Semi-Auto mode

[Anonymous]

How much does semi-auto sensitivity mode degrade your performance?. I see that many of you have sensitivity set in the upper 20's. When I try that, the cross hair hangs and return to threshold takes a while so it get confusing with multiple targets. I know different conditions require different settings, but with a setting of 27-28 in semi-auto I seem to get a bit more stability and faster return tho threshold. Would it be better to just use a manual lower threshold to get the same response? <img src="/metal/html/confused.gif" border=0 width=15 height=22 alt=":?">

 

[Anonymous]

If it works for you, use it. I have a couple of buddies who do really well with semi-auto. I have tried it myself and it works real good. However, the down side is you never actually know what level your running at. Listen to your machine in semi-auto and replicate that in manual. You can tweak it hotter if you want to. Thats what I like to do.
STICK SHIFTS RULE!
bing

 

[Anonymous]

David,
The crosshairs represent the last target seen before the threshold returns. Suppose you go over a nickel and the crosshairs move to the nickel position. Then suppose that you go over a quarter and continue to "wiggle" over the quarter without allowing the detector to return to the threshold sound. The crosshairs remain in the nickel position until you stop the "wiggle" and allow the threshold to return.
Also, suppose that the coil passes over a number of targets that are close together, but separated from each other by a couple of inches. The detector will produce the corresponding target tone as you pass over each target (if the sweep speed is slow enough), but the cursor MAY NOT change positions while making the pass over the targets. After passing over the last target, then the detector will go into a short null and recover to the threshold sound. Only then does the cursor move.
Hope this helps,
Glenn

 

[Anonymous]

Semi-auto can be a blessing and a problem. As an example if you have manual sensivitiy ON in a house where there is a lot of 60 cycle RF in the room then manual will be very noisy but will keep the sensitivity setting you select. If that is 28 then the sensitivity remains at 28 no matter how much (external noise stray RF) is induced into the receiver coil.
If you go to semi-auto then there is a problem becasue that RF is induced into the receiver coil and the auto adjustment will lower the sensitivity to the point were there is a good signal to noise ratio which can be so low that you only detect a quarter at 5 or 6 inches. Howvever, the threshold is solid.
Transfer this idea to the field and which to use, Semi-auto or Manual and it becomes a little more complex. The detector is very directional and the stray RF constantly changes in relation to how close you are to the source and even depending on the soil and direction you are searching in relationship to the source of RF.
Semi-auto will decrease the sensitivity looking for the best good singal to noise ratio in the field just like in our house and if the stray RF is very strong then it can reduce the sensivitiy so low that you don't get much depth. If you go to manual you then hear a lot of external noise and lack of stability.
You can search in manual and put up with the poor stability and noise or go to semi-auto and have greater stability but less sensitivity.
Consideration where there is very little stray RF then Semi-auto and manual will both do well. However, in an area where there is little stray RF then Semi-auto will give great depth and also help to reject ground noise from strong ground minerals. This give you great depth and a very solid threshold.
This is why you see so much debate as to which to use as it depends on soil minerals and stray RF. Both are simply grouped into EXTERNAL NOISE.
Using manual if you can would be a good way to go and then Semi-auto if external noise is too much of a problem. Also, in this area it is a must to be sure ground cancel is used to keep the external noise knocked down.
Another minor point is you the sweep speed is a litte lower in manual than semi-auto sensivity. The circuits that adjust semi-auto also requires a slightly faster sweep. It is almost no noticed but worth a mention. An addition minor point is semi-auto can for this reason make it a little more difficult to hear ultra deep faint targets so one might want to use DEEP ON if looking for ultra deep targets with semi-auto ON.
One lost point that may be of help. I have checked about 20 or so Explorers and the at a sensivitiy setting above 28 your start to hear internal noise from the high gain circuits. If you run the sensitivity above 28 then the above is not going to reduce the noise level as it comes from the audio circuits themselves (called shot noise from solid state devices).
Kirk, feel free to email and I will enjoy the communications. You might want to meet Jeff Foster who is a software engineer. He is around the forums and hangs out on the DFX forum most of the time.
Hope this helps,
Cody

 

[Anonymous]

Very enlightening. I guess the best situation is using the manual setting that's right for the ground conditions to get the max out of the machine. The cross-hair hang really bugs me for some reason, I just wish it would respond as fast as the sounds respond to targets. Is explorer II any different in that respect?.

