Licklider & Pollack, "Effects of Differentiation, Integration, and
Infinite Peak Clipping upon the Intelligibility of Speech", 1948
http://asadl.org/jasa/resource/1/jasman/v20/i1/p42_s1?isAuthorized=no
Previous experiments on the effects of distortion in voice
communication circuits have shown that intelligibility is impaired
surprisingly little by the type of amplitude distortion known as
peak clipping. It has been found, in fact, that conversation is
possible even over a system that introduces “infinite” peak
clipping, i.e., that reduces speech to a succession of rectangular
waves in which the discontinuities correspond to the crossings of
the time axis in the original speech signal.
The intelligibility of the rectangular speech waves depends
critically upon the frequency‐response characteristics of the
speech transmission circuits used in conjunction with the
“infinite clipper.” In the present experiments ... strong
interaction is evidenced by the fact that word articulation scores
of 97 and 15 percent were obtained with two systems consisting of
the same components in different orders. The components were (1) a
tilter with a frequency‐response characteristic rising 6 db per
octave (a “differentiating” circuit), (2) an infinite peak
clipper, and (3) a tilter with a frequency‐response characteristic
falling 6 db per octave (an “integrating” circuit). When these
distorters were cascaded in the sequence 1‐2‐3 the speech output
consisted of triangularly shaped waves which sounded very much like
normal speech and which were highly intelligible (97 percent). When
the reverse sequence (3‐2‐1) was used, the speech output
consisted of sharp pulses giving rise to extremely poor quality and
very low intelligibility (15 percent).
As with delta-sigma modulation, this encoding allows one to turn the
"bug" of limited frequency response in the output stage into a
"feature" which restores intelligibility.
-Dave
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