Quality evaluation of 32 kbit/s coded speech by means of degradation category ratings

Digital techniques are more and more often used in telecommunications networks. How would these techniques affect the quality of transmitted speech? The use of subjective testing is generally necessary to answer this question. In this paper a degradation category rating test, which is a speech-adapted procedure of the one used for image testing, is proposed to distinguish among good telephone quality codecs. A comparative study of the sensitivity of the method and that of a classic opinion rating test is reported. It shows that the resolution of the degradation category rating test is twice that of a classic opinion rating test within good telephone quality domain. Subjective performances of various 32 kbit/s A.D.P.C.M codecs, including the effects of transmission errors and tandem connections, using degradation category rating tests are also presented.