Signal-adaptive switching of overlap ratio in audio transform coding

Contemporary perceptual audio coders, all of which apply the modified discrete cosine transform (MDCT), with an overlap ratio of 50%, for frequency-domain quantization, provide good coding quality even at low bit-rates. However, relatively long frames are required for acceptable low-rate performance also for quasi-stationary harmonic input, leading to increased algorithmic latency and reduced temporal coding resolution. This paper investigates the alternative approach of employing the extended lapped transform (ELT), with 75% overlap ratio, on such input. To maintain a high time resolution for coding of transient segments, the ELT definition is modified such that frame-wise switching between ELT (for quasi-stationary) and MDCT coding (for non-stationary or non-tonal regions), with complete time-domain aliasing cancelation and no increase in frame length, becomes possible. A new ELT window function with improved side-lobe rejection to avoid framing artifacts is also derived. Blind subjective evaluation of the switched-ratio proposal confirms the benefit of the signal-adaptive design.