18/ Understanding “Below-Zero” Accepts
I take this term from the language used with the first programmable, digital machines that came onto the market in the 1980’s. These set a benchmark for all others since. Part of the reason for this was because although they balanced out mineralization in the ground, they also examined the iron in the ground to see if there were targets mixed in with it. If you can imagine a scale of minus 100 to plus 100 with “0” being the line between non-ferrous and ferrous, these machines used “below zero accepts”. By this I mean that they were pre-set to examine part of this iron range (usually the first “30”increments) instead of just knocking it out. This created much better depth (by the standards of the day). These machines also had the ability to pick targets from amongst this iron--by hearing it too rather than just suppressing it. This is also the Anfibio’s strength--but with much more overall sensitivity. So what you have with the Anfibio is a very sophisticated level of filtering and processing that:
• -first, gathers a full and very detailed picture of what is under the coil.
• -then filters this diverse signal down to sort the (quick-responding) consistent parts from the (slow-responding) less consistent ones.
This broad, detailed initial “look” at the ground means that the detector can separate out small, deep objects as well as those that are in very close to iron very effectively. In effect the Anfibio is “reaching” down into the iron range to give the machine more detecting power. This also means that desirable targets that are coming in partly within this range are more readily detectable. Things like stainless watches, corroded targets such as long buried brass also respond better than with other machines. The “trade-off “here is that the line between ferrous and non-ferrous is blurred. This is true of all high Gain detectors. Things like round bolts and some of the more solid iron objects can jump up to overwhelm the discriminate circuit. So you have a “two edged sword.”
Where you want to get better responses from low conductors such as gold chains and earrings--you can run a lower discriminate and / or first Tone Break. However, when you do this--more iron, steel (such as bobby-pins), and other weak conductors will respond. Rather than an actual ferrous / non-ferrous line, what you have is more like a “firewall”--and as you bring the Discriminate and first Tone Break up--this “firewall” is made thicker. Learning to manage this “rust line” is a very important skill that will let you select targets with more accuracy and stabilize the unit at high Gain settings.
A low Discriminate / Tone Break setting opens the machine up to more of this “below zero” range. This comes through in the form of “crackle” and other falses. A higher setting creates a bigger reject block between the two (plus and minus) “sides.” Understanding this--and being able to choose a correct setting is the essence of running a balanced signal--distributing the unit’s power evenly and—by standard, quietly. It’s important to recognize the fact that what a detector does is not to just punch down into the ground and alert on metal. Instead--as described above--it acts to separate ground from metal. What this means is that the iron and ground’s responses form part of the signal. A good way to understand this is to think of all metal signals as being a “peak” in the larger ground’s signal.
The more of this low range you want to inhibit (block off)--the fewer targets will be available. (For hunters interested in re-working “hunted out” type sites--opening up this low discriminate range is another option that can be combined with a slow recovery speed (Deep mode) or a faster one (3 Tone) or targeted hunt methods using one of the three Frequency options).
Remember--it’s not depth that obscures targets--it’s the surrounding “noise.” The more of this “noise” you are prepared to learn to “hear though,” the more you will find.
Alternately--creating a wider, more solid bottom reject block can also reduce noise--allowing targets to “jump” though as with the Beach mode’s high Discriminate setting of “15.” This type of setting can be used either to bring the machine’s processing up above the noise level of difficult ground or salt sand, or to reduce the noise of dense iron and bring up masked signals.
From: "Successful Treasure Hunting with the Notka / Makro Anfibio Multi Metal Detector" by Clive James Clynick (Prestige Publishing, 2020