F75 LTD Sensitivity, Tone ID and Depth

Been away from detecting for several months. Took my F75 LTD out to my test garden to reacquaint myself with it. I have a clad dime buried about 9" down. In bP mode, disc 6 and 80 sens the LTD would hit with a high tone in one sweep direction and a low tone coming back from the opposite direction. I decided to start lowering sensitivity and got all the way down to 20 at which point the "loudness" of the tone was much softer but I was getting a high tone in both directions. It appears the LTD doesn't lose "detection" depth simply by lowering sensitivity. Everything else equal, with a reduction in sensitivity only, it appears the tones become more modulated in terms of volume but the LTD is still acquiring the target. It also appears that tonal target ID becomes more accurate at the lower sensitivity. I actually like the modulated volume since it gives me an immediate indication of the depth of a coin sized target. Have others seen the same response to a reduction in sensitivity on the F75LTD? By the way my GB # was 71 with an Fe3O4 reading of .1 Thanks for replies.



Edited 1 time(s). Last edit at 02/11/2011 08:02PM by dgc.

dgc --

This is an interesting question, and one I would like to understand better as well. I am still trying to wrap my brain around exactly what "performance" effects occur when changing sensitivity and threshold. You probably already know this, but here is how Mr. Dankowski answered an earlier question I had about exactly what sensitivity and threshold do...

"Transmit power ....... the moment the unit is turned on...... is a 'constant'.
Sensitivity...... refers only to receiver-gain........ie... how small of a signal will be reported.
Threshold...... is like squelch, whereby the base carrier (audio 'hum') can be adjusted. No 'hum'....and it takes a stronger/larger signal to break squelch....and produce a audio response."

Using this information, it says that sensitivity should cut down the detectors "sensitivity" to smaller (weaker) targets. Along those lines, I wonder if it is possible that there was, say, a speck of rust or iron in proximity your dime, (though maybe at a different depth); then, when turning down sens, you "lost" the small low-tone target, and the machine simply reported the larger dime? I have heard others in other forums hint (I couldn't find anything to substantiate these statements though) that turning down sensitivity will NOT cause you to lose a coin-sized target (unless it was at the very fringes of depth capability of the machine, such that the target was so "weak" that the lowered sens. knocked it out). I guess this is how you lose "depth" by turning down sensitivity. It would be interesting to me to see if you ran the thing at super low sensitivity, how much depth would you actually lose (i.e. at what depth does a coin become "weak" enough, with respect to the machine's constant level of transmit power, such that lowering sens. can eliminate this "weak" target?)

I don't know if this helps your or not; I found it interesting to know that there is no adjustment possible to "transmit" gain; only "receive" gain. I also don't quite yet understand the subtleties w.r.t. sens. and thresh. Both adjust how "small" or "weak" of a target you will hear; in one place, I saw it said that if you are in a "mineralized" soil situation, turning up the sensitivity too far is detrimental, as what you are then doing is "amplifying" the mineral feedback coming back through the coil to the machine's processor.

Anyway, still trying to understand all of this myself, but maybe this will help some (or help get others to chime in.)

Steve

Running the LTD in disc mode, as I run all the time, eliminates the "threshold" variable as it is disabled.

Interesting test nonetheless. I'll try to repeat it in my red clay garden.

Really, go-rebels? You have no ability to adjust threshold when in disc mode? I didn't know that.

Steve

I have had a very simular thing occur recently. I was in my test garden which is fastly approaching 4 years old. I did not clean the ground out before burying numerous items at various depths. Now before anyone replies to this some what stupid idea, hear me out. At the time I new very little about masking, but came to the knowledge from many mis-read targets dug over the past several years. Well any way, I have one particular dime that is buried at 6" and laying perfectly flat to the top of the ground. For my LTD to see and give a high tone I have to swing the coil from a certain direction. Most of the time I will only get a high tone in one direction and either an iron grunt or no audio on the return swing. But if I reduce the sensitivity to 40 the audio improves to a repeatable high tone more often. Now my thoughts are this:
With the higher sensitivity settings the width of the signal actually increases the detection field coming from the coil (this can be proved by doing air test with different sensitivity settings). Now I know there is iron in close proximity to the dime which in turn robs some of the transmitting signal and preventing the dime from being detected. Reducing the sensitivity reduces the effects of the iron and improves the dimes response.

Steve,

Thanks for a very informative reply. You stated something I had never thought of before. Maybe by lowering sensitivity I was eliminating a small influence from from nearby iron that was just above my disc 6 setting. I also think that the lower sensitivity may have been "aiding" ground signal cancellation.

