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Spectral shaping of quantizing noise is an important issue for achieving "transparent" speech quality with low bit rate wave-form coding. To obtain optimum quality in an Adaptive Predictive Coder with Noise Feedback (APC-NF), we have defined a class of functional forms for the noise spectrum, controlled by three parameters, that could span as completely as possible the space of reasonable shapes it could assume. The shaping functions are composed of two factors: a time-varying one, related to the speech spectrum, which accounts for the masking mechanism, and a time-invariant one which accounts for the critical bands phenomenon in the human ear. This formulation led also to a filter design computationally elegant and stable. Subjective tests have shown that optimum quality is achieved when the variable term in the noise spectrum duplicates the square root of the speech power spectrum, while the fixed term consists of a slight pre-enphasis.