Digital encoder with dynamic quantization bit allocation

A digital encoder for compressing a digital input signal derived from an analog signal to reduce the number of bits required to represent the analog signal with low quantizing noise. In the encoder, a digital input signal representing the analog signal is divided into three frequency ranges. The digital signal in each of the three frequency ranges is divided in time into frames, the time duration of which may be adaptively varied. The frames are orthogonally transformed into spectral coefficients, which are grouped into critical bands. The total number of bits available for quantizing the spectral coefficients is allocated among the critical bands. In a first embodiment and a second embodiment, fixed bits are allocated among the critical bands according to a selected one of a plurality of predetermined bit allocation patterns and variable bits are allocated among the critical bands according to the energy in the critical bands. In the first embodiment, the apportionment between fixed bits and variable bits is fixed. In a second embodiment, the apportionment between fixed bits and variable bits is varied according to the smoothness of the spectrum of the input signal. In a third embodiment, bits are allocated among the critical bands according to a noise shaping factor that is varied according to the smoothness of the spectrum of the input signal. All three embodiments give low quantizing noise with both broad spectrum signals and highly tonal signals.