Current scalable audio codecs optimize each layer of the bit-stream successively and independently with a straightforward application of the same rate-distortion optimization techniques employed in the non-scalable case. The main drawback of this approach is that the performance of the enhancement layers is significantly worse than that of the non-scalable codec at the same cumulative bit-rate. We propose in this paper a novel optimization technique in the Advanced Audio Coding (AAC) framework wherein a cross-layer iterative optimization is performed to select the encoding parameters for each layer with a conscious accounting of rate and distortion costs in all layers, which allows for a trade-off between performance at different layers. Subjective and objective results demonstrate the effectiveness of the proposed approach, and provide insights for bridging the gap with the non-scalable codec.
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