Depth estimation is done by the loudness of the signal. Very easy to do, and many guys on even machines that have depth meters never even look at or trust that any only go by the sound of how loud the target is.
The VDI on the Sovereign is very fast. Very "instant". There aren't layers of heavy processing software between the audio and the VDI. Once the target conductivity is processed the machine just puts out a voltage leve on a scale of I think 0 to 2V to the meter. So, even though I've owned faster machines, I've never owned a machine with this fast of a VDI. What you hear you instantly see a VDI for. Some machines so highly process the audio and VDI that they get very detached from each other and out of phase, so you can be hearing one thing while working between trash while the VDI lags behind and says something else. Not a problem on the Sovereign, but it's still a good idea to always use audio as your first reason to dig and use VDI on any machine as only a further reference of thought.
Far as ID at 2" versus at 9". Sure, most of the time the ID is going to be the same, but you need to short fastly wiggle over a target at depth to pull the best ID out of it. Sometimes at fringe depth a coin might only be able to climb but never make it to 180. Does it climb and say stop and lock on say 156? Then that's going to be it's ID. But if it keeps trying to climb to reach higher and higher towards 180 and never locks onto a # then it could be a super deep coin. I've found on the GT that so long as the machine can see a super deep coin I can pretty easily get the ID to climb and lock on 180, but a hair deeper at fringe depth and yep...the ID might only try to climb and never make it. pay attention to the tone. Do you hear a high tone in there as it tries to climb? Dig it!
The Sovereign's ID meter is the best I've used in terms of holding true ID all the way almost to as far as the coil can see. There's only maybe a 4th of in inch or maybe a bit more of extra depth of the detection field IMO where the meter will no longer ID the coin and it will only try to climb on you without reaching the 180 #. That's just my impression so my numbers might be a little off. I can only tell you that when I've tested this there is just a very small amount of height where the ID wouldn't go 180. Like when the coil just barely starts hearing the target. But you must wiggle fastly over it to try to get the ID to climb. Other machines I've owned they were often much deeper than the ID was accurate. Not so with the BBS mchines. They tend to ID right down to the fringes of depth almost all the way. Part of that is because of how they handle and ignore the ground signal, but also part is that the ID isn't being exoctically processed into some strange new form and something is lost in translation.
All the meters I've ever read about are said to be just as solid as one another as locking onto a target. They are only volt meters so there isn't much difference in how stabile or good they are going to be if at all compared to each other. Only exception I ever read about was that the 550 scale meters were more jumpy due to their super high resolution, and that the later version of the 550 meter was said to be more stabile than the earlier version Minelab made. And also read that even though both meters can be converted to 180, that the early version converted still had a bit less stability than the newer version that people converted.
Another perk to the Sovereigns exclusively is that the meters have a calibiration POT, so you can tune the coil perfectly to ID just right on a target when you switch coils. On other machines if you change coils the ID can be just a "hair" oddly off on target IDs. Not so with the Sovereigns. All Sovereigns, and all meters, are going to be very close to each other in IDs due to the tuning pot to adjust for individual differences between both machines and coils. I really like that feature!
Read the VDI Meter Stick. Tons of other info in there for you on the meter questions. I just re-hashed a bunch of it for you but probably should have just told you to read that sticky and go from there.
