"Your assumption is that manufacturers are not interested in building a discriminating PI. Well, you couldn't be more wrong."
That is a big negative Reg. That isn't my assumption. Those are the direct words that were used in my emails back and forth to Minelab.
First, there should be no reason why the later TDI's respond differently than earlier ones.
That would be correct. There SHOULDN'T be a reason why they respond differently but THEY DO. I have personally owned 3 different TDIs over the past couple of years. All three of them behaved differently. Not only that, but shoot Carl an email about them and you'll see there have been several minor upgrades since the first release of them. If they are upgrading them to make them better as they go along, via software updates, etc, then they are going to behave different to one extent or the other. I wont base my findings on this from just 3 machines though. When I did the TDI videos it put me on the map so to speak as a "goto" guy for help on them. I attend the DIV relic hunts in Virginia and in the last few years, pulse machines have taken over up there. Out of 300 participants, I would estimate 225 of them have pulse machines now and most of them have TDIs. Since those guys and gals know me from the videos, I would get stopped quite frequently by folks that had questions about the TDI. So over the past 6 hunts or so, I have handled a bunch of other TDIs in showing their owners how to set them up right and trying to show the differences from target to target. From one machine to the other, they do vary...and in some cases, pretty big differences. Of the three I've personally owned, if you put them all on Pulse Delay 10, they all three would GB in the same spot at different places on the dial. I had one that would GB at 7 at 10 pulse delay....had one that would GB at 8 1/2 at 10 pulse delay, and the last one would GB at 9 right on the dot. Some simply ran smoother than the others too; this last one we had would run smooth threshold no matter what you done to it. The other two were wavery.
"Stick a DD coil on a TDI and much of the near surface ferrous trash becomes easy to recognize."
That is also a negative. I own the GPX 5000 and have had the opportunity to put several DD coils on the TDI now. Believe me I have played around. I've put the 11" DD Commander coil on the TDI, have put the 5x10 CoilTek Joey DD on the TDI, have tried the 8x5x11 CoilTek Platypus DD on the TDI, and also another of the Commander coils that was the 15x12 DD. So there you have 4 different DD coils of various sizes and manufacturers that I've tried on the TDI. In my test garden and in the field, there was no difference in audio on the TDI per targets. I actually was hoping there would be but at least on these 4 coils there was not. The only difference I saw was the 15x12 DD made it somewhat less sensitive to the square nail but the weight was the horrible trade off.
On a side note, I have also tried a couple of Mono coils on the TDI and was truly impressed with the overall depth gain of the RazorBack coils. I put a 8.5x11 RazorBack mono on there and it made the TDI come to life on depth to smaller things like buttons and such. It actually bests the 12" stock coil and is smaller and lighter weight. A friend of mine recently bought a 5" mono coil by RazorBack and it too is something to behold. BUT it was just a gain in depth/ground coverage vs the DD coils of the same size. If you put a straight mono coil on the GPX machines, they too will perform better in sensitivity and depth.
As you noted though, the GPX will only work in discriminate mode with DD coils. But that is no problem to me. The stock 11" DD coil on the GPX will blow a TDI with its 12" stock coil away in depth and signal strength. I've never dug a bullet at 18+ inches with the TDI but I sure have done so with the GPX. I will take a depth loss over the mono coils though, for the ability to discriminate iron out. After you dig 100+ bullets beyond 12 inches in a weekend, it makes the body tired
I have ran into a big slow down at work and have had to try and sell my GPX but as soon as things pick back up I will own another one. Unless something better comes along between now and then, they are the pinnacle of PI machines right now. Everybody that has one don't want anything less, and everybody that has anything else and has seen what the GPXs can do, they want one.