BAND WIDTH AND TRANSMISSION PERFORMANCE 



577 



when they occur at a frequency only twice the filter cutoff frequency. See 

 A in Fig. 34. If the sampHng frequency were increased from the minimum 

 permissible value, the consecutive step errors would still be unrelated to 

 each other, and more and more of the step error spectrum (noise) would fall 

 above the low-pass filter. This is shown in B. 



STEPPED 

 F- TRANSDUCER OUTPUT 



-250/U.S 



(a) SAMPLING AT 8 KG ] 



(B) SAMPLING AT 64 KC 



— *\ U — 31^/iS 



(C) SAMPLING CONTINUOUS 



\^^^^^sNJ — vvMtmMV'''^\wwMM^ 



^THE QUANTIZING NOISE CONSISTS OF THE RESPONSE OF A 

 3500-CYCLE LOW-PASS FILTER TO THE STEP ERROR 



Fig. 34 — Stepping and sampling an audio wave. 



Reduction of noise would occur in this way until the sampling frequency 

 became so high that a considerable number of samples are taken while the 

 wave crosses a step interval. Correlation between successive step errors then 

 begins to be apparent. When the interval between samples becomes vanish- 

 ingly small, the process is equivalent to transmitting the stepped transducer 

 output directly. This case appears in C. 



In an alternate Hne of thinking, one may regard the stepped transducer 



