detectors having discrimination, in wet sand

[bklein]

Last week I went to my first MD contest - Minelabbing day in Santa Barbara. A lot of fun but I wish could have had someone shoot some video so I could see the secrets of the guys that got 10 times as many targets as I did. I used a Tesoro Tejon and a 10"x12" DD coil. A bit hard pinpointing targets as some were way deeper than the contest targets. Yesterday, I took the Tejon to the beachfront of the Montage - an expensive hotel in Laguna and discovered all the sand was wet. The Tejon was rather miserable to use. I suspect the targets it found were all really deep. I did get a penny at about a foot. I have a Minelab Pro-find 25 pinpointer and it wouldn't find the targets - perhaps they were much deeper or it did not work in wet sand either(?). I got tired of digging rediculously deep holes while drawing a crowd and eventually not finding the target. So I got to wondering about the design/detector options that would work in wet sand and still discriminate out iron. I wonder why the Tejon saw the penny if it "wasn't supposed to". The "Inside the Metal Detector" book briefly mentions the problem on page 244 and says it is so complicated it is out of scope for the book. It says the ground and salt are both balanced out - but can you also then discriminate out the iron trash? Should the Propointer work in wet sand or do we need a better design? (I know a lot of potential topics here - ok with me if you tackle any of em).

Barry

[Davor]

Guess your Tejon is still under warranty and you are not supposed to poke with it's intestines. If not, you could (ab)use it's having two switch-selectable discrimination settings to toggle between sea and ground balancing.

[Qiaozhi]

On page 244, the part that is beyond the scope of the book, is an indepth discussion of multi-frequency design.
The salt problem is more fully described on page 83, in the paragraph entitled GEB.

[bklein]

Guess your Tejon is still under warranty and you are not supposed to poke with it's intestines. If not, you could (ab)use it's having two switch-selectable discrimination settings to toggle between sea and ground balancing.

The Tejon was my first detector - got a good deal on it with like 4 or coils, used off Craigslist. It is in new condition though. Its basic design has two alternating-switchable discrimination circuits. Too bad they didn't offer the salt ground balance or give enough adjustability to make a difference with it. I really wonder though if that were so overpowering why I could get a strong hit on a penny a foot down. The hits I got there were pretty strong but either went away after digging a hole or were still deeper than my hole. Any reference schematics anywhere that show salt/iron dual ground balance? I'm probably best selling this detector off and buying one that has it. Excalibur, Sovereign, and Fisher CZ are mentioned - any others? I'm just really hating iron at the beach. I guess I can force myself to skip any double-beeps with the Surfmaster PI...

[Davor]

My point is that you can force this detector to run at both sea and ground modes by a few simple tweaks, provided you are up to it, and you don't mind poking with it's entrails.

A difference between beach balancing and ground balancing is in a phase that is used for generating sound. The offending phenomenon must produce 0V in such channel. As there is a resistor in series with the threshold setting circuitry connecting it to the gain block that is in quadrature with Tx (a normal GB), it can be interrupted in a way that you select which channel provides signal for audio (quadrature - GB, or inphase - sea) and thus toggle between sea and ground balance. So you get both options. You would not even have to do anything to your front plate, as you'll just have to replace a SPDT switch with a DPDT (no difference from outside), and the second discrimination potentiometer would assume a function of sea balance.

[Carl-NC]

Barry,

Any 2F detector will have a salt canceling ground balance: all CZs, Excal, Explorer, Sov, DFX, V3, and BHID are all the ones I know of. Some 1F detectors have a "salt mode" that extends the GB range from ferrite all the way to salt. Most of these are automatic tracking designs, off-hand I don't of any that are purely manual. When you get into salt+ferrite (west coast) the GB can fall just about anywhere and move around a lot, so you really need a fairly fast tracker and a slower sweep. On 'clean' beaches (Florida) the ground is almost pure salt so 1F/saltmode detector may work decently even in manual mode. You could probably extend the Tejon GB range to go up to salt (different than what Davor suggested with disc) and put it on a toggle switch.

- Carl

[Davor]

As GB at Tejon is a part of discrimination setting criteria, it is unlikely that extending GB range would do much good. That's why I suggested extracting the tone signal from a discrimination channel, and using the innate Tejon feature of a double Disc setting to dedicate one of the Disc setups for sea balance. It provides a dedicated potentiometer for the sea balancing, a kind of logical approach.

I tried extending the existing GB with IGSL (salt-saturated terra rossa) to reveal it just gets increasingly odd with departure from ferrites. Trouble is that the way Tesoro's (and White's) single frequency rigs are designed, a significant departure of GB from ferrites invites trouble into a discrimination quadrant - it opens for the reverse phase ferrous responses. Therefore a separate GB that has nothing to do with discrimination criteria is in order. That will also enable automatic tracking for amateur builds. Eventually.

[bklein]

I think I need to re-read the book and do some hands-on building to get this stuff permanently into my brain
Have you guys ever messed with a detector design that assigned signal amplitude to audio frequency and target phase to filtering? Or perhaps a dual audio tone approach each with voltage controlled filtering etc. Something where all the information of phase and amplitude is presented audibly to the detectorist and leaves it up to his/her brain to make sense of it? I wonder if then we would recognize the salt/iron concentration patterns vs a target signature. It seems the target information is there (thus I found the penny) but the audio cancellation approach is too abrupt and my brain at least just gives up on digging it as I'm not sure if it really is a target or concentration - and no depth clue.

[Davor]

I kind of agree with this view, but in a somewhat less constrained technical way. Nature favours simple solutions. Phases do get maintained unless you poke with them too much, and our brains do "see" the targets in the ground by just allowing our cognition tie to the interface we use. I discovered so far that two hindrances are standing on our way of cognitive nirvana: antichatter filtering with binary sound, and diddle-doodle too-smart-for-its-own-good flying circus parade of intrusive multitone interfaces.

[bklein]

I know a guy that is perfect for your tasks. He was an Applications Engineer for Freescale for many years as well as many very interesting jobs before and during that involving DSP and binaural audio. He surely could do what you want but we'd have to get him interested in metal detectors first... or pay him.
He is no longer with Freescale and does contract work now.

[Davor]

Rather pay me instead

Truth is that all of the functions I'm describing are feasible in DSP, yet for an amateur struggling to obtain garden variety components a DSP is a joy comparable only with listening to Klingon opera. Until you can find a DSP that is wildly available at most shops, for over 10 years period, with free libraries, easily re-programmed ... the whole DSP thing remains firmly in professional development.

[bklein]

Oh yeah I know. A lot of guys in my other interest area - electronic music - are using the Microchip DSP33 in synth platforms. It requires both a programming and a math background which I don't have. I've attempted small 8 bit microcontroller coding but DSP is way beyond my capabilities. I suppose though once you have a goal you can get driven to accomplish them and learn whatever necessary to do it. Perhaps this should be a separate thread on just what the right DSP-powered detector would do with sound. There are some studio recording programs that allow 3D sound placement. Perhaps they could be used to test the experience using detection outputs for control. The way soft synths are evolving there could be a modular soft synth with 3D sound placement in the next few years or sooner.

[Davor]

Such spatial manipulation would be useful for complex signals such as speech, but for a couple of sinus sources I'd do it the simple way, even with a DSP. At a single frequency simple group delay is sufficient.

There is another "feature" of DSPs that I don't like much: power consumption. It makes sense if you squeeze everything into a single DSP, but it is not a common practice. Yet.