From Tom's information it stands to reason that since the detector is "outputting" the same power all the time, the signal coming "into" the coil from a target cannot be increased in amplitude (except by increasing sweep speed). Thus there would be some lower level micro volt input signal limit at which the detector electronics is unable to detect a target at all. I am now viewing sensitivity on the F75LTD as more of an input signal gain adjustment, having more to do with volume and clarity of a "detectable" signal rather than making a target detectable to begin with. I guess you could also view sensitivity as a threshold adjustment of sorts since, if the receiver gain is too low, the input signal won't be amplified to a volume we can hear.

By the way, there is no independent threshold adjustment in discriminate mode for the F75.



Edited 2 time(s). Last edit at 02/12/2011 02:51AM by dgc.

Mark,

Thanks for posting your test garden experience. Very similar to what I am seeing.

Very good.....and correct. Somewhere in one of my posts.....I reported on this exact topic. Yes, when Sens is dropped......it is the VOLUME of the targets that are reduced.......and this phenomenon is accentuated to a greater extent when Disc is on '4' and below. Also, in somewhat mineralized dirt......this phenomenon is also accentuated. This resultant is true with T2/F75/F70 platforms.......and not other units.

Your dirt certainly poses interesting (and correct) results. It does sound like mineralized dirt (and/or) a small fleck of iron off to one side of your buried dime.

Thanks Tom. It is good to know that running relatively low sensitivity on my F75 LTD doesn't mean that I have sacrificed any significant detection depth. I do like the modulated audio resulting from running low sensitivity.

UPDATE:

Went hunting today, ran the black LTD at sensitivity 40 and disc 0. Now here was the surprise for me. Machine was very stable at these settings. Now this is what happened next. After hunting for 30 or so minutes I got a low volume high tone that repeated in every directions. Decided to increase the sensitivity to 60 and was surprised at how loud the target sounded then. Dug over 8" to find a 1942 S silver nickel. I was totally amazed.

markg Wrote:
-------------------------------------------------------
> UPDATE:
>
> Went hunting today, ran the black LTD at
> sensitivity 40 and disc 0. Now here was the
> surprise for me. Machine was very stable at these
> settings. Now this is what happened next. After
> hunting for 30 or so minutes I got a low volume
> high tone that repeated in every directions.
> Decided to increase the sensitivity to 60 and was
> surprised at how loud the target sounded then.
> Dug over 8" to find a 1942 S silver nickel. I was
> totally amazed.

Congratulations Mark. Well done.

markg --

Now that is some really cool information. Was that in boost process, or no? If not, then hitting an 8" nickel with a solid tone in all four directions at 8", sens 40, thresh 0 is really impressive, to me.

And, by the way, CONGRATS on the silver -- nice job!

Steve

Thought I expand on the post above.

You LTD guys try this.
I live in the red clay dirt area of Virginia.
Most of the time my black LTD in boost mode chatters a lot.
This time I changed all the previous pre-conceived notions of high sensitivity and went with a more modest settings and found out something interesting.
I had read where the LTD has a fixed output signal and the sensitivity only changes the amplification of the receiving signal.
I first put the machine in all metal and switched between the different frequencies to find the most stable one. (Tom's findings)
Switched back to discrimination mode and started with a sensitivity setting of 45 and disc at 0 BP, 4H. Totally surprised at how stable it ran even close to some power lines.
One thing was noticed right away, might have been the lack of chatter, the sound volume of the targets were a lot lower in nature, but quite pleasing to listen to. Got a good sounding very low volume high tone that repeated. Stopped and increased the sensitivity to 60, yep the volume increased more than 100%, but so did the chatter. Dug a 6" plug and sweep over the hole again, yep still there. Pulled another 2" of dirt out and fired up my propointer to find the exact location. Carefully removed the remaining dirt to find a silver nickel at over 8" deep in my bad Virginia dirt. Ending up not using my head phones because of an open short in the cord, but did uses these setting for the rest of the day. Hunted an area where the machine was always unstable in the past and ended up with 5 wheat pennies 3 of which were well over 6" deep with a sensitivity setting of 45 or below. Confidence has increased a lot today.

I'm all smiles, Mark.

Mark, your experience/findings are correct.

Next time you find a deep target with a low Sens setting......try adjusting the Disc to '5'.......and see what happens!

happen with the disc at 5? I have the f 75 being shipped Monday. advance notice aabout the disc at 5 would be great.....

Today EMI was bad in my test garden so I found the quietest channel on my F75 LTD (#7) and began playing in Boost mode.

SENS=40? No need to run it so hot!

