20 BELL SYSTEM TECHNICAL JOURNAL 



circuit in the pulse regenerator which first doubles the length of each pulse 

 by adding thereto its own delayed reflection obtained from a short-circuited 

 delay cable, and then shghtly less than doubles it again by a similar process 

 using a longer cable. A final slicing, to eliminate amplitude irregularities 

 acquired in the lengthening process, yields square pulses as shown in line D, 

 with adjacent pulses merged into a single longer pulse. This is the final 

 output signal delivered to the intermediate-frequency modulator. Passage 

 of the modulator output through a shaping filter results in rounded pulses 

 (line E) suitable for transmission over the radio relay path. 



In this regenerating apparatus, provision is also made for introducing a 

 "framing control pulse," supplied from the timing bay and normally applied 

 only to Group 1, although any other group may be used if desired. This 

 pulse is about 1.5 microsecond long, and occurs once in each 8-kilocycle 

 frame, but has opposite polarity in successive frames. It is timed to syn- 

 chronize with the first digit of the Channel 1 code and is large enough in 

 amplitude to override the pulse or space put out by the coder in that position. 

 Hence in the final PCM output from Group 1, pulse 1 of Channel 1 is al- 

 ternately present and absent regardless of the audio signal. This arrange- 

 ment, used in automatic ''framing" of the receiving timing gear as described 

 in the following section, thus borrows the least significant digit from one 

 channel of the system, leaving that channel usable, but with 6-digit instead 

 of 7-digit quality. A 4-kilocycle tone of very low amplitude which it intro- 

 duces in that single channel is made inaudible by the low-pass filter in the 

 final audio output. 



Synchronization. The connection of a transmitting channel to its proper 

 receiving circuit in the time-division part of the system requires the two 

 terminals to be synchronized : timing operations at the receiver must follow 

 closely those at the transmitter. In a broad and general way this timing 

 matter amounts to getting a local clock to keep the same time as a distant 

 standard clock. Here the criterion of good timekeeping might be thought 

 fussy by some standards; we cannot work with a discrepancy as long as a 

 microsecond for the very good reason that incorrect routing of pulses would 

 then result, associated with intolerably large decoding errors. Three pro- 

 visions are made to take care of this situation. First, the framing is auto- 

 matically monitored at all times. Second, if the system is out of frame — as 

 it may be after transmission has been temporarily interrupted — the monitor 

 circuit hunts for and establishes synchronism. Third, whenever the system 

 is not properly synchronized and framed, all message circuits are cut off to 

 avoid resulting noise and crosstalk. 



For the purpose of this description we can use a mechanical analogy once 

 more and picture all the transmitting channels of a time-division group 

 arranged in order around a circle (Fig. 9). This time, however, we let each 



