Fisher Impulse And The " No Holes " Claim

[Steve Herschbach]

I have seen it said a few times now that the new Fisher Impulse AQ has "no holes". This started as a very specific mention on the part of the designer, and now has been expanded into a range of assumptions that do not match how I view the subject.

First, what is a "hole?" The classic definition originated around ground balancing pulse induction metal detectors. The first and most simple ground balance method is a basic subtraction. Determine where the ground is reading with the detector, then provide a control to subtract that signal.

Many people do not realize that ground signals and gold nugget signals within normal range are basically infinite and overlap 100%. This is due to the nature of natural gold nuggets, which occur in endless variety as far as size, shape, and composition. When added to the ground, they mix and blend with the ground signal, and in one way can be considered a naturally occurring component of a ground signal.

There is one simple rule you must keep in mind that makes all discussions of a "hole" simple. For every potential ground signal that exists, there is also a corresponding gold signal.

The ground balance control is just a special discrimination control. When you eliminate any one type of ground signal or hot rock, you automatically eliminate any gold signals that are identical to those ground and hot rock signals.

The nature of the electronics involved is not 100% in that the ground balance rejection point is not perfectly sharp edged, but covers a small range. Within this rejected ground range, you have the most intense rejection effect at the setting itself. However, items that fall close to the setting, while not eliminated, are weakened, and deliver less than normal depth. The effect is similar to that seen when black holes are diagrammed, and targets that are unintentionally eliminated because they fall into this range are said to "fall into the hole."

The classic metal detector ground balance "hole"

This can also be seen with many VLF detectors. Take a detector with a manual ground balance control, and grab a coin. Air test the coin depth, while running the ground balance control to both extremes. Depending on the coin or target you test, like a gold ring, you will usually find that the depth varies with the ground balance setting.

On a simple ground balance detector with a single ground balance "channel" the effect is quite pronounced. On the White's TDI the ground balance setting is normally around 8. This also corresponds with nuggets weighing approximately 1/4 ounce, though again, because of the natural variation of gold nuggets, the range is actually very large, and based on the time constant of the nugget, not any particular weight. Any nugget that reads at the setting of 8 is essentially eliminated. Nuggets that fall near to the setting have greatly weakened responses.

Minelab first addressed this issue with detectors that have two ground balance "channels". Channels have historically referred to detectors that have two separate ground balance "channels" but people are now misusing the term to refer to detectors that are returning dual audio results that can be separated into "channels." This is not the same thing. A dual channel PI has two ground balance sample points that are being compared. The Minelab SD 2100 has a switch that lets one chose one or the other or both ground balance channels. One favors long time constant targets (large nuggets) and the other short time constant targets (small nuggets). The ground result is roughly the same on either channel. By comparing and combining results from the two channels, emphasizing the one with the strongest nugget signal, the "hole" is largely eliminated. This was the major advance and secret to Minelab PI MPS "multi-period sensing" detector technology. The "timings" developed over time were progressively sophisticated comparisons of multiple channel results intended to address specific ground and hot rock situations.

No matter what though, the hole never goes away entirely. If you eliminate a certain hot rock, you lose gold that reads like that hot rock. People who set for and are willing to dig what others consider ground and hot rock noises, find targets with PI detectors that others miss. It's one of the secrets of the pros. At a minimum, on Minelab detectors, hunting with a different timing will reveal gold missed with another timing.

To sum up, a hole in metal detector terms is what occurs when a good item is unintentionally rejected when some undesired item is rejected or discriminated. This usually refers to good items lost due to the ground balance setting, but can include any items accidentally rejected due to a detector eliminating some undesired target or range of targets.

And that brings us to the new Fisher Impulse AQ and the claim of "no holes." As in all marketing claims, yes.... and no.

Alexandre Tartar has be quite firm since day one in emphasizing that the AQ is specifically designed to find gold ring range targets to the exclusion of everything else. In his early writings he says that the AQ does focus on this range, and that in that defined gold ring range there are no holes. This is true.

Unfortunately that statement has been extrapolated beyond the original intent. It all depends on how you want to define things. I am going with the definition above "a hole in metal detector terms is what occurs when a good item is unintentionally rejected when some undesired item is rejected or discriminated."

