advertisement
5
Display
Screens
VX 3 has two display screens which depend on the position of the trigger switch:
Search screen: Trigger neutral
Pinpoint screen: Trigger pulled or forward
Search screen
SpectraGraph VDI Icons Depth
Disc.
Mask
Status
Bar
Live Control Bar
The Search screen has several regions as labeled above. The central part of the screen is the SpectraGraph which plots the target response in terms of signal strength versus VDI. In the screen shown, there are three of these responses, one for each frequency. The top region shows the (composite) VDI readout, icons, and depth. The “status bar” shows a few status icons and other information. The Live Control Bar has been covered in previous chapters.
Page 5-1
Spectra VX 3 User’s Guide
VDI
As the loop sweeps over a target the received target signal rises, peaks, and then falls. Ideally the VDI
1
of a “wellbehaved
2
” target is independent of signal strength, but in reality weak signals will show errors in the VDI. For that reason, the most accurate VDI will occur at the peak of the target signal response. Therefore, as you sweep over a target (especially with a slow sweep) you may see the VDI rapidly change before it locks onto a final value:
Peak VDI
No VDI
Changing VDI
No VDI
In three frequency mode the VDI is a composite number based on the peak VDI readings of at least the two strongest responding frequencies. For that reason, you may occasionally notice that the VDI number does not exactly match the Spectra-
Graph response, especially for a weak frequency. In single frequency mode the VDI is obviously taken off the one active frequency.
Since VDI accuracy diminishes with depth, deep targets can produce erroneous VDIs. High ground mineralization can also shift VDIs; again it gets worse for deep targets. This is where experience determines whether you pass over a 1914D Lincoln because the VDI looked like a zinc cent. The best way to learn your detector is to dig a lot of targets, paying close attention to exactly what the detector was telling you before you dug it up.
1. Visual Discrimination Indicator; see Chapter 1.
2. Such as a flat round disc-shaped homogeneous metal target laying horizontally.
Page 5-2
Spectra VX 3 User’s Guide
Icons
Icons are simply a pictorial representative possibility of the
VDI number. Because many different targets can overlap in their VDI numbers, sometimes two icons are presented as possibilities. For example, pull tabs and gold rings often share VDI ranges. As you learn VX 3 , you should find yourself paying less attention to the icons and more to the VDI number and the
SpectraGraph bars.
Depth
The depth readout in normal search mode is only an estimate. With any detector, depth is calibrated for particular targets, usually certain coins, and many other targets may show a large error. For example, ring targets can be deeper than their displayed depth, and shallow stainless steel may display a deeper reading.
Furthermore, the depth displayed in search mode is a “live” depth reading where the signal has passed through several stages of filters. Live depth can vary with sweep speed, typically reading deeper as you sweep slower. Pinpoint depth is based on a static signal and is usually more accurate. Use the live depth reading with a degree of skepticism, and the pinpoint depth as more realistic.
SpectraGraph
The SpectraGraph ® display is a progression of White’s SignaGraph ® which was used in XLT and DFX. Besides the addition of color, SpectraGraph plots the response of each frequency separately, so you can see individual variations in the VDI responses, amplitudes, and decays. The highest frequency is always on top. Obviously, in a single-frequency mode only one frequency is plotted.
SpectraGraph plots target signal strength versus VDI. Note that the VDI axis is not linear; the negative half (-95 to 0) is compressed compared to the positive half (0 to +95). This is
Page 5-3
Spectra VX 3 User’s Guide
22.5kHz
7.5kHz
2.5kHz
VDI
Scale because we are mostly interested in what goes on in the positive half, where the majority of good targets respond. The negative half is mostly where iron falls.
Also note that each response bar is 7 VDI points wide. This means that the bars don’t represent a precise VDI number, but rather a small range of numbers. Because the negative side of the graph is compressed, bars in the iron region will appear to be more narrow, thought they are still 7 points wide. In some cases a target response will exhibit a VDI response that exceeds
7 points, and the SpectraGraph will show this as more than one bar. This can be seen in the example above for the 2.5kHz
response.
The “disc. mask” is the portion of the SpectraGraph region which shows the current discrimination mask. In the following example (Coin & Jewelry), everything below -5 is red
(rejected), and everything at or above -5 is green (accepted).
The response bars in the SpectraGraph are color-keyed to the disc mask. When you edit the discrimination pattern (see Chapter 3) the disc. mask and response bar colors will reflect those changes.
Rejected Accepted
Page 5-4
Spectra VX 3 User’s Guide
Ideally all targets would have tight responses that are repeatable. The closest you can get to that is to air test targets 3-
4” from the coil, and that is not a bad start to learn how different targets respond. However, just like we discussed with VDI numbers, as a given target moves farther from the coil, its response gets weaker and exhibits shifts and spreading in the graph response, and is not as repeatable. This gets even worse in mineralized ground.
Even with three frequencies and SpectraGraph, interpreting target responses can be tricky business. Experience is the best teacher (learn by digging lots of targets!), but there are a few tricks we can discuss.
