738 THE BELL SYSTEM TECHNICAL JOURNAL, MAY 1956 



code combination, relay cbkm is operated, ^emo^•ing a disabling bias 

 from the driver stage of the blocking oscillator. When in this condition, 

 each occurrence of the match pulse will trigger the blocking oscillator, 

 thereby producing the a gate-opening pulse once per drum revolution. 



(3) Slot-Spanning Pidser 



Whene^'er an a gate-opening pulse has acted to permit read-out of i 

 information from half of the proper a slot, it is also desired to read out ; 

 all the information from the next-following b slot. The first step toward : 

 doing this is to cause the a gate-opening pulse to trigger a single-shot , 

 multixibrator whose characteristic period is long enough to just bridge i 

 the time until the next slot appears. The output of this pulser is combined I 

 with the B sync pulses in an and gate so that the selected b pulse, cor- i 

 responding to the wanted b slot, can be used to trigger another gate- . 

 opening blocking-oscillator just as the match pulse was used to trigger t 

 the A gate-opening blocking-oscillator. j 



(4) The B Gate-Opening Pulse | 

 The outputs of all the reading amplifiers must be gated for the b slot. | 



Hence the b gate-opening pulse must operate twice as many gates as the . 

 A gate-opening pulse and must be correspondingly more powerful. This ■ 

 requirement is met by using the same circuit design with parallel output 

 tubes. 



Pulse Stretchers and Channel Detectors 



Fig. 13 presents a simplified schematic of one of the translator output 

 channels, together with certain of the relays in the decoder. Package-wise, 

 the pulse stretchers combine two functions: that of an and gate with two 

 inputs and a threshold feature, and that of a single-shot multivibrator for 

 amplifying and lengthening the short input pulse from the gate. A single 

 point-contact transistor provides the necessary gain for the monostable 

 action. The inputs to the and gate come from sources which supply nega- 

 tive-going pulses from a standing potential of +130 volts. When one or 

 the other, but not both, of these sources supphes a pulse, a larger portion i 

 of the current being supplied to resistor ri must be drawn from the non- 

 active source; this extra demand causes a small \oltage drop which be- 

 comes evident at the gate output. The resultant weak false signal is pre- 

 vented from affecting the transistor pulser by the action of threshold 

 diode VRi which is normally back-biased a few volts by the potential di- 

 \-ider r2, r3. Small negative-going signals from the gate will not over- 

 come the bias and will therefore be greatly attenuated; normal gate-out- 

 put pulses, occasioned by coincidence of pulses at both inputs will, 