 

[Anonymous]

The ExplorerII is pretty much the same. Although,it seems like the crosshairs responding as fast as the sounds would be good it would create some problems.
When we sweep the coil if we could see a slow motion tape we would see that we change the height and also the coil in reference to the target being centered in the electromagnetic footprint of the coil. Also, if you go to IM-16 you see how much refined iron there is in the soil. This causes problem with co-located targets in that we have a different soil matrix under the coil at any moment in time if the coil is in motion. The microprocessor is trying to pull the good targets from the contiminated composite signal in the receiver coil. Combine all this together and we would see the crosshairs jumping all over the place. It would be working correctly but becasue there are so many variable from sweep to sweep in many areas the crosshairs would be all over the place. There needs to be some kind of averaging for the crosshairs to indicate or it would drive us up the walls.
If we don't hit the target at the same angle and height but sweep over the target 10 times then you can take the average of the composite signals for the ten hits, as an example, to predict the most likely metal which is indicated by the crosshairs or digital numbers.
This is one reason why some users with very good hearing and headphones will suggest using tones as the primary method and then digital or the crosshairs as secondary. Once you know the tones they are the same no matter which mode you hunt in. Using tones when you get a hit then center the target and when there is a constant tone from sweeping over the centered target then I go to the crosshairs or digital display for a much cleaner reading.
Also, jumping crosshairs can be of value. As an example some Indian heads will give crosshairs that jump from the upper left to upper right of the screen. I associate the tones to the jumps and know there is a good chance I have a deep Indian head.
Another good thing about tones is you can measure the size and response of a target quickly from different angles. Coins have a much smaller sound pattern and since they are most often very nice and round will give a constant tone.
I am sure you know much of this so excuse the rambling.
HH, Cody

 

[Anonymous]

Hey Cody,
I was the one that suggested you try the tests indoors with household RF to see the effect on the sensitivity. You said you got the same results: in high noise environments semi-auto greatly reduces sensitivity from the set point. And that in semi-auto the sensitivity can be INCREASED or DECREASED from the set point depending on conditions. (Many people are under the misconception that the set point is a top limit, but in fact in semi-auto it is possible that you are actually hunting hotter than you think.)
Here is another thing to consider on the manual/semi-auto debate. I don't think that the explorer can discern RF EMI signals from the signals that you get when detecting in a trashy environment. When you sweep the coil over many targets the explorer will see that as noise and greatly reduce the sens in semi-auto. That is why many of us prefer to stay in manual, especially in trashy areas.
Any idea how to do a test to determine how quickly the semi-auto tuning algorithm tries to change the sensitivity? It seems to me that when it kicks down the sens it tends to stay down. I think Captain Kirk did an experiment where he switched to semi and did some detecting and came back to a target he could get in manual and could not pick it up at all until he switched back to manual. Do I have that right Captain?
Chris

 

[Anonymous]

Chris,
My memory is not so good anymore on details, so I can not recall whether the test referred to was posted by me. But, I have had similar experiences like the one you referred to regarding that test.
I almost always hunt in the manual mode, but I do experiment from time to time with the semi-auto mode. My general feeling is that manual provides better depth performance. I also believe that that things other that electrical interference (e.g. changing ground mineralization in the search area) have an effect on where semi-auto mode will set the threshold. I also think that even repetetive targets signals will reduce the threshold setting in semi-auto mode. One can run a test that seems to verify this feeling.
* Set the sensitivity to 10 in manual mode. This is low enough that the electrical interference probably does not become a factor.
* Do a fast "wiggle" of a coin back and forth over that coil at a distance that is right at the limit of the detection distance for this setting.
* While continuing the fast "wiggle" of the coin, switch from manual to semi-auto mode. One will note that the target tone goes away, suggesting that the threshold has been reduced.
This is not a very scientific test, but is rather interesting.
It is my belief that, for any given sensitivity setting, that semi-auto mode will never allow the sensitivity to be greater than the manual mode. I am not sure where this idea comes from.
Hope I have not confused you too much. HH,
Glenn

 

[Anonymous]

