Helium speech unscramblers--A critical review of the state of the art

The development of saturation diving has enabled man to work in the sea at great depths and for long periods of time. This advance has resulted, in part, as a consequence of the substitution of helium for nitrogen in breathing gas mixtures. However, the utilization of HeO2breathing mixtures at high ambient pressures has caused problems in speech communication; in turn, electronic aids have been developed to improve diver communication. These helium speech unscramblers attempt to process variously the grossly unintelligible speech resulting from the effects of helium-oxygen breathing mixtures and ambient pressure, and to reconstruct such signals in order to provide adequate voice communication. This paper presents a discussion of the effects of HeO2/P on speech and then describes some of the techniques used to "unscramble" the distorted speech. Included among the techniques are: 1) frequency subtraction; 2) tape recorder playback; 3) vocoder approaches; 4) digital coding; and 5) convolution processing. In addition, a generalized evaluation of these approaches is included.