and message and television signals all meet the ap- 

 propriate objectives. 



2.17 Frogging of Message Circuits 



When signals, either message or television, are transmitted over long 

 distances through many amplifiers in tandem, the accumulation of 

 modulation products along the line becomes an important system prob- 

 lem for two reasons: (1) the accumulation of certain types of third order 

 prcxlucts t(»nds to follow a direct or in-phase law and (2) the distribu- 

 tion of modulation products over the band produces more modulation 

 noise in certain parts of the band than in others. Both of these cumula- 

 lion problems are alleviated if, at intervals along the line, the signals 



