In the context of lossy audio coding, the power spectral density of stationary tones can be over/underestimated in some windows due to the time-shift sensitivity of the Modified Discrete Cosine Transform (MDCT), which leads to potentially audible coding artefacts. This paper discusses the advantages of using a nearly time-shift invariant regularized MDCT spectrum for the bit allocation in MPEG-AAC coder. We show how this modification applies to the standard iterative algorithm, as well as to a more efficient model-based framework. Objective and subjective results indicate that the overall quality is significantly improved when rich stationary sounds are encoded at low bit-rates, or when the coder operates in a variable bit-rate mode.
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