A New Bit Allocation Method for Low Delay Audio Coding at Low Bit Rates
A low delay variable bit rate audio codec, implemented for wideband arbitrary audio signals, combines inter-frame and intra-frame bit allocation in an adaptive scheme. An outer-loop uses a moving average noise-to-mask ratio (NMR) indicator and a bit reserve to adaptively allocate bits from frames of a lesser perceptual significance to frames of a greater perceptual significance. An inner loop allocates the available bits to each line of the spectrum via an adaptive algorithm based on a weighting function derived from the masking thresholds. Through informal listening tests, the proposed new bit allocation method resulted in an improvement in audio quality over most samples, as opposed to one using a single adaptive intra-frame loop. Particularly, these improvements were more perceptible at the lower bit rates of about 36 kbps as opposed to the higher bit rates of about 64 kbps. Numerical results also indicate a savings of 8 - 10% of the total bit rate.
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