A two-layered trellis approach to audio encoding
The fact that audio compression for streaming or storage is usually performed offline alleviates traditional constraints on encoding delay. We propose a rate-distortion optimized approach, within the MPEG Advanced Audio Coding framework, to trade delay for optimal window switching, resource allocation and selection of quantization and coding parameters for the entire audio file using a two-layered trellis. Stages of the outer trellis correspond to audio frames, nodes represent window choices, and branches implement transition constraints. The inner trellis operates within each node of the outer layer and has stages corresponding to scalefactor bands and nodes representing combinations of quantization and coding parameters. A suitable cost, comprising bit consumption and psychoacoustic distortion, is optimized via multiple passes through the two-layered trellis to achieve the desired bitrate. The procedure thus optimizes most of the encoding decisions involved in audio compression. Objective and subjective tests show considerable performance gains.