Hi Napper,
I am no expert but I copied the following post that was written by a person that is an expert. Hope you find it helpful.
Bob
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Explanation Of Span & Wrap
Without getting too technical - of which I am often "accused" .... here's a brief explaination of SPAN and WRAP ....
To understand these two features, it is necessary to understand the basics of "Correlation". With Correlate = ON, Spectra first looks at the target return from all three frequencies. It then picks the two that are hitting the hardest, and compares the information from these signals to see how closely they match (correlate). By adjusting SPAN and WRAP, you can tell Spectra how closely you want the target frequency information to match before it considers the target GOOD. This is an excellent tool if you find yourself in an iron-rich environment, and it also helps reduce the effects of EMI. With these basics in place, you can now better understand the explaination of SPAN and WRAP that follows .....
SPAN determines how much difference (how many VDI numbers) are "allowed" between frequencies for the target to be "accepted". In my program, I allow "20", which is based on a lot of testing of various types and sizes of targets from small bits of foil to large gold rings. After I got all the readings from all the frequencies for all the targets loaded into a matrix, I looked at the MIN / MAX / MEAN / AVERAGE of the universe, and then added about 30% to the WORST CASE scenario to account for bad ground, adjacent junk, etc. Increasing any further will allow more bad targets to be accepted, but that might be better if your soil is REALLY bad. Mine is "sweet", so I can actually run with SPAN = 15 w/o any kind of significant target dropout, etc. However, at that setting, adjacent junk (bottlecap next to a coin / foil next to gold) has the effect of causing "sputter". The extra 5 points is all I needed to clean up and restore the "good target" status. If you ran SPAN = 40, it probably wouldn't hurt anything in terms of performance, especially if you have a lot of mineralization to deal with. The range is 0 to 190. Zero says "everything is an exact match" and that makes no sense. 190 says "everything is a match". That's dumb too. Again, my setting of 20 was developed doing bench and field testing IN MY GROUND. Jack (CyberSage) runs his Span around 35 if memory serves, so if you cranked yours up to 40, you'd be close to both of us. Try it with some targets in your ground to get "the right answer" for YOUR area.
WRAP controls how far high end signals (+90's) can go from the +95 into the -90's region (wrap around) and still be considered a good target. If WRAP is set = OFF, any signal that "wraps" into the -90's is immediately disregarded. That's not getting the most out of the correlate feature. Soooo - by adding a wrap limit of -94, I'm giving something like a silver dollar, in high mineralization or next to a piece of junk the opportunity to "wrap" all the way to -94 before its kicked out of the pool. For what its worth, Jack (CyberSage) and I arrived at this setting independently. Range = -95 to -90. The HIGHER the number, the closer to +95, and the less wrap accepted. -95 IMHO is "too tight" and anything more is bringing in too much junk, which is counter to the reason we're using Correlate in the first place .... Good, bad or moderate ground, I don't think I'd go any further then -94 on the Wrap.
Hope this answers your questions, and hope this is helpful. Let us know how the Correlate program works out for you and what you did to optimize performance in YOUR ground. Thanks. Da FoX
I have gained a lot of knowledge from this forum as there are many smart contributors here. Thank you, Robert