By
Steve Herschbach
My first comment regarding discrimination with a ground balancing PI like the TDI and others is that people may be expecting things of the TDI it cannot deliver, and may be better off with a good discriminating VLF detector. If your priority is discrimination, no PI will discriminate like a VLF.
There are two ways to tune the TDI. The first and preferred option is to tune the detector for the best depth. On low mineral beaches ground balance "off" usually give the best performance and you generally have to dig all targets. The sensitivity control and pulse delay are set to the desired level of quiet operation. You have to adjust those settings for your conditions. Forcing it by using settings other people use is fruitless. Salt conditions do vary as does mineralization and electrical interference, and the machines themselves vary by a small amount.
I notice many people think lower settings will cause lost depth and therefore insist on higher settings, and then complain the machine is unstable. It simply is what it is. Adjust the machine for the conditions. If it does not perform to your satisfaction, use a different detector. You can't make a detector do what it does not want to do.
As ground mineralization increases on some beaches, there is a point where using the ground balance “on” gives you more depth. Only with experiments can a person determine which setting gives more depth on your beach - ground balance “on” or ground balance “off”.
If you use the ground balance, it BY ACCIDENT creates two audio classes of targets, those above the ground balance setting and those below the ground balance setting. These give either a low tone or a high tone. The resulting two classes of targets have only a little to do with what they are made of, but are based instead on the rate at which eddy currents decay in the target after the transmit pulse shuts off. Size has as much to do with the audio results as composition, just like on a VLF. The pulse delay sets the minimum level for this eddy current cut off or rejection. See Understanding the PI Metal Detector by Reg Sniff.
With ground balance “on” you would normally, just like with ground balance “off”, adjust the sensitivity and pulse delay for whatever level produces quiet operation. I usually just put the coil underwater and pump it in the water, and first try lower sensitivity. If that does not remove noise I raise the pulse delay a little and try again. In general I am trying to keep the pulse delay as low as possible and sensitivity as high as possible. Eventually through trial and error I find a combination of sensitivity and pulse delay that eliminates audio results when pumping the coil in saltwater.
The ground balance setting is determined by whatever setting gives no audio results when moving the coil up and down over the beach or the bottom when underwater. If basalt cobbles are present they may also need to be included in the ground balance tuning procedure. In extreme cases you may have to lower the sensitivity and pulse delay even more to get a proper quiet ground balance.
Again, once you have tuned everything for best performance, you will have a pulse delay setting and ground balance setting that ACCIDENTALLY creates two classes of targets. The only way to see the result is to test various targets. In the U.S. our coins are much more conductive than many European coins and retain eddy currents better, and therefore generally give a low tone while most jewelry will give a high tone. Large ferrous will give a low tone and small ferrous a high tone. Very large rings may give a low tone as will most silver rings. In Europe and other places the tones may vary from what we see with U.S. coins.
After experimenting to find out what items give what tones, you have a simple decision. You can dig one tone only, or you can dig the other tone only, or you can dig all targets. The results will be what you have determined by your experiments and if digging only one tone or the other loses items you do not want to lose then you must dig all items. If that is not acceptable, your should be using a VLF detector instead.
Now, if you are willing to give up some depth, you can try to purposefully misadjust the ground balance control to move the tone division point. Doing so may switch some items from one tone to the other for a better result as regards discrimination. This however puts the detector out of proper ground balance. In mild ground you can do this easily but in highly mineralized ground the machine will now signal when moved over the beach or the bottom or past hot rocks. The depth is lost as you compensate for this by again reducing the sensitivity or increasing the pulse delay. Again it all is a matter of experimentation. If a desired item that is giving a "wrong" tone can be made to give the opposite tone by misadjusting the ground balance you may benefit from this in milder ground. It may be that the lost depth or audio side effects from being out of ground balance are not be worth it. You have to decide.
To sum up, VLF detectors offer the best discrimination but may not get enough depth on mineralized beaches. A PI detector can get more depth, but any discrimination is an accidental byproduct of the ground balance system employed and will not separate targets like a VLF. In general you dig everything with a PI but in some cases you can derive benefits by digging some tones and not others, but you will without a doubt miss some class of good targets by doing this. That is just the way it is.
The first ground balancing PI (GBPI) I used for beach detecting was the Garrett Infinium. Next was the TDI and then Garrett ATX. All three have similar tone results, but the TDI has the additional benefit of allowing you to manually set the tone break point via the ground balance setting. I have also used similar tonal separation using Minelab GBPI detectors. I go into more in-depth detail on another post referencing the new Fisher Impulse AQ where I reference all my notes on all these detectors so I am going to link there for further study. The Impulse is basically a refined version of what the TDI is doing, with the ground balance control used first and foremost as a discrimination control, and as with the TDI there will be performance trades depending on the settings employed. Understanding one will help you understand the other.
This is a very complex subject for those wanting a simple VLF type discrimination system in a PI detector. They are however two different things, and you have to read and think quite a bit about how a PI detector actually works to get your head around all this. Or at a minimum do lots of experimenting and learn by observation. I have tried my best to explain things, but there is no magic tuning or answers I can provide that will make these machines do what people seem to want them to do - act like a VLF. They are not. If they do not discriminate the way you want I have no settings that will make it happen other than what I have tried to explain already. This is kind of a summary and along with all the other posts is really about all I can offer or have to say on the subject. I hope it helps!
Fisher Impulse AQ Discrimination Explanation
Where Will The Holes Be In The Fisher Impulse System
White's TDI SL Owners Manual
White's TDI Beachhunter Owners Manual
White's TDI Pro Owners Guide
Steve's White's TDI Review
By Steve Herschbach
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