Perceptual evaluation of two speech coding methods by native and non-native speakers of English

Abstract

The segmental intelligibility of two speech coding methods was evaluated using the modified rhyme test (MRT). In order to investigate how language knowledge and experience affects the perception of coded speech, we used both native and non-native speakers of English as listeners. We used 8 kb/s pitch predictive multi-pulse coding (MPC) and 50 kb/s μ-law PCM as coding methods, along with 120 kb/s unprocessed speech as a control condition. The results indicated that for native listeners, the intelligibility of unprocessed speech was best, followed by PCM and then MPC speech. However, for non-native listeners, the intelligibility of the two types of coded speech was much worse than the unprocessed speech when compared with the performance of the native English listeners. The difference in error rates between MPC and PCM was not significant for non-native listeners. In contrast, for native listeners, the error rate for PCM was significantly less than for MPC speech. Non-native listeners also had a tendency to confuse stops and fricatives more than native listeners, especially in the coded speech. These results suggest that language knowledge and experience play a much more important role in the perception of coded speech than in the perception of unprocessed speech. Furthermore, non-native listeners appear to be much more affected by some of the unique characteristics of coded speech than native listeners. The results have implications for the design and improvement of low bit-rate speech coding methods and for the development of new techniques for perceptual evaluation.