Frequency compression of 7.6 kHz speed into 3.3 kHz bandwidth

Telephone channels restrict the bandwidth of speech signals to approximately 0.3 to 3.3 kHz, with the consequence that the intelligibility of unvoiced sounds may be significantly impaired. To prevent this bandlimitation of unvoiced sounds while still confining the speech to the telephonic bandwidth, we propose a scheme which on recognizing the presence of unvoiced sounds extending to 7.6 kHz, frequency maps them into the band 0.3 to 3.3 kHz. Four mapping laws are considered and the unvoiced speech is compressed using each law. Frequency de-mapping is employed, and the law that has the best spectral match to the speech spectrum is selected. Voiced speech is bandlimited from 0.3 to 3.3 kHz. Results indicate that the adaptive frequency mapping algorithm significantly enhances the recovered speech compared to telephonic speech.