I happened to have the 5" DD on the end of the rod so I began playing around my 6" silver dime buried in red clay. EMI chatter began around SENS=55 so I dropped SENS to 40. DISC=4, tone=4h for all tests. The unit is absolutely quiet. I turned the volume on the F75 to 90% max and set my Killer Bee headphones to 75% max. Bang! No problem on that 6" dime. Dropped SENS to 30, then 20. Still no problem finding the coin in any direction. It was only until I dropped the SENS to 15 did the signal become too iffy to dig. Ran the SENS back up to 20 and manually set the GB 20 points too high and too low. Still no problem clearly hearing the coin with grossly offset GB. I reset the GB and the F75 screams out over my other 4" deep targets. The digital readout is consistent regardless of sensitivity.

I then switch to the 11" DD and begin dropping the SENS again. Now I can drop SENS to 5 before I lose the signal on that 6" dime. My 8" dime is easily found at SENS=20. The shallow targets are now uncomfortably loud. Running at SENS=20 and switching to variable tone produces a quiet, Minelab-like operation. Very sweet indeed!

I'll do that, thanks Tom

go-rebels,

Thanks for the test report. Excellent information in your post.

By the way, go-dawgs!

go-rebels = VERY good test. Isn't a test-garden priceless! Serious education...confidence builder.

Sonny = If you have time......read the threads titled:

COMPILATION #1
COMPILATION #2
Rcpt Ack of F75 LTD prototype

This should really help you.

markg -- really love the additional info there. This is great stuff...

go-rebels -- same thing. GREAT stuff. You directly answered my statement in an earlier post on this thread where I said... "It would be interesting to me to see if you ran the thing at super low sensitivity, how much depth would you actually lose (i.e. at what depth does a coin become "weak" enough, with respect to the machine's constant level of transmit power, such that lowering sens. can eliminate this "weak" target?)"

Can you do the exact same thing, but NOT in boost mode? That would directly apply to my F70 -- and would love the hear the results as I have not yet been able to get a test garden of my own set up.

Sonny -- yes, read the threads Mr. Dankowski mentioned; GREAT info.

Mr. Dankowski -- just to be clear, the F70 does indeed have the "higher sensitivity" or "lower threshold" or however you'd describe it in disc 0-4, then then "less sensitive" disc. 5-19, and then higher again 20+, same as the F75? Or no...

Thanks,

Steve

Steve, yes I have verified that the F70 has boosted performance when Disc is on '4' and below...........but could NOT find any gain with the Disc on 20 or 21 ..... IRT the F70 OR F75.

Repeated some tests today but had DISC=3 rather than 4... my error, and will result in an offset comparing to yesterday's data. Anyway, today I could see my 6" deep silver dime at SENS=3 in BP and at SENS=19 in DE. And in DE I needed a much quicker sweep speed else the target was invisible. The breakpoint at SENS=19 was identical in PF, JE and BC modes too.

My 8" silver dime was found at SENS=14 in BP and at SENS=40 in DE but at any sensitivity in DE the target sounded like iron; in BP the coin ID'd correctly as silver. I began to pick up EMI interference at SENS=50.

The LTD is really another machine moving slow in CL mode, delta-pitch audio with SENS=1, that's right... ONE! You lose the modulated volume compared to low sensitivity BP mode (which i miss) but the audio sounds clean and the deep silver jumps out! Setting SENS>10 in CL seems to distort the audio and gains little depth. But I can't ascertain that exactly as my 8" dime was the deepest target I could (sometimes) see with my trusty Minelab Explorer II with Pro coil. I have nothing buried deeper as nothing seems to settle lower than that level in my South Carolina red clay earth.

I discovered a new deep iffy target adjacent to my test bed running in CL mode that I will need to recover. Should be interesting...

go-rebels --

I need a test garden; need to build one ASAP!

VERY informative, thanks SO much.

So, 6" deep silver lowest sens 3 in BP and 19 in DE. Not bad. I assume you got good TIDs and tones on this dime in DE?

8" dime lowest sens was 14 in BP, but needed 40 in DE and it hit as iron? Was it "iron" in both tones AND TID? You say it was iron at any sens. -- but do I understand that you couldn't run sens any higher than 50? Wonder if higher sens would have ID'd it better?

This CL mode thing, I don't understand, as my F70 does not have "CL" mode. I assume this is a "slow" mode? My F70 has dE (default) mode, and sl (slow) mode, which is supposed to be much more sensitive BUT more subject to EMI (I probably should have tried it today with my 5" DD coil once I saw how quiet the DD was). Anyway, I found your info on this CL mode to be FASCINATING. BTW, by saying you "lose the modulated volume," you mean that volume does not vary with target size or depth? Anyway, I wonder why sens at ONE is so good for you in that mode? Fascinating! What made you think of doing such an unusual test (the CL mode and sens = 1)? Is that something you read about or something?

