One thing about unmasking ability...You can't change the laws of physics. Think of the detection field as a beam from a flashlight. It can't go around corners. If trash is directly over the coin, or even a good deal off to the side of the coin but much shallower than the coin (how far off to the side it can be depends on how much difference there is in depth between the two targets), the coil signal will hit the trash first and never have the ability to reach the coin to see it.
The flashlight analogy isn't exactly the best way to describe the beam, because a flashlight can hit a closer object and still have part of it's beam go right past the side of it and continue on...Hitting the further away object as well. A coil's detection field tends to be more broad the closer it is to the coil, and if any of this beam first hits the shallower object it in effect collapses the field and doesn't allow any part of it to continue further on to see the deeper object. IE: It can't shoot a straight beam of detection right past the shallower object that part of it is hitting and down further to the deeper target. That's why the greater distance between depths means the two can be somewhat further off to the side of each other and still in effect the deeper target never gets hit by the detection field.
Even a small but shallower item such as a staple (read a test about this) will stop the detection field from reaching the deeper coin. That's where even a fast recovery machine really has no better ability than a slow recovery machine (so long as you are sweeping the slower machine slow enough to make it's recovery between targets a non-issue).
Where the real ability lies is in just how sharp the detection field is of the coil in order to be able to separate targets. Something like an SEF 12x10 has a much tighter left/right detection field than a typical DD coil, making it laser like in it's ability to separate two targets sitting right next to each other. I'm fairly impressed in the abilities of this coil in that respect. It feels like you are using a scalpel to slice between targets.
Now, when a coin and a piece of trash are laying at the same depth and so close that they are even touching each other, resulting in both being washed in the magnetic field from the coil at the same time....That's when it's up to the machine's internal processing to be able to separate the two in some way if it can. I've used some machines that were poor at this, and would either discriminate out the combined targets, or at least severely average the signal to somewhere halfway between them on the conductivity scale. Other machines seem better at making distinctions between the mixed signal, and won't discriminate or at least won't severely average the two targets on the conductivity scale...At least as often as other machines tend to.
Ask yourself this...Would you rather use a fast recovery machine that's coil is putting out a blunt and wide beam into the ground, or would you rather use a slow recovery machine that is using a coil with a very sharp detection field that can "light up' one target while not the other in it's detection field? Don't get me wrong, fast recovery has it's advantages with the ability to swing faster without missing targets, but slow recovery is for the most part a non-issue so long as you keep your sweep speed at a proper slow speed. That, combined with trying to "sniff" around targets with proper coil use (wiggling and such) to see if there are other objects also present, makes it less of an issue.
That's were some (including me) are big fans in the separation ability of DD coils versus concentrics. The DD detection pattern is somewhat like a line going from the tip to the tail of the coil, giving it better left/right separation over the entire depth range. While a concentric tends to have more of a blunted or more broad detection field, at least at shallower depths, and so tends to have it's best separation ability at the deepest part of the signal where it narrows towards a point. Of course these are very broad statements, as various coils have very different detection patterns and qualities that somewhat overlap each other when comparing one style (DD) to the other (concentric), so these aren't hard and fast rules or correct by any means in all respects. But one of the big perks of DD coils is that, because of their general detection field, they ride on and "see" (again, in general) less ground matrix than a concentric...Which means they are submitted to and washing in less ground matrix than a similar sized concentric. Translating into machines being able to handle rougher ground matrix's than is typical for similar sized concentric. Again, not a hard and fast rule....But in general.
One of the odd things I've noticed is that a small (5.5") DD coil seems to hold onto it's depth abilities better than a small concentric. I'm simply amazed that I'm getting 7 to 9" on a dime with such a little DD coil while my 8 to 10" concentric coils on other machines could only muster around 7.5" in my soil. That just blows my mind. I know that (again, generally speaking) a concentric of the same size as a DD will tend to have slightly better depth (even though it usually is only one small point at the center of the field that reaches deeper than the DD), but this rule seems to reverse it's self when the coils get smaller than say about 6"
I know I'm way off topic and have rambled enough. Sorry...
This is all my theory anyway, and so I welcome any different views on all this that don't agree.
Think I'm finally home after a lot of years searching for my particular needs. Custom 1.5 lb lighter 
Sovereign GT (easy to do by just "building" lighter shaft) & SEF 12x10 as everyday coil. 8" Tornado (actually 7.25") as super heavy trash coil (Very deep & separates better than typical 8/9" true size coils). 13" Ultimate for large fields/beaches on certain days I want to grid faster than even the 12x10's nice improvement over stock for that. Stock 10" Tornado stays on water rig (stock shaft). 
ProPointer. Owned 15x12 & SunRay S-5 (5.5") at one time. Loaned a prior 13" Ultimate & SunRay S-12 (12.5").
Some Oldest Silver- 1835 Bust Dime, 1842 "O" Seated Dime, 1855 Seated Quarter, 1858 3 Cent Piece, 1891 Seated Dime, & another Seated Quarter too damaged to read date. Highest Value- 3 Cent Piece, Seated Quarter, & now 1921 F-12 Standing Liberty. Oldest Coin? Got to give large cents a good look & see. 2013 so far- 1 Seated Quarter, 1 Barber Quarter, 1 Standing Liberty, 2 Washingtons, 1 Barber Dime, 9 Mercs, 3 Rosies. 1 War Nickel. 19 silvers in total, & 1 1872 key date Indian, 2 Bufallos, 1 man's 14K gold medium sized ring & 1 gold chain, with possibility of few 8"+ deep mystery coins being Indians after cleaning.
What matters in life is the journey and not the desitination. How you handle that journey determines if you were a success, not so much what you achieved if and when you finally get there, especially if you made it by stepping on people along the way...
In loving memory of my long haired (wooly) husky Sky. She taught me just how beautiful and pure a soul can be, human or not...Edited 4 time(s). Last edit at 04/21/2012 12:12PM by Critterhunter.
And for the reasons they stated. Better machines, smaller coils and more experience make the difference. I actually find much more silver with my Omega than my older Whites Silver Eagle due to better technology and far superior coils (10" concentric/11" DD and 5" DD). In 1981, it was sweep fast, cover as much ground as possible. And it worked. Easier picken's. Just have to slow down these days and go to spots that have some potential. But, unlike the past, I no longer dig it all. Not worth it to me. Too much trash, especially aluminum. But I do leave my nickel notch open for gold in that range.
