Some time ago, I had a conversation with an avid treasure hunter whose instrument of choice was White's Eagle II SL 90. He described to me a technique with which he could discern pull tabs from rings, nickels, and other desirable targets by listening for some subtlety in the audio response. Then he asked me, "If I can do this, why can't you program the Eagle's microprocessor to do it?" My response was, "If you can do it yourself, why would you want the microprocessor to do it for you?"

I mention this as a means of illustrating what I consider to be the metal detector designer's fundamental dilemma, which is, as Prince Hamlet might have phrased it, "To beep or not to beep." More specifically, if you're faced with a target at some depth in badly mineralized ground and the detector has a hard time getting a solid reading on it, what do you do? If you design your detector to ignore the target, and then someone comes along with their El Cheapo brand detector and digs the target, which just happens to be a USD $10 gold piece....you're in big trouble. If, on the other hand, your customers find that they're spending most of their time chiseling through eight inches of hardpan and finding bent nails and wads of aluminum foil, you're not much better off. The point I was trying to make with the gentleman who had devised the clever pull tab discriminating scheme was, that if you put too much of that kind of "intelligence" into your metal detector, there are always going to be those targets that you miss because the machine got fooled.

Which brings me, of course, to the Spectrum XLT. The XLT has all the features, performance, and flexibility of previous members of the Eagle series, plus a new display which makes the instrument remarkably easy to use. It also makes use of a new way of displaying information about targets - the "SignaGraph®" or "Phase Spectrum Analyzer" - which shows the operator everything that we currently know how to display about the characteristics of metallic objects in the ground. In this way, we have gone a long way towards addressing the dilemma I mentioned earlier. The Spectrum XLT is a very "smart" detector, but it is also an "honest" one. Having done the best it can do to determine the probable identity of the target, the Spectrum XLT gives you all of the information you need to make your own decision (human beings are, despite what you might have heard, still a whole lot smarter than computer) to dig, or not to dig.

Before I begin to describe in some detail what the SignaGraph® is and how it works, I should emphasize that you don't need to know how it works in order to use it effectively, and that the best way to learn how to use it is in the field. I should also point out that even if you ignore the SignaGraph altogether, this instrument still has the audio discriminator and V.D.I. number that its predecessors had, plus the target icons, and significant improvements in terms of weight, physical size, and ease of operation.

For many years, White's has built detectors which identify targets based on a V.D.I. number (Visual Discrimination Indicator) which characterizes metallic objects according to their size, shape, and composition. The V.D.I. scale on the Spectrum XLT runs from -95 to +95. Large positive numbers typically indicate objects that are good electrical conductors; for example, silver dollars will come in at 92. Smaller positive numbers usually indicate objects which, because of their size, shape, or composition, are not as conductive; nickels will read about 20, and aluminum foil may come in near 5. Large negative numbers are typical of targets which are readily magnetized, but which conduct electricity poorly or not at all. Some sands or soils which have a high concentration of ferromagnetic minerals may read -93. Metals containing iron have both magnetic and conductive properties, which causes them to spread over a wide area of the scale, although most typically iron objects will fall in the range -30 to -75. (See Phase Chart).



The V.D.I. reading is an excellent way to determine the identity of most commonly occurring targets, although I might mention in passing that the only 100% reliable discrimination is called a shovel!

If all of this sounds confusing or mysterious to you, allow me to put your mind at ease. The Spectrum XLT is one of the simplest-to-operate detectors you will ever use. I shall describe just how and why it is so easy to use momentarily; but before I finish talking about the SignaGraph®, I want to say it one more time - you don't need to be a Nobel Prize candidate to figure out what the display is telling you.

What makes the Spectrum XLT so easy to use? The key is something that is known in the software business as a "menu-driven interface." To implement that, we have used what is known in the display business as "A True Graphics Display." What all of this means to you, the user, is that all of the controls and options are listed clearly in plain English on the display. A flashing arrow appears on the screen next to one of those options; you can move the arrow up or down with the two "arrow" keys on the 5-key touchpad. When the arrow is next to the control you are interested in, you push the ENTER key. That is everything you need to know to run this machine. If you are like me and you hate reading instruction manuals, I believe I can safely guarantee that you will be able to operate the Spectrum XLT successfully your first time out without having to open the cover - although the manual should be extremely helpful if you want to fine-tune the performance of your detector by adjusting any or all of the rather lengthy list of professional options. Incidentally, another name for this method of running a machine is the "point-and-shoot" method; you point at what you want, then "shoot' with the ENTER key to make it happen. Finally, for those in a hurry, there are a number of "shortcuts" designed to make accessing commonly used features as fast as possible.

What makes the Spectrum XLT even easier to use are the factory-preset programs (like those in previous Eagles) which you can load with just a few simple keystrokes by following the prompts in the display.

Any attempt on my part to detail all of the advanced features and controls which the XLT has to offer would probably leave me with blisters on both of my typing fingers. Most of the features are here because somebody asked for them - the moral of the story being, keep those cards and letters coming, and we will continue trying our best to give you the kind of detector you really want.

Mark Rowan is a Senior Engineer for White's Electronics, Inc. Mark holds degrees in General Science, and Electronics Engineering Technology, and is a graduate of the University of Oregon. His background includes satellite communications and RF test and measurement instruments.

Features and control settings of the Spectrum XLT

Coin discriminates (rejects), most common junk items like nails, foil, pull tabls and hotrocks, and responds to most coins and some items of jewelry. It is a good program for lawns parks, and playgrounds.

Coin and Jewelry offers settings similar to Coin, but provides less discrimination (less trash rejection). Coin and Jewelry is a good program to use if less less discrimination and more jewelry is desired.

Jewelry and Beach offers settings similat to Coin and Jewelry, but some of the Pro Options are changed to deal with salt water and other aspects of beach use.

Relic offers even less discrimination than Jewelry and Beach. It responds to all metals except small iron such as nails and some stainless steel. Suitable when the desire is to hear all significant targets and discriminate iron with the display.

Prospecting does not discriminate at all with the audio tone. However, the display will show a V.D.I. number only for metals that could be gold avoiding many iron targets.

Custom Programs 1-4 allow you to save and name your own special programs.

10 Basic adjustments
Target volume, audio threshold, tone(audio frequency), audio discrimination, silent search, mixed mode, A.C. Sensitivity, D.C. Sensitivity, Backlight, viewing angle.

29 Pro Options conveniently located in 5 menu areas.

Audio
Ratchet pinpointing, S.A.T. Speed, Tone I.D., V.C.O., Absolute value, Modulation.

G.E.B./Track
Autotrac, Trac view, Autotrac speed, Autotrac offset, Trac inhibit, Coarse G.E.B., Fine G.E.B.

Discrimination
Disc. edit, Block edit, Learn accept, Learn reject, Recovery speed, Bottlecap reject

Display
Visual disc., Icons, V.D.I. sensitivity, D.C. phase, Graph averaging, Graph accumulating, Fade rate

Signal, Transmit, Receive)
Transmit boost, Transmit frequency, Preamp gain