Steve

The 'cl' mode is a "cache" mode on the F75 SE / LTD......and is a MUCH boosted process. It is a extremely high 'snap-shot' sample-rate analyzation program (process). VERY slow to report/respond. In fact......the coil can be two feet PAST the target.....before it starts to report any audio......depending how fast you swing the coil. VERY deep mode; yet, MUST be used in exceptionally clean (target-free) areas. Designed for large target 'cache' hunting; BUT..................if you ever test a Type-1 U.S. $1 gold coin (1849 - 1854) in the 'cl' mode,,,,,,,you will instantly have extreme appreciation/amazement for this technological advancement that is incorporated into this detector. The Type-1 one-dollar coin is the smallest Federal Issue coin from the United States mint. It'll slightly overlap the head of a pencil eraser......when rested on top. In areas of minimum EMI.......and with the F75 in 'cl' mode.......it'll air-test detect this tiny gold coin to 17". There's plenty of detectors on the market that cannot detect a silver dollar to 17". I consider this Type-1 $1 gold coin to be a mandatory standardized calibration/validation target for the LTD's and SE's.

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steveg
So, 6" deep silver lowest sens 3 in BP and 19 in DE. Not bad. I assume you got good TIDs and tones on this dime in DE?

Yes, good tones and accurate TID.

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steveg
8" dime lowest sens was 14 in BP, but needed 40 in DE and it hit as iron? Was it "iron" in both tones AND TID?

Yes, in DE both the sound and TID were 'iron'.

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steveg
You say it was iron at any sens. but do I understand that you couldn't run sens any higher than 50? Wonder if higher sens would have ID'd it better?

I ran the sensitivity to the mid-80's until the background chatter was incessant and the ID was identical: 'iron'. This was very disappointing to me.

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steveg
BTW, by saying you "lose the modulated volume," you mean that volume does not vary with target size or depth?

The volume does not very much by depth. I don't have any experience regarding target size since all my actual targets in my test bed are silver dimes.

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steveg
Anyway, I wonder why sens at ONE is so good for you in that mode? Fascinating! What made you think of doing such an unusual test (the CL mode and sens = 1)? Is that something you read about or something?

I found some success hunting in CL when I absolutely wanted to go slow, like when I'm hunting around old foundations and there's ton's of roofing nails and other trash. In addition, it is a MUST to run low sensitivities as otherwise I find the audio and TID greatly distorted on shallow and medium depth targets. The ability to see around iron still exists, as the tight response of the Fisher DD coils remains. I only ran SENS=1 since it is the limit as to how far you can take the F75 and I can still easily hit my 8" deep silver dime. Plus, SENS=1 maximizes whatever little audio modularity exists, as anything above 5 or 10 sounds identical, that is, there is no discernible difference between deep and shallow targets. It's similar to running the Etrac with DEEP 'on'. But for general hunting, I like to crank the volume and run in BP mode with low sensitivities. It goes without saying that you need a good pair of earphones, and a good pair of ears to match.

Tom if you had a choice.
Best frequency chosen
Assuming the EMI would allow the following, which setting would choose?

1. Sens 45, disc 0-1, 4H, BP

2. Sens 80, disc 5-6, 4H, BP

Why?

What is the severity of the soils mineralization? Low, medium, high? Perhaps you can supply the Fe3O4 readings from your detector.

go-rebels, and Mr. D. --

With your "South Carolina red clay" , I wonder if the iron content biased the deeper dime toward iron in dE mode. Didn't I read somewhere that high iron soil will bias a weak target (even if high conductivity) toward iron? Reason I ask is, I have "Oklahoma red clay" here and I CERTAINLY don't want to ignore an 8" dime just because it badly ID'd. Don't know how to avoid that though.

Mr. D. -- is an 8" dime "iron" signal on both TID and tone sound normal to you? Probably that's getting close to the in-ground max of the machine for a dime, at least in somewhat mineralized soil -- as I air-test a dime to about 12" with mine. I want to find the deep silver, but obviously some will be too deep for the machine to "see." However, for the ones that the machine can manage to "see," I SURE don't want to skip those, thinking they are iron. The only thing I can think of is every time I get an iron tone, pinpoint it to see the depth (the F70 has no depth meter, only gives depth in pinpoint), and if deeper than 6" (per go-rebels' test), dig it, in the event it is a mis-ID'd coin. Only problem with that is, pinpointing EVERY iron tone would be VERY tedious!

Steve

FEO will read from .3 to max