INTRODUCTION

1. a) original signal b) synthesized signal using the standard encoding c) synthesized signal using the constrained encoding d) error signal between a) and b) e) error signal between a) and c) [ ] å 2 = - D x ( n ) g y ( n ) a 1 1 0 n standard codec modified codec standard codec modified codec INCREASING THE ROBUSTNESS OF CELP-BASED CODERS BY CONSTRAINED OPTIMIZATION Mohamed Chibani, Philippe Gournay and Roch Lefebvre University of Sherbrooke Sherbrooke, Quebec, Canada • INTRODUCTION • The use of the adaptive codebook (ACB) in CELP-like speech coders allows the achievement of high quality speech, especially for voiced segments. • The problem • The dependency of the ACB on the history of the signal makes it vulnerable to frame erasures. When a frame is lost, it affects not only the corresponding segment, but also the following frames due to error propagation, and the decoder may take several frames to recover. • The proposed solution • To reduce the error propagation when a frame is lost, we propose to lessen the inter-frame dependency by limiting the energy of the ACB contribution to the synthesized speech. The ACB gain by itself is not limited. 3. CONSTRAINED SEARCH ALGORITHM If the ACB energy contribution is high, a fraction of this contribution is forced to be left in the new target. The ACB contribution is estimated by comparing, in the perceptual domain, the energy due to the ACB excitation to the target energy. 4. EXPERIMENTAL RESULTS Constrained encoding improves the recovery time of the decoder after a frame erasure 6. CONSTRAINED JOINT QUANTIZATION OF THE ACB AND ICB GAINS The ACB contribution can be controlled more precisely during the gains quantization process, since at this stage both ACB and ICB excitation shapes are known. The constrained optimal solution can be found analytically using the Kuhn-Tucker conditions. The quadratic error relative to the quantized ACB and ICB gains • 2. EXCITATION SEARCH IN THE AMR-WB CODEC The standard search for the excitation parameters is done sequentially. It consists first in finding the gain and the delay for the ACB, then the shape (pulse positions) for the innovative codebook (ICB). Finally, the ACB and ICB gains are jointly quantized. The procedure may be summarized as follows : • Perform the LP analysis on the speech signal, extract the residual signal and build the target. • Search for the ACB parameters that match the target (in an analysis-by-synthesis loop), according to the error criterion : • Update the target by subtracting the ACB contribution. • Search for the ICB parameters using the updated target. • Jointly quantize the ACB and ICB gains (quadratic error minimization in the perceptual domain). A value of Rth1 as low as 0.55 preserves the overall quality in clear channel while significantly improving the robustness against frame erasures. NO PREFERENCE IN CLEAR CHANNEL STRONG PREFERENCE WITH A FRAME LOSS RATE OF 10% • 7. CONCLUSION • Limiting the ACB contribution, significantly improves the recovery time of the decoder after a frame loss. • In clear channel, the quality degradation due to the constraint is very small. • The modified coder is completely interoperable with the standard, it requires no extra bits and adds no extra delay. • Although the ACB contribution can be controlled more precisely at the gains quantization stage, modifying only the target for the ICB search is the simplest and most effective approach. • 5. SUBJECTIVE TESTS • Two subjective tests were carried out in order to compare the standard and modified codecs: in the clear channel condition (no frame erasures), and with a simulated frame loss rate of 10%. • Test features : • 20 sentences for each condition. • 12 listeners participated in the tests. • Listening using binaural headphones. • AMR-WB in mode 2 (12.65 kb/s). • Audio files on http://www.gel.usherb.ca/gournay/