4. A wideband speech signal reconstruction method comprising: a first step wherein an input narrowband speech signal is spectrum-analyzed; a second step wherein the spectrum-analyzed results obtained in said first step are vector-quantized using a narrowband speech signal codebook; and a third step wherein the quantized values obtained by said vector quantization in said second step are reconstructed to obtain a wideband speech signal through use of a representative waveform codebook.
5. The method of claim 4 further comprising: a fourth step wherein said input narrowband speech signal is up-sampled to compute sample values; a fifth step wherein frequency components outside the band of said input narrowband speech signal are extracted from said wideband speech signal obtained in said third step; and a sixth step wherein said out-of-band frequency components obtained in said filth step and said sample values obtained in said fourth step are added together to obtain a wideband speech signal.