Big Treble
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When you are using discrimation side of detector and accepting all vdis ( ferrous and nonferrous) you are indeed allowing detector to use filtering-- which will limit depth moreso vs using true All Metal.
F75 is a good example one of the originals,,,,you can run in disc mode use 0 disc,,and you can run in all metal--- different setup altogether,,,,with AM being deeper overall.
An operator also using the F75 example as I have,,,take a 9" deep dime,,,what a user is likely to notice,,,coil position using discrimation mode,,,far more critical to get dime to sound off. Running true AM coil position more forgiving,,,meaning far easier to locate this target based on audio.
Not all detector feature true all metal.
At Pro don't
CTX
Etrac
At Gold does
Deus does,,hard to run though
White's V3i does
Juat to name a few.
So the discrimination in the digital detectors, prevents the user from hearing it, it doesn't prevent the detector from seeing it. Hence hearing the threshold drop out on discriminated targets, you don't hear a specific tone, but your detector sees it. Discrimination does not change the transmit power on your detector on digital machines.
Here is one, I can find the other if you like,
http://www.metaldetectingworld.com/metal_detector_sensitivity_depth.shtml