Adaptive time-frequency resolution in modulated transform at reduced delay

Cosine-modulated transforms such as the Modified Discrete Cosine Transform (MDCT) are key elements in audio coding. They allow efficient energy compaction and perceptual irrelevancy reduction. The frequency localization can be adapted to the signal characteristics and fast implementations exist. All of this has made MDCT the most popular transform in audio coding. In this paper we solve a problem never addressed in the literature: in order to enable variable MDCT sizes in communication codecs, we demonstrate how frequency resolution can be adapted on the fly without using transition windows hence decreasing coding delay. A low complexity implementation of the method is also proposed.