Continuation of Gain/Sensitivity discussion from Explorer forum..

[Chris(SoCenWI)]

I've been reading the threads of a gain vs sensitivity from the main explorer forum, thought I would post some thoughts over here where it won't get buried as quickly.

I was impressed(as usual) with Charles(Upstate NY) and Captain Kirk's explanations of the topic. A couple of things gelled.

I've been doing lots of boulevard detecting with frustrating results. Just can't seem to get deep coins, where in many other locations I can. The biggest problem seems to be because of adjacent power lines I cannot run my sens higher than 20 or so, often much lower.

It seems to me that if the sensitivity setting sets a threshold that signal strength must be above to in order to be processed by the electronics, it would stand to follow that at lower sensitivity settings you would not get faint signals.

Let's say the gain is strictly a multiplier of processed signal strength (Deep ON would make this a non-linear function -faint signals are amplified by a bigger amount than stronger ones). If you were running max sensitivity - Manaul 32- so every signal no matter how tiny was accepted and processed you would hear real faint signals through the head phones. If you lower sensitivity and raise the threshold you will no longer be processing the real faint signals and the minimum beep you hear should be quite louder than at higher sensitivities.

In other words at lower sensitivities there should be no weak beeps. I would think this would be pretty easy to test, I'll try by ear but probably needs to be done by someone with an O'scope. But it kind of bears out my experience in high noise conditions; having to run low sensitivity is a real depth killer.

Chris

 

[Cody]

Chris, you have explained this exactly as I understand sensitivity vs gain. Gain is a ratio of the input to output of an amplifier while sensitivity sets the level of a signal on the input that will be amplified by the gain of an amplifier.

If the sensitivity is high so very small amplitude signals are amplified by the gain of the amplifier we can reduce the gain and still hear the small signals. There will be limitation but the small signals just sound less intense. Again you are correct in the gain settings are not linear in that a setting of 5 or less will amplify in a way that is often called modulated so we can tell depth by the intensity of the signal. This is true as far as I can tell from 6 to 10 but because the signals are kicked up so high even faint signals will sound off very strong. One the amplifier is saturated then that is how it is and it looks to me like that saturation is based on a faint signal at about 12".

Sensitivity is more critical then gain for detecting deep signals. If I run the sensitivity up in the 28 to 32 area the I can set the gain down to 3 or 4 with no loss in depth for my particular hearing and headphones used.

The problem as you know is if we have the sensitivity set high then we have problem from ground noise and stray RF. If we go to Semi-auto sensitivity the ground noise and stray RF is compensated for but can really kick the sensitivity down. I have found that there is a point, other seem to agree, where the manual sensitivity really starts to be a problem and that is below a setting of 16. Gain is going amplifier what we have so by reducing the gain down to 3, 4, 5 we modulate the signal and then use manual to get the machine as solid as we can for a good threshold AND the small coil helps. The situation you describe is where I use the small coil, gain of 3 or 4 and sensitivity as high as I can set it with a solid threshold.

You are also correct in we can bury tiny signals in a threshold that is too high or not get them at all if we have the threshold set too low such as below 0 for silent search. It is fairly easy to locate an ultra deep target and play with the threshold setting to see what I have posted is correct. It is fairly good advice from the User's Manual to set the threshold at the slightest sound that we can hear.

This may sound awful but we are using a detector that is designed to process a very wide band of frequencies which is a major factor in the noise problem. This is where a single frequency machine is nice to have around, I like the Whites 5900D, as a single frequency machine does much better when around those high power lines. The DFX is a little better in those situation as they process only the 3kz and 15khz components of the signals. If the multiple frequency detector will not do it in the situation you are hunting then it may be worth the cost of a good single frequency machine. I have used several that will hunt right under a power line with no problem. I think this is a very common problem for all of us right up there with a hot fence or up close to a wall where there is a TV or other strong sources of magnetic induction to our coils.

I have read Charles' comments and they are very good. I thought where I read them that this guy agrees with my thinking as much as any of the comments I have read. There are always minor areas where we disagree but he has pretty much nailed the operation as far as I am concerned.

 

[Captain Kirk]

Chris,

Here is a thought on the subject of how the Sensitivity signal effects the loudness of the weak signals. What follows is NOT based upon a great knowledge on the subject, but rather just some ideas.


THEORY

The signal strength sent to the processor MAY BE related to how much above the threshold the signal is. Consider the following example.


THRESHOLD SET TO 15

Signal #1 is significantly above threshold and gives a modest signal tone loudness

Signal #2 is just barely greater than the threshold and gives very low signal tone loudness.

Signal #3 is just barely below threshold and gives no signal tone at all.


INCREASE THE THRESHOLD TO 16

Signals #1 and #2 will produce greater signal tone loudness than for the prior threshold setting.

Signal #3 will now produce a weak signal tone loudness


CONCLUSION (IF THE THEORY IS CORRECT)

Increasing the SENSITIVITY will cause weaker signals to be louder than before. Targets not previously detected will now produce the very weak signal tones.

HH,
Glenn