Gaz, I think 17 is a fairly accurate "break point" for separating the two tone audio on ferrous properties represented by coins and jewelry here in the States. Like you, I've dug hundreds, if not thousands, of targets trying to "prove it wrong". But never did. Not to confuse the TTF discussion with more theories, but one of the things on my "Wish List" is to allow the user to program our own tones to what ever target properties we desired. For example, on the E-TRAC, TTF is the simplest approach to coin hunting sites that hold an abundance of iron trash. However, the preset levels for two tones isn't as flexible as I would like. I know we have the four tone option, which breaks at 07. But I don't find that helps me, a coin hunter. And I know I could implement multipe tone in ferrous. But that quickly becomes quite annoying. If I had the capability to program my own tones, I would assign a low tone to all ferrous properties greater than 19, and individual "higher" tones to those ferrous readings of 19 or less. In that scenerio, the majority of iron would still produce the same low tone that I currently ignore. But the ferrous readings of 01 through 19 could each produce their own unique higher tone. For example, I could make targets with ferrous readings of 11 - 13 one high tone and different tones for the other numbers (or groups of numbers) between 01 and 19. If I wanted to widen the range or assign individual high tones to each ferrous target registering below 19, (or even those 20 - 35) I could. If programmable tones were available on conductive tones as well, I could "lump" silver into one tone group and copper into another. I could even assign a single tone to all my coins, and different tones to all other conductive properties. Like I said, it's just a "Wish List" !
Your theory of audio blending on closely adjacent targets is interesting. I have to say I've not recognized that while in the field. I won't say it hasn't happened. I've just not noticed it. I believe the reason I have not noticed a blending is because I use a small coil most of the time. Especially when hunting in areas with an abundance of targets. And when I hit a suspective target, I work the coil around it from a multitude of directions. I know there are those who think that all DD coils separate equally. And I would agree that, for the most part, they do when the targets are either left or right of each other. But if the detection field of all DD coils can be thought of as a "blade" running from the front tip to the center of the rear "heel" of the coil, it seems logical to me that a small coil will separate better than the stock coil or larger when the targets are not left or right of each other.
I've made two simple diagrams that demonstrate that theory.
This one shows how adjacent targets (pull tab, coin and nail) can be separated with either coil, when they are aligned left and right of each other.
[attachment 189081 adjacenttargetsleftandright.jpg]
If you happened to be detecting in a different direction, this one shows how a large coil will mask the coin when swept across those same adjacent targets when aligned with each other. (heel and toe)
[attachment 189082 adjacenttargetstipandheel.jpg]
Granted, we can reduce the masking by hunting an area from multiple directions. But many simply don't take the time to do that.
JMHO HH Randy
