Underwater Voice Communications Research Articles

Data transmission in the hydroacoustic channel – experimental researches

ABSTRACTThe paper presents the results of experimental researches of data transmission in water using Direct Sequence Spread Spectrum technique and the RAKE receiver. The signal with the QPSK modulation was spread by factor of 4, 8 or 16 chips. Researches were made to verify that spread spectrum technique can be used to underwater voice communications. Static studies were conducted in a small laboratory water tank and in a harbour. Propagation properties were evaluated based on the estimated impulse response of the hydroacoustic channel.

Signal processing in underwater acoustic communication system for manned deep submersible “Jiaolong”

In this report, signal processing in underwater acoustic communication system for manned deep submersible “Jiaolong” is introduced. 1. Four communication methods are integrated to meet different needs: (1) coherent underwater acoustic communication, with a variable transmission rate from 5kbps to 15kbps, to transmit images. (2) Non-coherent underwater acoustic communication, with a transmission rate 300bps, to transmit texts, instructions, and sensor data. (3) Spread spectrum underwater acoustic communication, with a transmission rate 16bps, to transmit instructions. (4) Underwater voice communication, using analogy modulation method to transmit human voice. 2. Signal processing method in coherent communication mainly consists of concatenation of decision feedback equalizer and Turbo decoder, and wavelet based image compression with fixed length coding. In the equalizer, Doppler compensation, multichannel combining and equalizer coefficients updating are all using fast self-optimized adaptive algorithm. 3. A linear hydrophone array is lowered from the mother ship to certain depth, and spatial diversity combining technology is adopted. From July to August 2011, diving trial of “Jiaolong” was carried out in the Pacific Ocean. The communication distance can cover nearly all ocean depth. The covering conical area is wider than 100 degree. An optical/acoustic image could be transmitted in 7 or 14 seconds.

Underwater voice communications using digital techniques

A digital signal processor (DSP) based voice communication system for divers with a 2400 bit/s transmission rate is described. Each speech signal frame is analysed using linear predictive coding (LPC) and the essential parameters such as amplitude, pitch period and digital filter coefficients are extracted. These parameters are transmitted through a water channel at a rate of 2400 bit/s by employing digital pulse position modulation (DPPM)

The intelligibility of simulated underwater voice communication as measured by the NSMRL Tri‐Word Test of Intelligibility (TTI)

Our previous paper suggested the TTI for use in assessing the effect of temporal distortions on speech intelligibility. Here, the TTI is applied in an investigation of the phenomenon of multipath, a temporal distortion which can severely limit the intelligibility of underwater voice communication. Evidence suggests that intelligibility may be improved if speech is time‐reversed prior to underwater transmission. Results are presented for simulated forward and time‐reversed speech conditions across an echo delay range of 50–500 ms as a function of the amplitude of an echo relative to that of the original signal. Asymmetric intelligibility functions are demonstrated which are dependent on the length of echo delay interval. These results are discussed in terms of temporal masking and perceptual fusion effects.

Diver-to-Diver Speech Intelligibility of Underwater Communication Systems

The purpose of this research was to evaluate speech intelligibility of underwater diver-to-diver communication systems. To accomplish this, a second DICORS was designed for two diver/listeners, who could respond to speech produced by a diver/talker situated in the original DICORS. The research was conducted in 30 ft of water at NRL's Underwater Sound Reference Division's Bugg Springs facility. Speech materials consisted of Clarke multiple-choice lists; responses were recorded on an IBM keypunch, at the surface, coupled to specially constructed multiple-choice response switches activated by listeners. Five experienced diver/talkers and up to eight experienced diver/listeners (the average talker/listener totals were 36) were situated 30 ft apart and back to back. All systems evaluated were rigged with the Bioengionics “Nautilus” muzzle and a double hose regulator. Several units could not be kept in operation, however, and that equipment could not be evaluated. Mean scores for the other four units ranged from 57% to 64%. Our scares are higher than those previously reported for these units under diver-to-surface protocols; however, they probably reflect (1) experience with this type of research of both talkers and listeners, (2) the use of multiple-choice word lists (which elevate scores) instead of PB25 word lists, and (3) use of the Nautilus mouthcup and a double hose regulator (which has been shown to be the optimum available support configuration). In any case, it can be concluded that (1) while reasonably good diver communication systems are available, they are not yet of sufficient quality to allow for adequate underwater voice communications, and (2) no single approach (modulated, hardline, acoustic) is currently superior to the others. [This research was supported by Physiological Psychology Branch of the Office of Naval Research.]