Normal quantization or requantization noise is white, but the ear's sensitivity to low-level broadband noise is not uniform with frequency. By adopting a suitable weighting curve to represent low-level noise audibility, one can design dithered requantizing noise shapers to approximate the inverse of the audibility curve and hence achieve the least audible noise penalty. If Fielder's modified E-weighting curve is adopted as a model of the 15-fon audibility curve, a reduction of 10.9 dB in perceived noise is possible by the use of a simple second-order noise shaper. This result is already within 0.6 dB of the theoretical minimum set by information theory, and almost a 2-bit gain in apparent signal-to-noise ratio. Even greater perceived noise reductions are possible if one adopts an audibility weighting curve which more closely approximates the ear's precipitous high-frequency rolloff, and incorporates a higher-order filter into the noise-shaper's feedback loop. In face, a 20-dB apparent reduction in the requantizing noise is then possible with filters of modest order, but the penalty is a significant increase in the total noise power. This paper explores these questions and illustrates some of the available options. Such noise shaping will soon get advantageous in order to preserve on the 16-bit compact disc the lower noise floor of an original 18- or 20-bit master recording.
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