176 THE BELL SYSTEM TECHNICAL JOURNAL, JANUARY 1951 



circuits. They are due to the transmission of reflected energy from points of 

 impedance irregularity. They are troublesome factors whenever the time of 

 transmission between the point of reflection and the disturbed subscriber is 

 appreciable, especially when the use of repeaters prevents the attenuation 

 of the reflected energy from being negligible in magnitude. Increasing the 

 velocity of wave propagation is a sure procedure for reducing echo effects. 

 Control of the impedance irregularities which cause them is also important, 

 but cannot be carried to a sufficiently fine degree in long, low- velocity cir- 

 cuits. For the satisfactory control of echo effects in very long, four-wire 

 type, high- velocity, (H44-25) loaded circuits, the use of auxiliary devices 

 known as "echo suppressors" was found to be very advantageous.^^ 

 (e) Basic Networks and Filters: The previously mentioned basic networks^^ 

 which simulate the impedances of the loaded circuits are vitally necessary 

 features of the balancing lines that are used with the repeaters employed on 

 two-way speech circuits. Relatively simple types of networks provide satis- 

 factory impedance simulation up to a high fraction of the cut-off frequency, 

 in loaded circuits having regular coil spacing and uniform coil inductances. 

 At frequencies near the loading cut-off, however, it is not feasible to provide 

 basic networks which satisfactorily simulate the impedances of the loaded 

 circuits. Consequently, in order to avoid large repeater unbalances which 

 would cause objectionable singing at these frequencies, it is necessary to 

 associate with the repeaters low-pass type electric wave filters^^ which have 

 cut-off frequencies appreciably lower than the loading cut-off frequencies. 

 Usually this cut-off frequency differential is of the order of 10% or more of 

 the loading cut-off. The reduction of transmitted band width caused by 

 these filters aggravates the frequency attenuation distortion effects pre- 

 viously discussed. This use of filters with the repeaters thus became a 

 contributory factor in the need for raising the loading cut-off frequency in 

 long repeatered circuits. 



By reducing the transmitted frequency band width, however, the filters 

 used with the repeaters have favorable effects in reducing the high-frequency 

 velocity distortion caused by the loading. The filters used with the repeaters 

 on long H44-25 circuits, subsequently described, had cut-off frequencies 

 substantially below the loading cut-off frequencies primarily for the purpose 

 of controlling velocity distortion impairments. 



7.3 Improved Loading Systems 



(a) General: To sum up the foregoing, the theoretical analyses of the limi- 

 tations of the standard toll cable loading pointed definitely towards an in- 

 crease in cut-off frequency and in the velocity of wave propagation. Since 

 the state of the art was such that theoretical studies"- '^ alone could not 

 determine the magnitudes of the changes that would be required, extensive 



