Statistical model for the quantization noise in the MPEG Advanced Audio Coder. Application to the bit allocation algorithm

In an MPEG Advanced Audio Coder (AAC), for each frequency subband, the quantization parameter called scale factor controls the quantization noise and the bit rate. Tuning this coder would require a characterization of the rate-distortion function per subband, which seems to be available only in a parametric manner. In this paper, we show that an accurate model for the quantization noise, depending on the scale factor value, can lead to a simple expression of the relation between the scale factor and the error power per subband. In other terms, we are now able to obtain an explicit writing of the distortion-rate function. In a second time, we apply these results to the coding algorithm in case of a fixed output bit rate and propose a new coding algorithm. Unlike the one described in the informative annex of the MPEG-2 standard, it makes use of a single loop and happens to be faster and more flexible towards psychoacoustic criteria.