Mixed Analog-digital Speech Communication for Underwater Applications

In some cases, a speech communication link is necessary in underwater environments. While in terms of data communication advanced techniques are investigated and employed, mostly traditional (analog) forms of speech transmission are used for speech communication (at least in commercial products). In this contribution we present a mixed analog-digital approach, that tackles this problem by combining digital and analog approaches. Both transmission paths rely on each other in order to lower the necessary bitrate for the digital part and enhance the signal properties of the analog part. The proposed transmission scheme uses linear predictive coding (LPC) filtering combined with a codebook to whiten the input speech. Afterwards, the resulting signal is power normalized and transmitted in the analog transmission part and the quantized normalization factor is transmitted together with the codebook index in the digital transmission part. This leads to an analog transmission signal which is strongly reduced in dynamics.