16 BELL SYSTEM TECtiNlCAL JOURNAL 



pacity. This inefficiency can be avoided by deriving the pulsing rate from the 

 pulse train through the use of a narrow band filter. Framing may then be 

 effected by using in each frame just one digit pulse, which is given a distinctive 

 repetition rate. With this method, a certain amount of time is required to 

 establish synchronism when the system is started up. In the system to be 

 described the framing time is less than one-tenth second — a tolerable value. 



III. Experimental System 



In the light of the foregoing discussion, the block diagram of the experimental 

 system shown in Fig. 6 is believed to be largely self-explanatory. It will be 

 noted that for microwave transmission the modulated intermediate-fre- 

 quency signals are simply translated in frequency to the 4000-megacycle 

 band. The shaping filters and group selection filters shown have approxi- 

 mately Gaussian characteristics, in accord with transmission considerations 

 noted earlier. The band widths shown for these filters apply between points 

 one neper down from the midband loss. Band widths given for the ampli- 

 fiers, on the other hand, refer to their regions of essentially flat response. 



Overall measurements are facilitated and the amount of experimental 

 apparatus minimized by looping the radio path through a non-regenerative 

 microwave repeater 21 miles away in New York. Both ends of the 24 

 channels are thus made available at Murray HiU, New Jersey, the location 

 of the apparatus pictured in Fig. 1. With this arrangement, and using 

 conventional 4-wire voice frequency terminating sets, 24 people are able to 

 engage in 12 simultaneous conversations through the system. 



PCM Transmitter. The transmitting equipment for an individual 12- 

 channel group is shown schematically in Fig. 7. Each audio input^ is passed 

 through a 3400-cycle low-pass filter and through a limiter which chops off 

 the positive and negative peaks of any signal exceeding a prescribed maxi- 

 mum amplitude. This limit is chosen to penalize the loudest talkers to the 

 degree customary in toll system practice. The inputs then enter a "col- 

 lector" circuit, which assembles samples of the channels in time division 

 multiplex on a common lead. Although it functions electronically under 

 control of pulses from the timing bay, the circuit so resembles a mechanical 

 commutator that this analogy has been used in the schematic. The period 

 of rotation of the "contact arm" is 125 microseconds (8 kilocycles), and a 

 conducting path is formed to the common multiplex lead from each channel 

 circuit in turn for yV of this period, or 10^ microseconds. It should be 



•In telephone terminology, these 4. wire inputs are normally at the —13 decibel level 

 point; i.e., 13 decibels below the transmitting level at the toll test board. Strap connec- 

 tions are provided, as in the Western Electric A-2 channel bank, for adapting the system 

 to inputs 3 decibels smaller. Similarly the final 4-wire outputs are delivered at the -f-4 

 (or -H7) decibel point. The normal gain through a link is thus 17 decibels but can be 

 set as much as 6 decibels grtater. 



