Speech Recognition over Mobile Networks

This chapter addresses issues associated with automatic speech recognition (ASR) over mobile networks, and introduces several techniques for improving speech recognition performance. One of these issues is the performance degradation of ASR over mobile networks that results from distortions produced by speech coding algorithms employed in mobile communication systems, transmission errors occurring over mobile telephone channels, and ambient background noise that can be particularly severe in mobile domains. In particular, speech coding algorithms have difficulty in modeling speech in ambient noise environments. To overcome this problem, noise reduction techniques can be integrated into speech coding algorithms to improve reconstructed speech quality under ambient noise conditions, or speech coding parameters can be made more robust with respect to ambient noise. As an alternative to mitigating the effects of speech coding distortions in the received speech signal, a bitstream-based framework has been proposed. In this framework, the direct transformation of speech coding parameters to speech recognition parameters is performed as a means of improving ASR performance. Furthermore, it is suggested that the receiver-side enhancement of speech coding parameters can be performed using either an adaptation algorithm or model compensation. Finally, techniques for reducing the effects of channel errors are also discussed in this chapter. These techniques include frame erasure concealment for ASR, soft-decoding, and missing feature theory-based ASR decoding.