Variable rate coder

Abstract

A variable rate coder for transforming an input signal into a low bit rate digital signal, comprising: a plurality of coding units each comprising an output quantizer having a different number of output bits, and each independently transforming the input signal into a respective compressed digital signal. A plurality of bit rate determined circuits are provided to determine an output of one of the plurality of coding units as a candidate output of the coder by a respective individual determining method, in which method, the quality of the signal which is regenerated from the output of each coding unit is compared with an individual predetermined quality evaluating standard, and then the candidate output of the coding unit having the smallest number of output bits, among the candidate outputs of coding units satisfying each standard, is determined as the actual output of the coder. One candidate output, among the plurality of the determined candidate outputs and, which is further determined by a most suitable determining method relative to the level of the input signal, thus is selected as the actual output of the coder. Further, when the number of the output bits in each of the coding units can be variably set, the most suitable number of output bits for the actual output of the coder is calculated based on prediction gains of the coding units, a required signal-to-noise ratio of the coder, and an ADC signal-to-noise ratio at the analog to digital converter, and the number of the output bits in each of the coding units is set based on the above calculated value, before the above operation by the bit rate determining circuits.