The direct use of vector quantization (VQ) to encode LPC parameters in a communication system suffers from the following two limitations: 1) complexity of implementation for large vector dimensions and codebook sizes and 2) sensitivity to errors in the received indices due to noise in the communication channel. In the past, these issues have been simultaneously addressed by designing channel matched multistage vector quantizers (CM-MSVQ). A sub-optimal sequential design procedure has been used to train the codebooks of the CM-MSVQ. In this paper, a novel channel-optimized multistage vector quantization (CO-MSVQ) codec is presented, in which the stage codebooks are jointly designed. The proposed codec uses a source and channel-dependent distortion measure to encode line spectral frequencies derived from segments of a speech signal. Extensive simulation results are provided to demonstrate the consistent reduction in both the mean and the variance of the spectral distortion obtained using the proposed codec relative to the conventional sequentially designed CM-MSVQ. Furthermore, the perceptual quality of the reconstructed speech using the proposed codec was found to be better than that obtained using the sequentially designed CM-MSVQ.

This paper presents an improved mixed excitation linear prediction (MELP) coder. The MELP is the linear-prediction-based speech coder that was chosen as the new 2400 bps US Federal Standard. Even though the MELP is quite good, there are still some perceivable distortions, particularly around non-stationary speech segments and for some low-pitch male speakers. The key features of our new coder include a robust pitch detection algorithm, a new plosive analysis/synthesis method, and a post processor for the Fourier magnitude model. Formal quality tests are used to show that the new MELP improves the quality of the US Federal Standard MELP coder while requiring only a small increase in algorithmic delay and while also retaining compatibility with the Federal Standard MELP bit-stream specification.

This paper describes our enhanced mixed excitation linear prediction (MELP) speech coder which is a candidate for the new U.S. Federal Standard at 2.4 kbits/s. The new coder is based on the MELP model, and it uses a number of enhancements as well as efficient quantization algorithms to improve performance while maintaining a low bit rate. In addition, the coder has been optimized for performance in acoustic background noise and in channel errors, as well as for efficient real-time implementation. Listening tests confirm that the enhanced 2.4 kbit/s MELP coder performs as well as the higher bit rate 4.8 kbit/s FS1016 CELP standard.

This paper presents a new video coder that operates below 16 kbps and other rates below 64 kbps. The coder uses a quadtree-based hierarchical structure and a distortion measure that controls which levels of the hierarchy are selected. This paper also introduces a cache motion predictor that is based on the principles of designing cache memory. The motion cache can significantly reduce the bit-rate in coding motion vectors while also reducing the computational complexity when compared to the full-search block matching algorithm. The 10 frame/sec QCIF color Miss America sequence was coded with an average Y-PSNR of 35.52 dB at 16 kbps.

A hierarchical coder (HC) is constructed from a number of simple coders for image blocks arranged in order of increasing bit rate and reproduction quality. This paper discusses several aspects of hierarchical coders, including: a general description of the structure of a HC, its advantages and important design considerations, a discussion of how threshold selection can affect the performance of the coder through a simple example, and an introduction of a simple procedure for selecting optimal (in the rate distortion sense) decision thresholds which control the levels of the hierarchy.>