Underwater Helium Speech Unscrambler Communication System

The complete inside story

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Underwater Communication system development The Dangers of Deep Sea Diving: In commercial diving, the need for a clear underwater communication system between the diver and the topside supervisor is obvious. The modern underwater communications equipment we use today has been developed over many years to increase the safety for  the modern day diver. The most difficult area of the underwater communications system to develop, has been the helium speech unscrambler communication unit for the heliox diver, this is the diver who breathes a mixture of helium and oxygen when saturation diving. The term helium voice decoder or helium speech unscrambler are not the best terms to describe the helium voice correction process as the helium voice is not encoded or scrambled in the true sense of the word. To explain why, let us first explain why deep sea divers need to breathe a special helium oxygen gas mix. For a diver to be able to successfully breathe underwater, the air or special gas mix he has to breathe needs to be supplied at the equivalent pressure to the surrounding water pressure which he is diving in. If this air or gas mix is not supplied at the same pressure as the water pressure then the immense water pressure on the chest of the diver would prevent the diver from expanding his lungs to fill them with life giving air or gas mix. What this means, is that the deep diver always breathes at pressures many time greater than that found at the water surface. This presents many hidden dangers. Once divers spend any considerable time diving over 140 feet there are risks which have to be resolved for the divers safety. When diving, a divers body tissue absorbs more air or special gas mix the deeper he dives, which creates many dangers to the well being of the diver. A deep sea diver needs the same oxygen level in his blood during his dive which he has at the water surface. But because a divers body absorbs more air or gas mix the deeper he goes, the diver can easily develop an oxygen toxicity due to the body absorbing more oxygen than it needs to function. Inert Gas Narcosis Oxygen toxicity can be avoided by using a special inert gas mix for the breathing-gas mix, this gas mix may cause inert gas narcosis. This is referred to as nitrogen narcosis, since nitrogen may induce a narcotic effect when divers breathe it for more than a few minutes at depths greater than 140 feet. this narcotic effect, has also been called "rapture of the deep," as the inert nitrogen gas is dissolved in the fatty components of the divers nerve cells. Saturation divers use helium as the major component of the special breathing-gas mixture. This gas mix is called heliox (a mixture of helium and oxygen) Helium is an inert gas which is estimated to be 15 times less narcotic as nitrogen. Helium however, requires a longer decompression time because the gas has to be removed from the divers tissues more slowly than nitrogen does to reduce the rate of gas bubble formation, which could cause "the bends". Other disadvantages of helium are the "Donald Duck" effect on the, voice, which complicates the communication problem, and the more rapid body heat loss due to the greater conductance of helium.

To avoid the effect of oxygen toxicity the helium oxygen mix ratio is adjusted for the different diving depths. The normal surface air ratio is about 21% oxygen and 79% nitrogen, the nitrogen component is substituted for helium and then this ratio is changed yet again to suit the divers depth. A diver at a depth of 600 meters breathes mainly helium with only small percentage of oxygen in the supplied gas mix. For this reason helium speech unscramblers have a variable depth control knob, which is used to vary the level of speech decoding required.

The helium voice and diver communication:
Because the mass of helium gas is less than that of nitrogen gas the vocal cords vibrate at a higher rate in the voice box, this coupled with the higher speed of sound for helium results on a very high pitched voice when the diver breathes the helium oxygen gas mix.

The individual characteristics of the divers voice remain fully intact, only the frequency of the voice pattern has been linearly shift to a higher key. When correctly frequency corrected using a high quality helium speech unscrambler, the divers voice can be recognized as the voice pattern characteristics are also restored.

Early helium speech unscramblers created a lot of distortion which made it difficult for reliable diver communication, but over time along with more high speed advanced digital integrated circuits the modern helium unscrambler has become very reliable and successful.

Inside the helium speech unscrambler communication unit:
As stated earlier the term helium speech unscrambler is not the best term to describe the helium voice correction process as the divers voice is not scrambled in the least, but only changed in pitch.

To recover and restore the natural voice of the diver, it is only necessary to slow down the voice to change its frequency. If the divers helium voice is recorded on a tape-recorder then re-played back at a slower speed than it was recorded at, the voice would be perfectly normal in pitch and tone although the diver would sound like he was talking more slowly than normal.

This indicates the only real problem is the time component, in that it takes longer to replay the tape than the diver took to say the sentence in the first place. From this perspective it would be better to call the unscrambler a helium voice pitch frequency correction device.

To overcome this time problem, the modern helium unscrambler continually digitally records into memory chips short samples of the divers helium voice up to four or five times every second, then at the same time, continually digitally replays these recorded helium voice samples back at a much slower speed.

The recorded voice samples are very short, rarely over one second in length when played back. The continual replaying of these short samples at a slower speed provides the ability for real-time communications with the diver.

A typical high quality helium speech unscrambler  printed circuit board as fitted to the Divelink  communication unit operating on 12vdc 3w.

The helium speech unscrambler can be visualized as a revolving wheel of memory data, where the digital voice pattern of the helium breathing diver is recorded into a memory wheel (memory I.C.) up to four times faster in speed than the continual memory reading process used.

This means that both recording and reading of the memory chips occur simultaneously in that the helium voice is being continually  recorded into the memory wheel (memory I.C.) , while at the same time the helium voice is being read out and replayed at a slower speed.

Not all of the recorded helium voice is replayed as the recording process continually updates the memory several times a second.

While this process sounds simple in principle, it is a very complex in that when the recording sequence catches up with the reading sequence the recording process must cease to  write to the memory IC until the phase of the already recorded signal in memory and the new helium voice data to be recorded are in the correct phase.

The reading of the already recorded data continues to occur irrespective of the state of the recording process. The reading process never catches up with the recording process.

This enables a smooth transition between the old and new recordings contained within the memory wheel.

This information is generally not publicized by diving communication companies. The above Divelink digital helium unscrambler printed circuit board design works using exactly the processes that have described above.

This is a very oversimplified block diagram showing the basic operation of a helium unscrambler. The special interactive digital phase comparators and some other internal control circuits are not shown for clarity.

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