A digital signal processing fish tracker, investigation and design

Sullivan, Michael A. (1994) A digital signal processing fish tracker, investigation and design. Masters thesis, Memorial University of Newfoundland.

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Abstract

This thesis describes an investigation into the design of a digital signal processing ultrasonic fish tracker. This work was initiated to overcome problems with an analog system presently used at Memorial University to track individual cod. Consisting of a local oscillator and several superheterodyne receivers, the analog system now works intermittently and gives unreliable results. A study into a more compact receiver design based on recent technological advancements was then commenced. The new receiver will connect to the existing four hydrophones, interface through a PC ISA bus, fit on a single PC proto-board, and track three frequencies simultaneously from each of the four hydrophones. Position resolution under 1 m is achievable, over a 500 m range. -- For this study only passive fish tracking systems are investigated where a transmitter is attached to the fish and a grid of detecting hydrophones are placed around the area 01 interest. These systems are reviewed from beat frequency receivers used in the 1950's, to computer controlled superheterodyne receivers in use today. Analog designs are prevalent throughout all these systems. -- To understand the received signal, a review of sonar fundamentals is presented. This includes the transmitter, the transmission medium (water) and the receiving hydrophone. The sonar equation is introduced as a means of calculating the detection level at the receiver. Background noise and problems caused by working in shallow water are also considered. -- An undersampled technique is used to reduce the data rate and the computational load on the Digital Signal Processing (DSP) chip. Signals from each of the tour hydrophone channels are multiplexed together and then digitized at 41 kHz. Digital data from each hydrophone is transferred to the DSP chip and grouped into blocks of 64 points. A fast Fourier transform (FFT) is used to convert each block into the frequency domain. Transmitter frequency is deemed detected if the magnitude response at one of the transmitter frequencies is greater than the threshold. -- Undersampling equations are developed to give upper and lower bounds on the sampling frequency for each of the frequency folds. In the frequency domain, the effect of smearing on adjacent transmitter frequencies is investigated as a function at sample rate and bandwidth. Bit growth problems when using fix-point FFT calculations are also discussed. -- The system is designed wherever possible with existing components, in order to keep the cost of a prototype low. Finally, simulations of the DSP source code confirm that the system performs as designed.

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