Subjective quality of the same speech transmission conditions in seven different countries

Because of the diversity of testing methods, it is not possible, in general, to compare speech quality measurements from different laboratories. A recent experiment had the aim of determining whether comparable results can be obtained when the same test is performed in several different countries. Speech samples in the native languages of Britain, Canada, France, Italy, Japan, Norway and the United States were processed by 38 communications circuits at Bell Laboratories. Recordings of the processed speech were returned to each location for evaluation, on a five-point scale, by native listeners. The communications circuits included 22 references and 16 adaptive differential pcm (adpcm) test circuits. An important part of the data analysis was estimation of the quality of the 16 adpcm conditions at one location given measurements of adpcm quality elsewhere. The results indicate that a reasonably accurate estimate of adpcm quality at location B is the quality measured at A adjusted by an additive constant. The accuracy of this estimate is equivalent to that of a subjective test at location B with about seven evaluations of each transmission condition.