Smearing
A “well-behaved” target will have a response that closely follows a constant VDI as the loop is swept over it. The vector response for such a target would start at zero, follow a straight line on a constant VDI up to the signal peak (where the loop is centered on the target), and return to zero as the loop is moved away. It will graphically look like
3
:
0
Quarter +83
-95 +95
This might represent a somewhat shallow quarter, and would have a SpectraGraph response with nice tight bars. Note the black dot, which indicates the peak signal strength; this is also the point where the VDI number is calculated. The same quarter buried deeper begins to exhibit a small amount of curvature in the vector response:
3. See Ch. 1 for background info on vector responses.
Page 5-5
Spectra VX 3 User’s Guide
0
Quarter +83
-95 +95
Now the signal response starts at zero and still reaches approximately the same peak point (perhaps resulting in the same final
VDI), but shows some curvature on the way up and on the way back. Because the SpectraGraph response is taken from the whole of the signal response and not just the peak, this can result in a “smearing” effect in the bars, as shown here:
Pay attention to the amount of smearing compared to the depth and the composite VDI number. If it seems like it might be a good target that’s just deep, then that is exactly the sort of target you want to dig.
Trash targets, especially iron, can exhibit significant smearing. To illustrate why, let’s look at the vector response of a bottle cap:
+24
0
-95
Bottle cap
+95
Page 5-6
Spectra VX 3 User’s Guide
As the loop is swept over the bottle cap, the phase response varies wildly. This will not only result in smearing of the Spectra-
Graph plot, but will also cause it to jump around significantly before it locks on to a final reading. Such a final reading might look like:
Notice the peak point from which the composite VDI is calculated; it is clearly in the non-ferrous (good) region, even though bottle caps are typically steel. The point of all this is to pay close attention to what VX 3 is telling you all the time, not just the locked-in response. When you initially detect a target, go back over it with deliberately slow sweeps and watch the dynamic responses.
Unaligned bars
Because VX 3 normalizes the VDI responses of each frequency, the SpectraGraph bars tend to be vertically aligned. In some cases they are skewed, indicating a possible “problem” with the target. For example, in the prior SignaGraph response of the bottle cap, the bars have a very poor correlation, caused by the ferrous properties of the bottle cap.
However, like everything else, there is no hard-and-fast rule at work here. Mineralized soil can also affect frequencies differently such that deeper targets have unequally shifted phase responses. In the “deep quarter” SpectraGraph on the previous page, there is some skewing of the bars. Again, look at the dynamic responses, and also pay attention to the repeatability of the target responses. If every sweep across the target produces a different response, then it is questionable (but if it is very deep, consider digging it!).
Page 5-7
Spectra VX 3 User’s Guide
Mixed response
Sometimes you will see response bars that include both green and red:
This simply means that the target is on the edge of the rejection/acceptance point. Because each bar is 7 VDI points wide a target that is accepted can still show some red in the response bar.
In the above graph, notice that the 22.5k and 7.5k frequencies are nicely aligned, and the 2.5k response is both slightly skewed and broader. This generally indicates that the 2.5k
response is less reliable, as it had trouble “locking” to a particular VDI.
Wrap-around
Occasionally targets will “wrap around” between +95 and -
95. This can be due to a high conductive target (a silver dollar, we hope) getting shifted upward by mineralization. Or, on the down side, it could be due to a “cold rock,” a mineralized rock that is below the ground balance point. Again, paying attention to how other targets are behaving
4
will give you a strong clue as to which of these is more likely.
Status Bar
The final part of the screen to discuss is the Status Bar. To the left is a group of icons which depict (1) the trigger switch mode, (2) wireless headphone status, and (3) backlight status.
4. Are dimes and quarters tending to read high? This could be due to mineralization.
Page 5-8
Spectra VX 3 User’s Guide
The status bar also displays the currently loaded program. If
Autotrac is enabled, the program name may occasionally get overwritten by a “tracking” message, indicating that VX 3 is adjusting its tracking point.
Pinpoint Screen
22.5kHz
7.5kHz
2.5kHz
The Pinpoint screen has regions similar to the Search screen, and is accessed by pulling the trigger switch. The central part of the screen is the Pinpoint bars which show the live signal strength of the received signal, regardless of what causes it. In the screen shown, there are three of these responses, one for each frequency, again with the highest frequency on top.
The top region shows depth. The same status bar and Live Control Bar are also displayed.
In 3-frequency mode the pinpoint bars show which frequency has the strongest response. In the example above,
2.5kHz is strongest. The outlining rectangle around each bar serves as a signal “peak” indicator; that is, the rectangles show the maximum peak the bars have achieved during the pinpoint activity. This is useful in determining the exact position of the target.
Page 5-9
Spectra VX 3 User’s Guide
Salt Mode Anomaly
Normally in Pinpoint mode all targets will produce an audio response as well as the screen response. However, targets that are below the ground balance point will produce nulled audio response. In normal ground, this is not a problem as targets are never below the ground balance point
5
. But in saltwater hunting, the ground balance point can be close to a VDI of 0, meaning that the entire iron range could be below the balance point.
If you are saltwater-hunting and find that a target “disappears” in pinpoint mode, it is likely below the ground balance point, and it is likely iron. You will notice, however, that the
Pinpoint bars still properly respond to the target, so you can still pinpoint it and dig to verify.
5. Except for cold rocks.
Page 5-10
advertisement
Related manuals
advertisement