COMBINED SOURCE AND CHANNEL CODING OF SPEECH FOR TELECOMMLNICATIONS
Guowen Yang · 1990
Many efficient speech coding techniques to achieve high speech quality have been developed for bit rates between 4.8 kbits/s and 16 kbits/s. Among these techniques, Code Excited Linear Predictive (CELP) coding is a potential technique for providing high quality speech at very low bit rates. In the absence of channel errors, the CELP coder can produce high quality speech at bit rates as low as 1.8 ~&s/s. In the presense of channel errors, however, the speech quality degrades dramatically. In order to improve speech quality without increasing the transmission rate for given channel conditions, we study combined source and channel coding of speech using CELP coding and Punctured Convolutional (PC) coding in this thesis. Based on the informationtransmission theorem, this thesis derives the performance b o a d of the CELP coder in an Additive White Gaussian Noise (AWGN) channel and an interleaved Rayleigh fading channel. This performance bound provides a reference for the performance of a practical combined CELP and PC coder. To arrive at an efficient combined coder, different levels of error protection are applied to different bits of the CELP coder's output according to bit error sensitivities. Simulation results show that at the bit error rate of the combined coder can obtain up to a 10 dB improvement in the AWGN channel and a 12 dB improvement in the Rayleigh fading channel, with respect to the CELP coder without channel coding a t the same transmission rate. These improvements are measured in terms of the segmental signal to noise ratio of the reconstructed speech.