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Conductive Soil Subtraction And Addition...what Is It?


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I am still having some brain fog from Covid or maybe I am just old.

This may have already been explained and I missed it.

What are Conductive Soil Subtraction and Addition?

Are these two happening automatically in the background using FMF or do they refer to picking the correct FMF program with their different frequency weighting for the soil conditions encountered. So basically is CSS/A done automatically or by user trial and error?

This paragraph from the XP Deus ll FAQ section of the XP website does not clarify it for me. Maybe it is perfectly clear for some of you.

DEUS II is unlike many other multifrequency detectors that usually offer multiple fixed frequencies, the DEUS II implements real combinations of different frequencies depending on the selected programs. For example, some will use low to medium frequencies (4 to 14kHz), and other programs will include higher frequencies up to 24kHz or 40kHz. To these combinations of frequencies specific signal processing is applied in order to best adapt to the terrain. These frequencies can be subtracted to remove electrically conductive soils or added to favour a wider range of targets if the soil is not too wet. 

So again, that last sentence refers to: subtracted or added by whom, the detector or me?

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I was stumped on that too. My guess is that XP is attempting to describe their programs as using different algorithms than SMF without giving out too much proprietary information, but with enough info to show how FMF is different from SMF.

I think what they are saying is that SMF algorithms just add frequencies (example 4kHz, 14kHz, & 24kHz) to the transmit signal and do not filter or process those (I do not know if that is how SMF works), whereas, FMF may process or filter to reduce certain frequencies to compensate for noisy ground and add certain frequencies to enhance different kinds of targets. 

I think those are probably fixed frequency algorithms within each program and can't be user edited outside of the normal user controls.

It may actually be exactly how SMF works too, but I don't know.

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Frequency subtraction is how salt notching works in an MF design. Salt has a weaker response at low frequency and stronger at high. So if you subtract them in the right proportions then salt disappears. In older MF detectors this was done all the time. The V3 gave the option of turning it off because it also notches out really small gold. I assume newer models only subtract salt in the beach modes. If there is no salt then it's also possible to add frequency components which (I think) will improve the resistive response across the board, which is the signal used for determining the presence of a target.

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Hi Carl,

I am not totally sure but the paragraph quoted above seems to refer to all of the FMF modes on Deus 2 not just the saltwater modes

Deus 2 has manual salt sensitivity and iron sand settings adjustments on top of whatever CSS and CSA are........

The Equinox does all of that automatically in the background using its beach modes, especially Beach 2 from my understanding.

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I have no idea. If it were my design then only beach modes would do salt subtraction by default, although I might include the option in other modes as highly alkaline soil can also be a problem. Maybe that's what Deus 2 is doing. I may get the chance to borrow one, if I do then I'll make a salt wand for it and test it.

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50 minutes ago, Geotech said:

I have no idea. If it were my design then only beach modes would do salt subtraction by default, although I might include the option in other modes as highly alkaline soil can also be a problem. Maybe that's what Deus 2 is doing. I may get the chance to borrow one, if I do then I'll make a salt wand for it and test it.

What is a salt wand and how to make one?

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Wind a coil of wire and then "short" it with a resistor. This allows you to accurately make any tau you want. For salt you want a really low tau which allows for a fairly high R. So start with a small gauge wire, something small enough it is not detected alone, maybe 30awg. I would start with 5-6 turns @ 3-4" diameter. Then use a pot as the R-element and adjust the pot until you get the tau you want. Measure the pot, replace with a fixed resistor.

If you don't know the tau you're shooting for then make a guess and wave in front of the detector (assuming MF with salt notch enabled). When you get it right the detector should have no response. BTW, the pot/resistor needs to be mounted well away from the coil. Extend the coil wires 12" or so down a handle and put them at the end of the handle. Make sure the wires down the handle are twisted pair.

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4 hours ago, Jeff McClendon said:

I am still having some brain fog from Covid or maybe I am just old.

This may have already been explained and I missed it.

What are Conductive Soil Subtraction and Addition?

Are these two happening automatically in the background using FMF or do they refer to picking the correct FMF program with their different frequency weighting for the soil conditions encountered. So basically is CSS/A done automatically or by user trial and error?

This paragraph from the XP Deus ll FAQ section of the XP website does not clarify it for me. Maybe it is perfectly clear for some of you.

DEUS II is unlike many other multifrequency detectors that usually offer multiple fixed frequencies, the DEUS II implements real combinations of different frequencies depending on the selected programs. For example, some will use low to medium frequencies (4 to 14kHz), and other programs will include higher frequencies up to 24kHz or 40kHz. To these combinations of frequencies specific signal processing is applied in order to best adapt to the terrain. These frequencies can be subtracted to remove electrically conductive soils or added to favour a wider range of targets if the soil is not too wet. 

So again, that last sentence refers to: subtracted or added by whom, the detector or me?

Sounds to me like it adds or subtracts automatically according to what the next to last sentence says in your quote from the manual.  It says “specific signal processing is applied in order to best adapt to the terrain.”  To me the specific signal processing sounds like a filter that either adds or subtracts frequencies based on the ground reading.  But, heck I don’t really know. That’s just how I interpret it. 

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3 hours ago, Geotech said:

Frequency subtraction is how salt notching works in an MF design.

So are you saying for subtraction modes the high frequency is only really used for salt identification within the algorithm?

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5 minutes ago, Dan Fox said:

So are you saying for subtraction modes the high frequency is only really used for salt identification within the algorithm?

No, it's also used for target detection & ID.

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