22 BELL SYSTEM TECHNICAL JOURNAL 



kilocycle frequency is recovered from the input pulse train by means which 

 include filtering in a narrow-band crystal filter. The filter band is narrow 

 enough to select the base frequency with negligible modulation and noise. 

 That is, sidebands produced by pulse keying are attenuated to negligible 

 proportions, as is the background noise. It is this output which serves to 

 step the brush arm around the frame at precisely the transmitter frequency. 



With the transmitter frequency recovered, both hands are stepped around 

 their dials at the same rate. While this is a necessary condition for running 

 the receiver, it is not a sufficient one since most likely the system will not 

 be framed. In fact the odds are 83 to 1 against the two clocks indicating 

 the same time if connection to the receiver is initiated at random times. If 

 we had to deal with ordinary clocks, both in view, the resetting procedure 

 could be accomplished by moving one clock-hand to agree with the other 

 at one fell swoop. But in the PCM case resetting has to be done more dis- 

 cretely since only one dial position per frame is viewed in the framing re- 

 ceiver. Accordingly, an orderly procedure is set up for locating the framing 

 pulse which consists in examining digit positions one by one until the framing 

 pulse is reached. After any one position is viewed long enough to establish 

 the absence of the framing pulse, the receiving clock is set back one digit 

 position and the next position viewed. 



This resetting or framing procedure is governed by the framing receiver 

 through its control of a switch which connects the recovered base frequency 

 to the driving mechanism of the clock. If the channels are correctly routed, 

 so that it is the framing pulse which is being viewed by the framing receiver, 

 the switch is left closed, and the 672-kilocycle wave steps the clockhand 

 around the dial without interruption. But if the system is not correctly 

 framed the framing receiver does not get its distinctive pulses. In this 

 case the switch is opened every little while for the duration of a single pulse 

 interval, stopping the local receiver clock during that interval while the 

 transmitter clock advances one digit position. In effect the receiving clock- 

 hand is set back precisely one digit interval with respect to the standard. 

 Thus the next digit pulse is brought into the framing receiver. If again the 

 monitored input turns out to be other than the framing pulse, the stopping 

 process is brought into play once more; this process is repeated until the 

 system is framed. 



As pointed out in the last section, the framing pulse is alternately absent 

 and present in successive frames, corresponding to a 4-kilocycle rate. This 

 is readily distinguishable from any of the message pulses, which in practice 

 are found to have little energy content at this frequency. The framing 

 receiver accordingly includes a resonant circuit tuned to four kilocycles. 

 In the hunting process, eight frame periods are allowed between successive 

 interruptions of the clock drive, to give the resonant circuit sufficient time 



