Apparatus for expanding narrowband speech to wideband speech by codebook correspondence of linear mapping functions

An apparatus for recovering wideband speech from narrowband speech, said apparatus comprising:

• a linear predictive coding analyzer for performing a linear predictive coding analysis on an inputted narrowband digital speech signal to thereby obtain a set of narrowband spectral envelope parameters and a residual signal;
• a spectral envelope codebook having a plurality of spectral envelope codes, wherein each of the plurality of spectral envelope codes is a predefined set of narrowband spectral envelope parameters;
• a linear mapping function codebook having a plurality of linear mapping functions for linearly mapping the set of narrowband envelope parameters to a set of wideband spectral envelope parameters which correspond to the plurality of spectral envelope codes on a one-to-one basis;
• a distance calculation means for calculating a distance between the set of narrowband spectral envelope parameters and each of the plurality of spectral envelope codes contained in said spectral envelope codebook;
• a weights calculations means for calculating weights for the spectral parameters based on, and corresponding to, each of the distances calculated by said distance calculations means;
• a linear mapping function calculation means for calculating a plurality of sets of wideband spectral envelope parameters using each of the plurality of linear mapping functions contained in said linear mapping codebook and the set of narrowband spectral envelope parameters directly obtained from said linear predictive coding analyzer;
• a linear map result adder for weighing the plurality of sets of wideband spectral envelope parameters using the weights calculated by said weights calculation means and for summing the weighted sets of transformed spectral envelope parameters to obtain a set of wideband spectral envelope parameters;
• a residual converter for converting the residual signal into a wideband residual signal;
• and
• a linear predictive coding synthesizer for synthesizing the set of wideband spectral envelope parameters and the wideband residual signal so as to obtain a wideband digital speech signal.