I am not sure what I said before as I was testing to see but I agree with you. I try to not get locked into "my setting" but am more interested as I think we all are in what works the best. If I find something is not working as I want it to and find another way I change in a second. I fall into that group of people that is constantly looking for better detectors and better ways to use them so mess with the control and setting all the time.
I think for the first 25 years I just wanted to find all the coins and jewelry I could. I have been into how detectors work and messing with the controls a lot over the last 15 years or so as it adds another level of enjoyment for me in the use of detectors.
HH, Cody

 

[Anonymous]

I think you are correct but I am too lazy to mess with the external noise to be in manual. I get very good depth in Semi-auto so use it but also will go to manual and play with that. I do like manual when searching very slow around an old tree and have found some nice deep coins. I feel like the Explorer is so hot in both that I don't in the field worry that much about which one I use. If there is a lot of external noise that bother me then I check to see if I am in semi-auto and will go there and it helps but may be costing me those ultra deep targets.
HH, Cody

 

[Anonymous]

Hey Glenn,
I'm sure that semi can go higher than the manual setting. Turn the sensitivity down to 1 and switch between manual and semi-auto. Much better depth in semi. In the real world at normal detecting sensitivities I would expect that semi is almost alway less than manual however.
Guess what I'm trying to ask is how agressively does auto adjust the sensitivity level when you are detecting? When you turn on the detector is semi or switch there from manual does it find a level that it is happy with and just keep it there until you turn the unit off or is it continually trying to bump the sensitivity level up or down depending on conditions? My gut feeling is that it is not particularily aggressive.
Some of my work involves PID tuning loops for servos or temp control. You can tweak variables to track conditions, too aggressive and you overshoot or ring. Wonder if similar phenomena is going on with semi-auto tuning.
Just curious how the ##### thing works.
Chris

 

[Anonymous]

Chris,
#1 About achieving greater sensitivity in semi-auto vs manual.
It may seem like it would be a good idea for semi-auto mode to adjust for optimal sensitivity regardless of whether it is greater or less than the manual setting. But, the question arises as to "What is optimal?".
The Explorer II manual does not address this question in any great detail. The manual only states (on page 43) that, relative to the manual setting, that in the semi auto mode "... the Explorer will maintain the actual sensitivity setting AS CLOSE TO THAT VALUE AS POSSIBLE". To me this suggests that the semi-auto sensitivity will not be set to a greater value than the manual setting. <STRONG>But, refer to #4</STRONG>
#2 Does the semi-auto mode determine the optimal sensitivity setting and then leave it there?
Again referring to the Explorer statement above, the word "maintain" suggests that the Explorer is continually adjusting the sensitivity as required.
#3 <span style="background-color:#ffff00;">Some of my work involves PID tuning loops for servos or temp control. You can tweak variables to track conditions, too aggressive and you overshoot or ring. Wonder if similar phenomena is going on with semi-auto tuning.</span>
I did my graduate work specializing in servo systems, so I have some understanding about that subject. Overshoot and ringing is related to the control loop phase shift at the frequency where the loop gain falls below unity. If the phase shift is 90 degrees or less, then, generally, there is no overshoot and ringing. At 180 degrees or greater, then the loop becomes unstable and oscillates. A general rule is that anything less than 135 degrees is acceptable, depending upon how tightly the gain and phase shift are controlled. Having said that, it is not always desirable to make the response as fast as possible. Doing so may defeat the purpose of rejecting "noise" and accepting "good" targets.
#4 <span style="background-color:#ffff00;">Turn the sensitivity down to 1 and switch between manual and semi-auto. Much better depth in semi. </span>
I conducted that test (air test) and agree with your your results. In fact, I repeated the test up to a manual setting of 10 (using the 7.5 in coil and a penny). Even at 10, I found it to be true that that greater depth is achieved using semi-auto. In semi-auto the depth is about 6.75" while in manual it is only about 6".
<STRONG>Why am I getting different results now than I stated in my prior post?</STRONG> In that test I concluded that a sensitivity setting of 10 would probably rule out any effects from electrical interference. <STRONG>I was wrong in that conclusion. Since then, I realized that my prior test was run with a small fluorescent desk lamp turned on. That produced sufficient electrical disturbance to produce greater sensitivity in the manual mode.</STRONG> With the lamp turned off, then the semi-auto mode produces the greater sensitivity.
I hope that you are able to follow my ramblings.
HH,
Glenn

