230 THE BELL SYSTEM TECHNICAL JOURNAL, JANUARY 1957 



Crosstalk. 



The minimum equal-level crosstalk attenuation should be 61 db for 

 all sources of potentially intelligible crosstalk; this was accepted as a 

 target for both near- and distant-end crosstalk. Although go-to-return 

 crosstalk is not important for telephony (it appears as sidetone) and a 

 limit of 40 db is satisfactory even for voice-frequency telegraphy, it 

 assumes great importance for both-way music transmission ; also, it was 

 desired to be non-restrictive of future usage. 



Assessment of Requirements 



The design of the high-frequency path to meet the agreed require- 

 ments involves consideration of: 



(a) Noise, including fluctuation (resistance and tube) noise and inter- 

 modulation. 



(6) Wide-band frequency characteristics, including the effects of the 

 directional filters at the terminal and in the repeaters. 



(c) Variations of (a) and (6) in respect of temperature and aging. 



The noise requirement is by far the most important factor in the 

 design of the line system. 



The choice of route and cable having been made, the total loss was 

 known and it was necessary to determine the minimum number of re- 

 peaters to compensate for this loss and to meet the noise requirement 

 with adequate margin for inaccurate estimates of cable attenuation 

 after laying, temperature variations, aging and repairs. An attempt to 

 achieve the necessary gain with too few repeaters would result in exces- 

 sive noise. 



Design of the amplifiers in the British repeaters is such that, with 

 both forward paths in operation, the overload point is about +24 dbm, 

 and with a loading of 60 channels in each direction, this permits 

 planning levels of about —4 dbm at the amplifier output after allowing 

 reasonable margins for errors and variations. ^^ Previous experience 

 shows that, at such output levels, intermodulation noise can be neg- 

 lected and the full noise allowance allotted to fluctuation noise. The effect 

 of tube noise is to increase the weighted value of resistance noise by 

 about 1 db to —137.5 dbm, or —53.5 dba, at the input to the amplifier 

 in each repeater. 



At the highest transmitted frequency the equalizers, power filters and 

 directional equipment introduce losses of about 1 db and 4 dh at the 



