Fig. 3 is a simplified block diagram. The input signal voltage is commutated to the storage condensers by successive closures of the input switches at a rate set by oscillator I, and commutated thence to the output terminal by successive closures of the output switches a t a slower rate under control of oscillator 2. The latter runs a t a fixed rate of 16 000 Hz, a frequency .great enough to avoid generation of audible components due to commutation. Oscillator 1 has a higher frequency which is adjustable. The frequency demultiplication ratio is equal to the ratio of the oscillator frequencies; for example, it i s 50 percent with oscillator 1 set at 32 000 Hz.Both switch control circuits begin each reset cycle simultaneously with the first input and output switches closed. Then the switch closures progress to the right in Fig. 3, the input rate being greater than the output. With no glottal synchronizing signal present, the input circuit reaches its last position and waits there until the output circuit reaches its last switch. Then the next pulse from oscillator 2 resets both circuits and the cycle repeats. If a glottal synchronizing signal is received, the circuits are reset immediately regardless of how far the current storage-readout cycle has progressed.Evaluatiork of this unscrambler was performed with taped helium speech stimuli, as well as in on-line operation during dives. Highly successful performance was demonstrated with divers a t pressure depths as great as 1000 ft. Objective word intelligibility test results were 88 percent a t 500 ft and 78 percent at 800 ft, as compared with previously reported intelligibility of 5 percent for uncorrected helium speech a t a depth of 600 ft . ACKNOWLEDGMENT

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A new method of modulating a signal, using Sigma Delta modulation in conjunction with switching-and sub-sampling mixers, is presented. Besides the normal functionality provided by the mixer an additional mixing or modulation function can be achieved with a small modification to the existing mixer. This modification can be carried out on a big amount of mixers including the above mentioned. Although the principle was meant to be used to down convert analog signals the principle is general and can be used in digital circuits too. This paper verifies the new mixing scheme and compares it to the traditional mixing scheme. Also the most suitable applications of the different mixing schemes are discussed.

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