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One more question. This one on the threshold

M

Mike Hillis

Well-known member
UPS tried to deliver but no one home. Have to pick it up tomorrow during my lunch break. But at least now I have a manual to read, thanks to Joe. :thumbup:

After reading the manual I have one question and it revolves around the threshold settings. Page 36 gives a good pictorial discription of how a threshold operates and how it will operate on the Xt-70. It shows that a positive threshold tone would never saturate a target response. Is that really the case?

Thanks,

Mike
 
I had a DSP at my disposal. The diagram below is how I would attack the problem. Think of the combiner as a switch, you are listening to the threshold, when a signal is detected, the switch cuts off threshold audio and changes over to target audio. This is what I have been trying to get across in multiple posts, this ain't yo daddy's analog threshold.

While no targets are detected, I could have "you" listening to a stored MP3 of "God Save the Queen" as a threshold audio instead of a tone. Then "God Save the Queen" is switched off, and the target audio switched on when we hit a target.

In practical experience at the ocean, winds howling, waves crashing,(man this sounds like a scene from "On the Beach", I gotta start writing screenplays), I have had the threshold cranked to 8+ just to hear it, and it still sounds to me like it just gets switched off when hitting a target.

Caveat---The diagram below is speculation on my part, ML wouldn't let me look under the hood(bonnet), I'm still bumming over that.:(

Happy DSPing
BarnacleBill
 
Maybe ML has buried subliminal messages in the threshold tone.

<h2>buy more coils...</h2><h3>buy more coils...</h3><h4>buy more coils...</h4><h5>visit your dealer today!!</h5><p> Just joking around of course.
 
HF Concentric

LF Concentric

HF Small Concentric

MF Small Concentric

LF Small Concentric

HF Large Concentric

MF Large Concentric

LF Large Concentric

HF Double D

MF Double D

LF Double D

HF Small Double D

MF Small Double D

LF Small Double D

HF Large Double D

MF Large Double D

LF Large Double D

Kellyco look-a-like off size concentrics?
Kellyco look-a-like off size Double D's?
SunRay.....
 
But like you said, DSP allows what ever they want. The Coinstrike taught me (and still is) a lot about threshold operation. There is even a place for threshold saturation and I am curious if the XT-70 could do that too. Guess I'll be finding out tonight.

HH
 
lower right hand looks like maybe threshold could be set high enough to cause compression/clipping/distortion or whatever you want to call it. I'm not sure about this. There maybe a max audio; you may not want the threshold to use most of that level.
 
If you look at all 3 illustrations, on pages 36 and 37, it appears that they are telling us that Threshold will, in fact, play a role in degree of target response. By looking at the picture on page 36, it clearly says that having a Threshold in a negative value, small target signals will not produce a signal big enough to go above the Threshold of audibility. The associated illustration indicates that the Audible Target Response is less than "half way" to the top horizontal line. In the middle picture, with Threshold set to 0, the Threshold audio/sound is disabled and the detector will produce target signal's sounds only. Notice in this illustration that the Target #2 now has an Audible Target Response above "half way" to that same horizontal line. And, also note that the Volume is still set at 30. Now look at the picture on the right. It has the Thereshold set to a positive value, and the detector will produce target signals and a Threshold sound. The illustration for this setting still has the Volume set at 30 and the Threshold at 10. But the Audible Target Response is clearly much greater than with the other two Threshold settings. In fact, it surpasses the horizontal mark, and actually would have gone deeper if the target had been deeper.
Maybe Minelab Engineering will have to answer this for us. But, based solely on the illustrations, it appears that the Audible Target Response is constant. In other words, the value of the target response will not change in relation to where it started. The depth of detection appears to be based on where the "starting point" is. In other words, using Threshold level of 0, the target response point is defined. If you set the Threshold lower, the starting point for the target response will be lower and you will have a lesser Audible Target Response. And, if you set the Threshold higher, the Audible Target Response will be greater. I think the key is to hunt with the Threshold level that is comfortable for you. If you chose to hunt in "silent search" mode, recognize that you may not be maximizing the value of your Audible Target Response. Expecially if you set your Threshold below 0. I have found that I can just barely hear mine when it is set at 4. At 7, it is still comfortable. So, that is where I have mine as I want to have my "starting point" as high as possible without having it set so high that I can't distinguish audio changes that represent targets.
Again, I don't know. Just basing my opinion on the illustrations and verbage of the manual. HH Randy
 
The threshold level settings diffidently have an effect on how a target is HEARD on the 70.
Try this for a quick experiment and you'll soon see and hear for yourself what the effects are when adjusting the threshold level while air testing.
Power up the 70 and pull up the threshold volume adjustment.
Turn it up to 10, now wave a coin over the top of the coil from about 8-10" away and listen to how hard that coin hit's.
Now from the same distance, start backing down the threshold volume while waving the coin over the coil and see how fast the sound starts to fade.
B I G difference huh ?

Mike
 
Kind of reflects that "starting point" that I was referring to. I know that soil composition etc etc etc make a difference. And, I also know that some of the folks who read this pooh-pooh air testing results. But, from what I have gathered over the past few hours, checking all possible variables, I think I will keep my threshold at a level that gives me a very slight background tone. Just enough to let me know it is running, and not so much that it overwhelms any slight change in audio. Thanks Mike. HH Randy
 
Actually what you wrote is probably very true. I know when I purchased my first Sovereign, I ended up spending at least an additional $400.00 on accessories. Then when I upgraded to the Explorer, again the same thing- but this time as about $800 in accessories. Possibly the mentality is similar to one taking their family to the movies. The tickets cost around $8.00 a pop, and if you have four people totals $32.00. Now remember those concession stands as we have to get our "pop-corn, and soda's" more than likely that cost will be about the same or more than the price we paid for the movie. So needless to say, that movie better be worth it! In this case, hopefully the extra $200 to $400 one spends on extra coils will be worth it, so the total cost of the package will now be over $1000.00. Such an expensive hobby...but a darned good one at that! :)
 
I take air testing with a grain of salt even when doing them for myself and ESPECIALLY on a multi frequency unit like the Explorer or Sovereigns.
But ... in this particular case I think the results are pretty obvious and I'll be running a threshold with it's volume set at least to 7-8 and just see what the results are in the field :)
Mike
 
I'm with ya Mike. I don't hold a lot of stock in comparing different detectors or for depth of detection. But for comparing coils on the same detector AND for the Threshold test in question, it is sure worth considering! You know where I'm running mine! Keep us posted on those finds! HH Randy
 
A 9" DD MF & a 9" DD HF (the foil might drive me nuts) for the beach and, a 5" DD LF, 5" DD MF & a 9" DD MF for Coin shooting. Wonder what my bill is?
 
wave a coin about 10" away and it hits very softly, increase your wave speed and it hits hard, have you tried that on the 70?....John
 
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