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THE BELL SYSTEM TECHNICAL JOURNAL, MAY 1957 



The instantaneous compressor envisioned herein is, in essence, a non- 

 hnear pulse amphfier which modifies the distribution of pulse amplitudes 

 in the input PAM signal by preferential amplification of weak samples. 

 A satisfactory compression characteristic will have the general shape 

 shown in Fig. 2. Thus the amphfication factor, {vje) , varies from a large 

 value for small inputs to unity for the largest amplitude (F) to be ac- 

 commodated, so that the distribution of pulse sizes may be modified 

 without changing the total voltage range. Fig. 2 also illustrates how uni- 

 form quantization of the compressor output produces a tapered array 

 of input steps similar to those already considered in connection with 

 Fig. 1(b). 



A complementary device, the expandor, employs a characteristic in- 

 verse to that of the compressor to restore the proper (quantized) distri- 

 bution of pulse amplitudes after transmission and decoding. Taken to- 

 gether, the compressor and expandor constitute a compandor. 



The resolution of tapered quantization into the sequential application 

 of compression, uniform quantization, and expansion is operationally 

 convenient,^ as well as logically sound. Since there is a one-to-one corre- 

 spondence between step size allocations and compression characteristic 



INPUT VOLTAGE, e 



Fig. 2 — • Curve illustrating the general .shape of a suitable instantaneous com- 

 pression characteristic. All continuous, single-valued curves connecting the origin 

 to the point (F, V) and rising from the origin with a slope greater than one, i.e., 

 {dv/de)e=o> 1, are potential compression characteristics. The symmetrical nega- 

 tive portion [v{e) = —v{—e)] is not shown. The production of a tapered array of 

 input steps (Ae)i by uniform quantization of the output into steps of (equal) size 

 Az^, is also represented. 



