Optimality and Complexity Considerations
We propose a low complexity unit-selection algorithm for ultra low bit-rate speech coding based on a first-stage n-best pre-quantization lattice and a second-stage run-length constrained Viterbi search to efficiently approximate the complete search space of the fully-optimal 1-pass DP based unit-selection algorithm described in the previous chapter. By this, the n-best low complexity algorithm dealt with here, approaches near-optimality with increasing n, in terms of rate-distortion performance while having highly reduced complexity. The sub-optimal segmental unit-selection algorithm of Lee and Cox described in Chap. 3 is a 1-best special case of the algorithm proposed here, with the proposed n-best lattice based algorithm generalizing to a larger search space and hence having a significantly improved rate-distortion performance towards the fully optimal 1-pass DP unit-selection performance.
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