Speech processing for voice over IP applications

Voice over Internet Protocol (VoIP) is a technology that allows telephone calls to be made over Internet. In this technique, the voice signals are converted into the coded digital signals and sent over Internet Protocol (IP). While this system brings many advantages in terms of cost and network usage, there are unsolved issues in the quality of service such as internet connections, reliability and sound quality.In this study, the coding of speech and echo cancellation which affects the quality of sound are considered. One of the speech coding models which is based on a mathematical approximation of the varying acoustic filter is linear predictive coding. Linear prediction based vocoders are designed to emulate the human speech production mechanism. Such a vocoder is Code Excited Linear Prediction (CELP) and the CELP based coder and decoder is simulated in MATLAB platform. In order to evaluate the performance of the coding system, the sound quality and the computational complexity of the system are discussed. According to the results based on two different quality metrics, it is found that, quality of the proposed algorithm is nearly same as the reference algorithm. Compared in terms of the computational complexity, the proposed algorithm takes approximately quarter of the time spent by the reference coder.Also, an echo cancellation software by using adaptive filtering technique has been implemented. As a result of the experiments, echo is cancelled successfully by using frequency-domain adaptive filter.