Xavier,
The original question on this thread was this; which DIY PI detector is best for nugget hunting.
Your question was how to differentiate metals with audio.
Now, my assumption was you were wanting information about the two issues mentioned above. That is why I asked about the PI you built. Unfortunately, you didn't answer the question, so I couldn't try to explain answers in greater detail.
Now, as for the TDI and TDI Pro, the disc feature on both of those is based on the fact the detector has a specific designed form of ground balance. That design along with the design that allows one to select one or the other tone is the fundamental concept of how the TDI can differentiate metals with quite a bit of success.
The high conductor mode does better because most ferrous junk can be ignored while high conductor objects such as US silver and copper coins will still sound off. For gold nugget hunting, this setting will only allow one to detect certain nuggets, generally greater than 3/8 oz or so. Smaller nuggets would be ignored in the high conductor mode.
In the low conductor mode, most smaller gold nuggets, nuggets smaller than about 3/8 oz, could be detected while ignoring a lot of ferrous junk. If the ground balance control and the delay were set right, then most thicker ferrous junk would be ignored. What wouldn't be ignored would be small pieces of ferrous junk, pieces of tin can type ferrous metal, many ferrous bottle caps, and other very small ferrous items such as bobby pins, etc would still be detected right along with lead, brass, and other low conductor objects that were small enough.
Some objects such as nails, Civil War size Minnie balls or other larger lead, brass or other larger low conductive metals could sound off either as a low conductor or a high conductor depending upon the settings of the detector.
Again, the features found on the TDI are there because of the basic design and the fact that design has a form of ground balance, which is something not found on DIY detectors.
Can a form of ground balance (GB) be added? Yes, but it takes additional circuitry, etc. Since people have built the Goldscan 4, I mentioned this detector earlier because it contains much of what is needed to build a detector similar to the TDI. In the case of the GS 4, two settings were built in that would allow the detection of certain coins found in England while allowing one to ignore most nails. So, in essence, the GS 4 had or has a form of single tone built in.
If you need more information on the TDI, I can probably help you with the generalities, but can't divulge certain confidential information, including the actual specific design.
How do I know as much as I do about the TDI? Well, at one time the single tone feature found on the TDI only existed on one detector and single tone development happened just before the TDI was introduced. That one detector was/is a Goldscan 5 and it was/is mine. Yes, the TDI's fundamental design is based upon the design of the Goldscan 5.
Reg
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