APPARATUS FOR COMPRESSING A DIGITAL INPUT SIGNAL WITH SIGNAL SPECTRUM- DEPENDENT AND NOISE SPECTRUM-DEPENDENT QUANTIZING BIT ALLOCATION

An apparatus for compressing a digital audio input signal to provide a compressed output signal. A circuit derives plural spectral coefficients from the input signal and groups the spectral coefficients into bands. A distributing circuit distributes a total available number of quantizing bits between signal spectrum-dependent quantizing bits and noise spectrum-dependent quantizing bits. A circuit determines a magnitude for each band, and, in response thereto, allocates the signal spectrum-dependent quantizing bits among the bands. This allocates to each band a number of signal spectrum-dependent quantizing bits for quantizing each spectral coefficient in the band. A circuit determines a subjectively-allowable noise spectrum and, in response thereto, allocates the noise spectrum-dependent quantizing bits among the bands. This allocates to each band a number of noise spectrum-dependent quantizing bits for quantizing each spectral coefficient in the band. A quantizing circuit quantizes the spectral coefficients in each band using a total number of quantizing bits determined by summing the number of signal spectrum-dependent quantizing bits allocated to the band and the number of noise spectrum-dependent quantizing bits allocated to the band. The distribution circuit sets the distribution ratio in response to a circuit that calculates the smoothness of the spectrum of the input signal. In this manner, optimum quantizing for both musical signals and single sine wave signals are given by a single processing operation, without the need to repeatedly adjust the quantizing bit allocation.