Fast Algorithms for Low-Delay SBR Filterbanks in MPEG-4 AAC-ELD

The MPEG committee has recently completed development of a new audio coding standard “MPEG-4 Advanced Audio Coding-Enhanced Low Delay” (AAC-ELD). AAC-ELD is targeted towards high-quality, full-duplex communication applications such as audio and video conferencing. AAC-ELD uses low delay spectral band replication (LD-SBR) technology together with a low delay AAC core encoder to achieve high coding efficiency and low algorithmic delays. In this paper, we present fast algorithms for computing LD-SBR filterbanks in AAC-ELD. The proposed algorithms map complex exponential modulation portion of the filterbanks to discrete cosine transforms of types IV and II. Our proposed mapping also allows to merge some multiplications with the windowing stage that precedes or succeeds the modulation step. This further reduces computational complexity. Our presentation includes detailed explanation and flow-graphs of the algorithms, complexity analysis, and comparisons with alternative implementations.