 

[Anonymous]

I found a flaw in the conclusions for my prior testing.
HH,
Glenn

 

[Anonymous]

This may miss the boat by a mile but I don't think it is how far above or below the set point. I think Semi-auto hunts for the best signal to noise ratio. Also, the hunt speed has to be within the sweep speed parameters or it would always lag the new data loaded into some kind of buffer from the constant data that changes from the detection window. In additon the hunt speed has limitation as it will tune out very faint ultra deep targets. On my opinion this is the primary reason for the DEEP function. In semi-auto it kicks those signals up so we don't miss the quick changes to the threshold as semi-auto compares the SNR and constantly adjust the sensitivity.
Stry RF and ground noise can be identified fairly easy. If you use edit accept with a black screen and hold the detector in the air near RF you can see the RF pop all over the screen which give a signature. Ground minerals are constant in comparison to targets. The RF and ground noise is compared to targets, accepted and rejected, and an SNR calculated. Semi-auto attempts to maintain the best SNR based on the manual setting. So I am not sure it is how far it swings from the set point of the manual sensitivity. If we are set for 28 and a good SNR is obtained at 26 then that is as far as it swings. If it has to go all the way to 20 then that is how far it swings. However, it has to be within the parameters of a good sweep speed and those are set by normal and FAST. There is a constat update becasue there is going to be external noise. Again, if no signals at some specific level is fed into the receiver within a specifi time then semi-auto returns to the manual set point for an SNR or as close as it can get to find one.
HH, Cody

 

[Anonymous]

Cody,
What follows is a rather deep technical discussion of no interest to most folks. I hope that I have explained this in an understandable manner for those who may be interested.
I understand the concept of signal to noise ratio. What I do not understand is how do you measure the SNR if you do not know what the signal is?? When you are sweeping the coil over the ground, how does the detector know what part is signal and what part is noise?
This is what I think. The detector is sampling return signal at a very high rate. If we assume that most of the sweep represents an ambient condition (e.g. soil mineralization) with no targets present, then the detector learns to treat that condition as a no target ambient. As the ambient condition changes gradually during the sweep, then the detector simply adjusts to the gradual change and treats it as a changing ambient. If the return signal shows an abrupt change, then that is treated as a target. That is why coil movement is required for discrimination.
I think that this is why the noise cancel function requires that the coil remain fixed during the noise cancel process. Under this condition, there should be no changing signal signature* received by the coil. If there is a changing signal signature, then it is treated as noise. The detector adjusts the coil driving signal such so that the driving signal does not have the frequency components that exist in the noise source (a correlation detection process).
HH,
Glenn
* A GENERAL DISCUSSION ON SIGNAL SIGNATURE AND NOISE CANCELLING
Of course the recievied signal changes with time because the transmitted signal is time dependent. By "signature" we mean that the recieved signal is correlated to the transmitted signal in a specific non-changing manner and looks the same for each successive cycle of the transmitted signal. With this in mind, the detector looks for a changing signature for the received signal. For a noise source that has a fixed set of frequency components, then it is possible to improve the signal to noise ration by adjusting the frequency components of the transmitted signal so that the cross-correlation products are reduced to a minimum.

 

[Anonymous]

<span style="background-color:#ffff00;">I understand the concept of signal to noise ratio. What I do not understand is how do you measure the SNR if you do not know what the signal is?? When you are sweeping the coil over the ground, how does the detector know what part is signal and what part is noise?</span>
I think you are right on it in this post in that what you do know is the external noise when you ground cancel and sweep the coil. That is one part fo the SNR and where you set the sensitity is the other. <span style="background-color:#ffff00;">Any signal that is not noise is good signal</span>. In a sense the SNR is the ratio <STRONG>of any signal to noise.</STRONG>
HH, Cody

 

[Anonymous]

Cody,
I would very much like to carry on this discussion with you. But, I do not want to burden the forum with a long thread that is not of any interest to anyone else. I suggest we carry on with this discussion via email. CHECK YOU EMAIL
HH,
Glenn

 

[Anonymous]

I understand as it is getting involved.
HH, Cody

 

[Anonymous]

[No message]

 

[Anonymous]

Chris,
You are more than welcome to be involved with the email exchange. Just email me (from my post) and I will copy you on the email exchanges.
Regards,
Glenn