Universal successive refinement of CELP speech coders

Many speech coding standards are based upon code-excited linear prediction (CELP), and it is desirable to develop layered coding methods that are compatible with this installed base of coders. We propose a layered speech coding structure that is universally compatible with all CELP-based coders. This structure encodes the reconstruction error signal from layer 1 using a lowdelay, adaptive tree coder based upon the mean squared error (MSE) criterion. We note that rate distortion optimal successive refinement is achievable using two different distortion criteria and we derive expressions for the rate distortion function under autoregressive Gaussian assumptions on the source and the two different distortion measures. We demonstrate the universality of the approach by developing two-layer coders for a 3.65 kbps CELP coder, G.723.1, and G.729. We show that our layering method is favorably competitive with the MPEG-4 layering method at 8.7 kbps for both clean and noisy speech. Using tree coding and the MSE criterion in layer 2 improves speech naturalness when coding noisy speech.