Under that definition the Impulse AQ has two holes. They have simply been redefined as not being holes, but being something else. The ground balance control has been hijacked on the AQ and is employed as a discrimination control. Instead of being set to reject a certain ground signal, it is being used to reject a certain target range, specifically many ferrous targets. In the process, most U.S. coins except nickels are also rejected. So we have called the ground balance a disc control, and are attempting to eliminate ferrous items. The unintentional side effect is that many high conductors including most U.S. coins are rejected. Note that if this setting is high enough, heavy large gold rings can be eliminated. The ground balance hole is there, and in a big way. We are going to define it out of existence however by calling it a discrimination control.

The Volcanic Mode, by eliminating intense beach mineralization, will have an adverse effect on most targets. It has to. You can't eliminate a ground signal that severe without an unintended adverse effect.

The Impulse AQ is also locked into salt rejection mode at all times, even in "all metal" mode. Well, almost all metal mode - I wish there was a switch to shut off the salt rejection. There is still a filter engaged, so this is not a pure pulse mode. The machine is always set to eliminate salt signal. This means that small gold items like thin gold chains, small ear rings, aluminum foil, and small gold nuggets, are also going to be unintentionally eliminated. You catching the drift here? Unintended side effects, trade offs deemed necessary to get a desired end deemed more important. Welcome to metal detecting. So we have another hole in the extreme low end, but we have redefined it as being a preset salt rejection mode.

So does the Fisher Impulse AQ have "holes" in the detection pattern? It simply depends how you want to look at it. If you define the only genuine desired range of the detector as being a narrow range focused on gold rings, there are indeed no holes in that particular range. But if you consider the entire range of possible targets that can be found with a metal detector, the AQ is explicitly said to reject at least two target ranges of concern to some detectorists depending on the settings. It always is in salt rejection mode, and so will always miss low conductor targets. Engaging the disc mode will also lose many targets of interest to coin hunters.

So are those holes or are they not? You decide. Fisher does not define the term anywhere to my knowledge. It can mean whatever they want it to mean. Leaving it undefined means people just plug in their own assumptions. Frankly, none of this should come as a shock or a surprise to people who have been paying attention. I in fact have already written a long article, the Fisher Impulse AQ Discrimination Explanation, that has put all this out there for anyone with ears to listen. I have concerns that the marketing folks seize on simple catchword phrases, that make nice blurbs for people who don't really understand the technology and the limitations. People read what they want into such statements, and this in turn creates unrealistic expectations. This inevitably blows back when disappointed customers feel they were mislead. Frankly, I have always believed in underselling a product and letting it prove itself. Setting up unrealistic expectations is a setup for failure in my opinion. Keeping expectations realistic results in pleasant surprises and better long term outcomes. My goal here is therefore not to knock the Impulse AQ, but to educate people into having realistic expectations. Will you buy this detector and just go dig rings while digging no junk? No, that is not going to happen. Read the article at the link above. Are there items that the AQ will not detect as an unintentional side effect of eliminating undesired signals. Absolutely.

Finally, let's talk the upcoming Fisher Impulse Gold model. The Impulse AQ fails as a gold prospecting detector because the ground balance control is being employed as a discrimination control, and because the detector is locked in salt rejection mode. Obviously the salt rejection needs an ability to be turned off. I'd prefer this as a control so salt mode can be engaged on salt alkali ground in the desert, or for some hot rock rejection. It would be a shame to eliminate it entirely, but that may be their solution. If we had a switch for that on the AQ it would have been nice also for dry beach and fresh water use. In any event, salt rejection must be gone or optional on the Impulse Gold, or all that small gold the machine is designed to find is lost.

The disc control has to go back to being a ground balance control. Discrimination capability outside of perhaps a dual tone effect common on ground balancing PI detectors will therefore be unlikely on the Impulse Gold. What also will happen with an adjustable ground balance will be the inevitable hole that follows with such a control, Fisher can minimize the hole as much as possible, but again, you can't reject a certain hot rock without losing the nugget that has the exact same time constant.

The ultimate constraint on the kind of gold a PI detector can find has never been pulse delay. There are commercial PI detectors designed to find pin-sized targets. For gold prospecting the constraint is always the ground. If you make a detector with a 1 uS pulse delay, all it will do is light up the ground like the largest target ever found. The key is the ground balance technology employed, and how efficient it is at eliminating ground and hot rocks, plus alkali effects in some areas, while losing as few gold nuggets as possible.

