Vector quantization (VQ) is widely used in many high-quality and high-rate data compression applications such as speech coding, audio coding, image coding and video coding. When the size of a VQ codebook is large, the computational complexity for the full codeword search method is a significant problem for many applications. A number of complexity reduction algorithms have been proposed and investigated using such properties of the codebook as the triangle inequality. This paper proposes a new fast VQ search algorithm that is based on a multi-stage structure for searching for the best codeword. Even using only two stages, a significant complexity reduction can be obtained without any loss of quality.

A low delay tree coder for coding speech at 8 kbit/s that incorporates a new backward coefficient adaptation structure is presented. The coder has an algorithmic delay of 2.5 ms, produces good quality speech over ideal channels, and maintains robustness to bit errors for independent bit error rates up to and including 10/sup -2/. The tree coder consists of a randomly populated fractional rate tree, a weighted squared error distortion measure, the (M,l) tree search algorithm, an incremental path map symbol release rule, a long-term predictor, an all-pole short-term predictor, and the newly-obtained parameter adaptation algorithms; The sensitivity to channel errors is reduced by using the receiver excitation sequence for adaptation of the short-term predictor coefficients, but the ability to track rapid changes in the speech is retained by shaping the excitation sequence with an all-zero filter. The output speech of the tree coder is evaluated using segmental signal-to-noise ratios (SNRSEG), narrow band spectrograms, a paired comparison with pulse code modulation (PCM), and an equivalent noise paired comparison using modulated noise reference unit (MNRU).>

Simulation results are presented comparing the performances of four backward adaptive lattice algorithms for updating the short-term predictors in adaptive predictive coding (APC) and differential pulse code modulation (DPCM) code generators for low-delay tree coding of speech at 16 and 9.6 kb/s. The algorithms studied are the least-squares lattice, the exponential window lattice, the signal-driven lattice, and the residual-driven lattice. Ideal channels are emphasized, and comparisons are based primarily upon frequency-weighted signal-to-noise ratio and subjective listening tests.>

A report is presented on the design of CELP codebooks matched in some ways to multipulse sequences that produce toll-quality speech. The authors designed a multipulse LPC system that achieves toll quality at 15 kb/s. They then studied the properties, especially the statistical properties, of these successful multipulse excitations for a variety of speakers and speech segments. CELP codebooks were developed which match the properties of the multipulse sequences to yield a final CELP coder for operation at 7 kb/s.>