Telephony speech enhancement by data hiding

The current public switched telephone network (PSTN) is only able to deliver analog signals in a relatively narrow frequency band, about 200-3500 Hz. Such a limited bandwidth causes the typical sound of the narrowband telephone speech. In order to improve intelligibility and perceived quality of telephone speech, we propose using data hiding to extend the PSTN channel bandwidth. Based on the perceptual masking principle, the inaudible spectrum components within the telephone bandwidth can be removed without degrading the speech quality, providing a hidden channel to transmit extra information. The audible components outside the PSTN bandwidth, which are spread out by using orthogonal pseudo-noise codes, are embedded into this hidden channel and then transmitted through the PSTN channel. While this hidden signal is not audible to the human ear, it can be extracted at the receiver end. It results in a final speech signal with a wider bandwidth than the normal PSTN channel. Using both theoretical and simulation analysis, it is shown that the proposed approach is robust to quantization errors and channel noises. Although we cannot physically extend the transmission bandwidth of PSTN, the telephony speech quality can be significantly,improved by using the proposed data hiding technique.