There is a point where making the pulse delay short enough basically duplicates what a high frequency VLF does, and it creates all the same problems with ground and hot rocks you get with a high frequency VLF. Short pulse delays and air testing is totally meaningless. The Impulse Gold will have to ground balance effectively, and doing that will bring its performance parameters into line at best, in my opinion, with the limits we have already seen explored for those who are familiar with and have used current available technology. The main low hanging fruit waiting to be picked here is high performance in a light weight, ergonomic, and affordable package. Anyone expecting massive improvements in existing gold prospecting technology probably does not understand the constraints involved, and is likely to be disappointed. My advice? Be realistic and look for a new benchmark for what a gold prospecting detector can do without breaking your wallet or your back, and you are going to be happy with the new Fisher Impulse Gold.

That is how all this works folks, time constants. PI detectors know nothing about ground or gold or ferrous or non-ferrous. Conductivity is loosely involved if at all. You can accept targets and reject targets or ground based on their time constant, and for every good target there is a bad target that has an identical time constant, and vice versa. That is the reality of the technology, and getting your head around it is the key to knowing what these detectors can and cannot do. Does the Impulse AQ exhibit any detection “holes?” My answer is define first exactly what you mean when using the term, then apply that to the AQ to get your own answer.

UNDERSTANDING THE PI METAL DETECTOR by Reg Sniff

For anyone who wants to dispute any of the above, please provide an exact definition of what a “detection hole” is, and how it does or does not apply to the Fisher Impulse AQ.

[Geotech]

The traditional subtraction GB design subtracts the outputs of 2 channels to eliminate ground. That is, ground cannot be heard, ideally at all. That also creates a hard notch in the target conductivity range, whereby there is a hard notch where a particular target (or small range of targets) cannot be heard. Traditionally, targets on the low side of the GB point produce a high tone, and on the high side they produce a low tone. But right at the GB point particular targets produce no response at all.

In the AQ the GB subtraction is modified to no longer produce a hard notch. Therefore it can no longer provide a true GB function, but it also doesn't hard-notch targets, either.

In Tone mode as target conductivity approaches the transition point from the low side, targets respond with a high tone, then progressively you start hearing some low tone mixed in. Right at the transition a target produces equal amounts of high-low tone. Higher conductors eventually produce a low tone. But nowhere should any conductivity produce a hard notch in the audio response. That is what "no target hole" means.

Now, you can switch to Mute mode and any low tone responses are eliminated. This means that, around the transition point, targets produce a broken high tone audio. Targets above the transition produce no audio at all, and you can certainly call this a massive target hole but that's not within the spirit and normal definition of what is meant by a target hole. In Tone mode, the AQ lets you hear all targets and you can decide whether or not to dig them. With a target-hole detector, particular targets are simply not detected.

On the salt issue, there is no salt rejection mode other than pulse delay. At 7us you can definitely hear salt but even then I found it reasonably manageable, at least in wet salt sand. In active surf I think I had to back off to 8us, and in deeper water it may need to be higher still. There are limitations to what the AQ can detect at the very lowest conductivities, but that again is not in the spirit and normal definition of what is meant by a target hole. Much like it cannot detect a ring at 3 feet; that's not a target hole, it's a fundamental limitation.

 

 

 

[ALEXANDRE TARTAR]

Hello,

Regarding a metal detector model for finding gold nuggets, I would not comment today.

Regarding the IMPULSE AQ, there is no problem in the conductivity scale of the low conductors.

Let us take an example, on the conductivity scale of a White XLT Spectrum, the non-ferrous going from 0 to +94 and the ferrous 0 to -95.
The AQ pulse detects all targets with a VDI ranging from 0 to 58 as being valid targets.
And the IMPULSE AQ will minimize the loss of depth on the highest VDIs. (almost 55)

VIDEO HERE (Sorry for the rude English)

 

[Steve Herschbach]

50 minutes ago, Geotech said:

On the salt issue, there is no salt rejection mode other than pulse delay. At 7us you can definitely hear salt but even then I found it reasonably manageable, at least in wet salt sand. In active surf I think I had to back off to 8us, and in deeper water it may need to be higher still. There are limitations to what the AQ can detect at the very lowest conductivities, but that again is not in the spirit and normal definition of what is meant by a target hole. Much like it cannot detect a ring at 3 feet; that's not a target hole, it's a fundamental limitation.

