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Perceptual bit allocation for low rate coding of narrowband audio

In this work we consider adaptive bit allocation for perceptual coding of narrowband audio signals at low rates (down to 8 kbit/s). Two different strategies are used to shape the audible noise spectrum. In one approach, the quantization noise spectrum is shaped in parallel with the masking threshold curve. This way the noise is equally audible in different frequency bands. The other approach generates a flat noise spectrum above the masking threshold. The noise power is not equally distributed over the frequency range, hence it is audible to various extents at different frequencies.