Sparsity compensation for speech coders

Sparse excitation from fixed codebooks provides for efficient search algorithms and high-quality speech. However, as the bit rate decreases, sparsity causes hearing effects that may be phase-compensated. A procedure is described for training the impulse response of a phase compensation filter over a large database. Two four-pulse ACELP (algebraic code-excited linear predictive) coders operating at 5.3 kbit/s have their compensation filters trained. One of the coders searches the fixed codebook with the focused algorithm and the other one uses the joint position and amplitude search (JPAS) algorithm. Listening tests were performed involving both compensated and uncompensated versions of these coders at 5.3 kbit/s and the uncompensated version at 8 kbit/s. The lower rate coders provided with trained sparsity compensation are perceived to deliver speech quality equivalent to that achieved with 8 kbit/s coders. Additionally, the beneficial hearing effect is more pronounced with the computationally efficient joint position and amplitude search