Wideband quality speech encoders with bit rates of 16-32 kbits/s.

Telephone channels restrict the bandwidth of speech signals to within the range of 0.3 to 3.4 kHz and in doing so cause significant attenuation and distortion in unvoiced sounds with resulting loss of intelligibility and quality. A bandwidth compression system is proposed in this paper, whereby 0.3 to 7.6 kHz band limited speech is frequency mapped into the 0.3 to 3.4 kHz telephonic bandwidth range and is subsequently digitized prior to transmission. The received signal is decoded and spectrally expanded using a corresponding frequency de-mapping process. Consideration is given to the digital encoding of the compressed signals for bit rates between 16 and 32 kbits/s. We find that the quality of the recovered speech is improved compared to band limited telephonic speech.