Noise reduction and echo cancellation front-end for speech codecs

We present an enhancement front-end for speech codecs, which consists of the integrated elements of noise reduction and echo cancellation. By including these elements, the front-end performs the task of mitigating the objectionable effects of the two major factors, i.e., noise and echo, which adversely affect the quality of most transmission systems, especially when low bit rate codecs are used. The use of this front-end is demonstrated with the 7.4 kbps IS-641 codec (enhanced full-rate standard for IS-136 systems). The integrated speech-processing unit has the advantage of utilizing the synergy among its components: the voice activity detector in the speech codec, the noise reduction, and the echo canceller. This synergy manifests itself both in the form of a reduction of the overall computational complexity by the use of a number of shared elements among the unit's various components, as well as an improved performance resulting from these components working together. The system displays high performance in both clean and noisy environments and it works well with low bit rate codecs.