Chuck Husick: Techno-Talk, January 2004, BoatUS Magazine - current as of January 2009

Digital Signal Processing (DSP)
Does a newly installed piece of electronics on your boat indicate that the device uses “Digital Signal Processing” (DSP) circuitry? Unless you are a circuit design engineer or a ham radio operator, it is unlikely that the DSP label will register as something worthwhile. However, DSP can make a substantial difference in the way a device works. Depending on the equipment you have, the performance of your fishfinder, HF radio and radio fax receiver may already be benefiting from DSP technology. In fact, you may already be using DSP in your car radio or home audio system.

DSP can greatly simplify the operation of equipment by reducing, or even eliminating, the need for manual adjustment of controls by automatically providing optimum results. You can see the benefit of DSP most clearly in devices that produce an image.

DSP-equipped fishfinders, for example, can deliver exceptionally clear images of all that lies below the boat, differentiating between fish targets, fish and sea surface turbulence, and fish and the bottom.

In a conventional fishfinder design, the performance of the sonar transmitter and the receiver is determined by the unchanging electrical characteristics of the various resistors, capacitors, inductors and transistors used in the circuit.

DSP works by substituting a software controlled, special-purpose microprocessor, the DSP chip, for a collection of conventional electronic components in signal processing circuits. The processing of data is determined by software, not hardware. A multitude of operating choices are available and can be changed instantaneously to optimize the final result.

Specific attributes of DSP technology can be clearly seen in the Raymarine DSM 250 black-box fishfinder. In a conventional unit, the rate at which successive pulses of sonar energy (pings) are transmitted, the length of each pulse and the power used is adjusted by front panel controls. In the DSM 250, all of these variables are controlled by the unit’s software, managed by the DSP chip. The system evaluates the result of the chosen settings and automatically makes changes to improve the outcome — clear visibility of fish with minimum interference from both surface and bottom reflections.

In the DSM 250, an echo received by the sonar transducer is converted into a series of 36-bit digital words and processed at rates as high as 56 Mhz. Operation at these speeds allows the system to optimize each one of the transmitted pulses of sonar energy and to make thousands of adjustments to the receiving circuit as each successive sonar reflection reaches the transducer. Depending on the system, more than 200 separate adjustments may occur in a fraction of a second.

While a conventional fishfinder in the hands of a skilled operator can perform as well, the average user will find that the automation provided by DSP produces superior results.

Used in a radio fax receiver, DSP will produce a clear image of a weather map, or the satellite data, where a conventional receiver might, at best, deliver a minimally useable image.

Successful use of a radio fax often depends on the user’s skill in precisely adjusting the receiver’s tuning and signal filtering circuits. However, even when all adjustments are tuned to optimum, changes in atmospheric conditions can degrade performance. In a DSP enabled receiver such as the Furuno FAX 30, the filter circuit performance is automatically and continually optimized to produce the clearest possible signal. The optimization process is extremely rapid and remains in step with even the changes in conditions.

The value of DSP can be heard in many of the newer car radio systems where it is used to create a synthetic listening environment that can mimic the acoustics of a concert hall or jazz club in the confines of the vehicle. The similarity of a boat’s interior acoustics to that of a car or van make it likely that audio entertainment systems sold for use on boats will soon adopt DSP technology as a way of improving the listening experience on board.

By Chuck Husick

Chuck Husick is a pilot, engineer, sailor and former president of Chris Craft Boats.

© Copyright BoatUS Magazine 2004