Temporal Tile Shaping for spectral gap filling in audio transform coding in EVS
At low bitrates, next generation audio coders apply waveform preserving transform coding only for the perceptually most relevant parts of the signal. The resulting spectral gaps are filled in the decoder through techniques like Intelligent Gap Filling (IGF). IGF is currently being standardized in MPEG-H 3D-Audio and also in 3GPP Enhanced Voice Service (EVS). In IGF processing, spectral tiles are copied from a spectral source location into a target location and subsequently adapted by parameter steered post-processing to best match relevant properties of the original signal. Important properties include the spectral and temporal envelope. Since IGF operates on Modified Discrete Cosine Transform (MDCT) spectra of rather long time blocks, temporal envelope shaping is not trivial. In this paper, Temporal Tile Shaping (TTS) is presented. TTS is based on linear prediction in the MDCT domain for shaping the temporal structure of the gap filling signal in the target tiles with sub-block granularity. A listening test demonstrates the advantage of the proposed method.