10 BELL SYSTEM TECHNICAL JOURNAL 



we need less apparatus and less space if we use multivibrator step-downs. 

 Another advantage is that noise in the base frequency supply produces propor- 

 tionately less phase jitter with sub-harmonics. While multivibrators do not 

 have high inherent stability, they are capable of great precision when suitably 

 controlled. For these reasons frequency step-downs are used to generate ail 

 the timing waves of the system. 



Timing and gating operations require accurate time alignment of pulses, 

 which may be accompHshed by suitably delaying one set with respect to the 

 other. For this purpose use is made of delay networks or cables, or delay 

 multivibrators. Pulse durations may be varied through the use of interference 

 effects between given pulses and their delayed replicas. Additional timing 

 and gating wave forms are required for regeneration and for assembly in time- 

 division multiplex. All such control pulses are economically generated at a 

 single common point rather than by a number of local generators, and can then 

 be supplied to the message equipment by common power amplifiers via shielded 

 cable. 



Transmission 



In this section we are to consider the general factors entering into satisfactory 

 reception of on-off pulses including such limitations as those on band width 

 and noise. These are to be viewed while keeping in mind the procedures 

 available for pulse regeneration. 



If we start with a rectangular pulse like one of those generated at the trans- 

 mitter, the corresponding frequency spectrum exhibits lobes extending indefi- 

 nitely on the frequency scale, with progressively decreasing amplitudes as 

 indicated in Fig. 5a. When such a pulse is passed through a linear phase 

 filter^ which discriminates against frequencies beyond the first lobe, the re- 

 sulting pulse is practically of the sinusoidal form shown in Fig. 5b. Reducing 

 the high-frequency response to the extent prescribed rounds the corners of the 

 transmitted pulse, its duration at half value remaining equal to that of the 

 original input pulse. In practice, transmission characteristics depart from 

 phase linearity and low frequency cutoffs exist. Both effects introduce irreg- 

 ularities into the pulse response. While actual pulses therefore differ slightly 

 in detail from the idealized picture given above, that picture will be retained 

 for simplicity in discussion. 



The band width needed for good pulse transmission can be minimized by 

 making pulses as wide as possible. But since a specified number of pulses have 

 to be put into a given time interval (84 pulses in -^^-^ sec.) we must limit our 

 broadening at a value where the presence or absence of a pulse may be clearly 



' A suitable filter is one with a Gaussian characteristic, the loss in decibels varying as 

 the square of frequency and having a value of 1 neper (8.68 decibels) at a frequency equal 

 to 1/T. 



