26 



BELL SYSTEM TECHNICAL JOURNAL 



described. These include sampling circuits, the instantaneous compressor, 

 slicers, and the PCM coder and decoder. 



Sampling Circuits. The function of opening and closing a circuit at pre- 

 scribed instants, represented by rotating switches in the block schematics, 

 is actually performed^ by one or the other of the devices shown in Fig. 12, 

 employing diodes and triodes, respectively. 



The diode type (Circuit a) is normally biased ''open" by rectified charges 

 stored on the two large capacitors. While in this condition it presents an 

 extremely high series resistance between the low-impedance input and the 

 load. But when a fiat-topped pulse is impressed upon the pulse transformer, 

 the aforesaid high impedance changes to a low value (of the order of 100 



"O 4 8 12 16 20 24 28 32 36 



FREQUENCY IN HUNDREDS OF CYCLES 



Fig. 11. Equalization for the aperture efifect. 



ohms), for the duration of the pulse. The upward-sloping tops of the pulses 

 sketched in Fig. 12 represent predistortion to allow for transmission through 

 the pulse transformer, for the purpose of obtaining rectangular pulses at the 

 tube itself. 



In the other variety (Circuit b) the plate-cathode paths of the two triodes 

 are connected directly in parallel, conducting in opposite directions. The 

 grids are both arranged to be biased below cut-off during the ''open" con- 

 dition of the sampler by grid rectification of pulses, and to be driven strongly 

 positive by a pulse when a low-resistance conducting path is required be- 

 tween source and load. 



These two types have their respective advantages. For exami)le, tlie low 

 capacitance to ground which the diode type affords across its load makes it 



' A single exception is the case of switch A of Fig. 10. In this case two gated slicer 

 circuits are used, as described subsequently. 



