Waveform compressing apparatus, waveform decompressing apparatus, and method of producing compressed data

In a waveform compressing apparatus, a trial mode selecting portion selects a trial mode having the highest compression rate from a plurality of candidate modes which have not been selected before as a trial mode for generating a residue code, the selected trial mode comprising a scalar quantization mode or a vector quantization mode. A waveform data compressing portion compresses a given data amount of original waveform data according to the selected trial mode so as to generate the residue code, the data amount being determined in correspondence with the selected trial mode. A waveform data restoring portion generates a restored waveform data from the compressed data using the generated residue code. A determining portion measures an evaluation value of a quantization error contained in the restored waveform data relative to the original waveform data, and determines whether the evaluation value is equal to or smaller than a predetermined allowable value. A mode change instructing portion outputs a mode change instruction for instructing the trial mode selecting portion to select another trial mode when the evaluation value is not smaller than the predetermined allowable value.