12 BELL SYSTEM TECHNICAL JOURNAL 



although the narrower band would admit less extraneous noise, margins over 

 noise and interference would be reduced. Widening the band, on the other 

 hand, would allow pulses to build up and decay faster, but would not increase 

 the pulse height. Thus the same signal would result, but the widened band 

 would pass increased noise, and again the margins would be reduced. The 

 optimum band width represents the most useful compromise in efforts to re- 

 duce noise and interference and to increase signal. 



The filter characteristic we have been discussing is that of the entire link 

 taking in all selectivity inserted between the practically rectangular pulses 

 originally generated at the transmitter and the pulses delivered to the PCM 

 receiver. Filters at both transmitting and receiving terminals of the link are 

 included, the greater part of the overall selectivity being located at the receiver. 

 With about 1.5 microsecond available per pulse at half amplitude, filters spaced 

 1.5 megacycle apart accommodate the double sideband and keep the inter- 

 ference between groups within tolerable limits. 



To establish the presence of pulses we can set up an amplitude threshold equal 

 to half the normal pulse amplitude, and test to see if that threshold is exceeded 

 at a time near the center of the assigned pulse position. Selecting the threshold 

 at half amplitude provides equal margin against the possibility of noise and 

 interference bringing the full pulse amplitude, or mark, below threshold and 

 bringing the nominal space above threshold. Testing at the pulse position 

 midpoint maximizes this margin. 



The amplitude threshold is set up by slicing a thin section horizontally out 

 of the pulse at its half-amplitude level by means of an amplitude discriminator. 

 Evidently, this procedure restores the fiat top of the pulse. To complete re- 

 generation by restoring the pulse epoch to a standard value the sliced pulses 

 are gated at the midpoints of their proper intervals with narrow pulses supplied 

 by the timing equipment. By these two pulse regenerating processes — slicing 

 and gating — noise and interference are made impotent to produce errors until 

 they attain a substantial fraction of the pulse amplitude. With the effects 

 of noise and distortion thus eliminated, the only noise inherent in the system 

 is that of quantization. In long systems having many repeater points, re- 

 generation has to be practiced at spans short enough to permit cleaning out 

 noise and distortion before it piles up above threshold. Thus in this system 

 transmission impairments have to be considered only for the span between 

 regeneration points; with their effects limited below threshold they are not 

 carried over from one span to the next. 



Where PCM groups are multiplexed by frequency division, amplitude non- 

 linearity of the system must be kept within limits. Otherwise intermodulation 

 products may fall within transmission bands, adding to interference. Over- 

 lapping of neighboring filter bands also must be kept within bounds to reduce 

 direct crosstalk between the pulse trains. With pulses arranged exclusively 