Perhaps I misunderstood. When I inquired previously of Alexandre about being able to get full use of the 7 Us in all metal mode, he explained that the AQ was specifically designed to reject small foil items, and that thin gold chains, ear rings, and other micro jewelry targets would also be lost due to this intention to reject short time constant/low conductive targets. The Gold model were said to eliminate this low end limitation. Again ,perhaps something lost in translation. If all metal mode is a pure PI mode nobody will be happier than I.

Not to sure about the spirit stuff.  I fully admit there are different ways at looking at the subject, and none are "wrong" per se. Or maybe I am flat out wrong... I can live with that if good info comes of it. It is semantics on one side, and what the machine does or does not do on the other. No matter what.... I'm all ears for sure on the great responses. Nothing like a challenge to bring good info to light! 😉

When it settles out and when/if I am proven in gross error, I will edit my post to suit.

[Geotech]

I'm the one who approved the "no target hole" claim, based on my understanding of what "target hole" means. It has been discussed extensively with the TDI to mean any target conductivities that fall into the GB notch and are therefore not detected. I assumed that the same definition has applied to Minelab models, though admittedly I have not been engaged in many Minelab discussions. If I'm mistaken, then we can certainly change the claim to better match accepted terminology. Like you, I'm really opposed to introducing bogus redefinitions of widely-accepted terms just for marketing purposes.

 

[Steve Herschbach]

Well I fully admit to shaking the tree here to see what falls out. I’m not aware of Minelab explicitly addressing the issue anywhere beyond the original SD2000 explanation provided by Candy in an old magazine article I have on hand. They’ve always kind of implied and let people assume there are no holes, when in fact every machine they make has some. I’m not aware of any official definition of holes anywhere, just loose talk tossed around mostly among people like Reg Sniff or Eric Foster or you or me or whoever. But nothing really in-depth definitive anywhere I have seen. Any really always mostly around the idea of a ground balance point taking good targets with it. Back to my assertion... the ground balance here is relabeled a disc control, and moved to a certain location to take explicit advantage of the “hole”. Was the goal to eliminate most U.S. coins also, or was that an unintended side effect? If I was marketing I would say a guy like Steve does not want to waste time digging coins at the beach, he wants to dig gold. That is 100% accurate. So for me it is a benefit and could have been designed that way for me by intent.

Or it is an unintended and unavoidable side effect of eliminating the ferrous? Ok, that’s life. But there really are many beach hunters who like the coins. Lots of complaints now about coins drying up due to closures. This inability to find coins while discriminating ferrous would be a big deal to them.

I’m not trying to make a legal case and certainly not trying to step on toes. My main goal is.... what does the detector detect, what does it discriminate, and what gets lost as a result?

I’m really interested in what others think here. It’s not a fight... let’s figure out what a hole is, what it is not. Is a no holes claim valid... what do you all think? Am I being a picky jerk? Do I owe Carl an apology... I’m all in and will be more than happy. But let’s talk about it some more. Great subject.

I will say again... I like the chosen AQ target range and focus, and am very happy to pass on coins! I’m just being pedantic and argumentative this morning. 

[ALEXANDRE TARTAR]

I repeat :
Let us take an example, on the conductivity scale of a White's Spectrum XLT, the non-ferrous going from 0 to +94 and the ferrous 0 to -95.
The IMPULSE AQ detects all targets with a VDI ranging from 0 to 58 as being valid targets.
And the IMPULSE AQ will minimize the loss of depth on the highest VDIs. (almost 55)

It will therefore detect between 59 and  94 the targets as invalid

If your coin is at 75 it will not detect it.

An 18K gold signet ring of 20grs is only 45 therefore detected !

Can be that a signet ring of 45 grs 18k will have a VDI of 70 (I don't know I have never seen one) then it will not be detected in MUTE mode and it will make a low tone in TONE mode. (But there will not be the double beep which shows more than 95% the presence of a ferrous)

 

[Desert Dawg]

I thought that holes in detection were only evident in single channel pi detectors and when they went to multi channel pi detectors that hole was closed up mostly???

[Steve Herschbach]

5 minutes ago, Mxt Sniper said:

I thought that holes in detection were only evident in single channel pi detectors and when they went to multi channel pi detectors that hole was closed up mostly???

Mostly is the key word.  Name one timing on the GPX that misses no nuggets that can be found with some other timing. How does the GPZ find gold the GPX misses? There is also some gold the GPX does better on then the GPZ.

[vive equinox]

I don't know if I understood correctly, according to the table we lose the item number 16 with the AQ, it’s a common gold ring that we dont want loose it, did i misunderstand or can you explain ?