Micron Modem
0630-SOM-00001, Issue: 03
6
© Tritech International Ltd.
1. Introduction
The
Tritech International Ltd
Micron Data Modem provides a means of transferring data
acoustically through water. Operation is point to point, between a pair of Micron Data
Modems, at operational distances of up to 500m horizontally and 150m vertically at a data
rate of 40 bits per second.
Devices are addressed through a serial electrical interface, which may be controlled directly
from a personal computer with a simple, half-duplex, terminal program.
Spread Spectrum Technology
The quality of acoustic data transmission in water using conventional single frequency
systems suffers considerably from multi-path phenomena. Sound transmitted from the
sending modem arrives at the receiving unit via the direct path, and via a series of secondary
paths, due to reflections from the sea surface and sea bottom. This can often result in the
loss or corruption of transmitted data.
In addition, conventional systems have poor immunity to the continuously varying background
sea noise (such as wave noise).
Tritech Spread Spectrum technology does not concentrate the acoustic energy in one
waveband, but produces a transmission which is linearly varied between 20kHz and 24kHz
(known as a CHIRP waveform). By correlating the received signals with the CHIRP waveform
it is possible to achieve superior performance in challenging multi-path environments.
Identification of a unique transmission signature allows signals to be detected in extremely
noisy conditions, to the extent that communication is successful even when the signal to
noise ratio is as low as -6dB. This means that data streams can be successfully detected
which are considerably below the background noise level.
