Biologically inspired algorithm for enhancement of speech intelligibility over telephone channel

This paper describes a method to increase speech intelligibility when the speech signal is being transmitted over telephone lines. In order to detect all factors which affect speech intelligibility, we use telephone simulation tool in ITUT Software Tools Library release 2005 (STL2005) to identify the most problematic telephone-channel deteriorations. Of the various effects considered, additive noise and bandwidth of telephone channel seem to have the greatest influence on human´s recognition accuracy. To reduce this degradation, a Lombard production and perception effect model, which represents the spectral changes of speech signal uttered and perceived in noisy environment, is proposed. This model is realized by a threestage modification procedure involving time-scale, pitch-scale, the formants and the consonant-vowel energy ratio modifications of the normal speech signals. From listening test conducted, it is shown that the proposed compensation algorithm increases the intelligibility of speech in telephone channel environments at 0 dB Signal-to-Noise ratio (SNR) and -10 dB SNR by nearly 5.5% and 13% over the unprocessed signals respectively.