Variable rate coder/decoder system

Abstract

A variable rate coder/decoder system in which a quantizer, under control of a quantization controller, quantizes a signal obtained by subjecting discrete cosine transformation of a difference signal between an input signal and a signal from a frame memory storing therein a previous frame with a first quantization step width calculated based on a power value of the input signal calculated by a power calculator and a predetermined first signal-to-quantization noise ratio. In the system, further, a re-quantizer quantizes a difference signal between a signal being not quantized by the quantizer and a signal after the inverse quantization of an inverse quantizer with a second quantization step width calculated based on the power value controlled by a re-quantization controller and a predetermined second signal-to-quantization noise ratio. Variable length encoders subject the quantization output signals of the quantizer and re-quantizer to variable length encoding, decrease discard priority for the quantization output signal of the quantizer, increase discard priority for the quantization output signal of the re-quantizer, and then transmit them in the form of fixed-length cells or variable-length packets whereby, even when the amount of input data varies, its quality can be maintained constant and encoding and decoding can be realized with a high efficiency.