SIGNAL BAND EXPANDING METHOD AND APPARATUS AND SIGNAL SYNTHESIS METHOD AND APPARATUS

A bandwidth expanding method and apparatus in which frequency characteristics of high-frequency components of broad band signals can be adjusted to the liking of the user, overflow due to addition is prevented from occurring without power variations being perceived by a user, the number of broad band formants is reduced, and emphasis is attached to the rough structure of the spectrum, so that the produced broad band speech signals can be improved in quality. To this end, in a speech bandwidth expansion device, frequency characteristics of the frequency components not less than 3400 Hz are adjusted by preset alterable parameter values and summed to the original narrow band speech components. If overflow has occurred in a sample, the high-range gain of the sample is lowered to a level below the overflow level before proceeding to addition. Also, broad band autocorrelation &ggr;w is generated and inverse-transformed in an inverse parameter conversion unit to produce broad band linear prediction coefficient &agr;W to synthesize the broad-band speech in a linear predictive coding synthesis unit.