Gain

Jerry-Wi said:

Gain is a figure of merit for an amplifier circuit. With context to metal detectors, it is probably referring to the sensitivity setting. Sort of like turning the volume on your radio up or down.

Jerry

Click to expand...

Even with Metal Detectors, gain and sensitivity are not the same thing.

Gain amplified the signal coming out of the coil before being processed. Sensitivity adjusts the threshold at which the processors consider anything above to be a signal. When you increase sensitivity, you are lowering that threshold. When you decrease sensitivity, you are raising the threshold.

Gain has the problem of also amplifying ground noise. If gain is set too high, you are going to get a lot of noise. Sensitivity, if set too high will make bad targets and ground noise appear to be good targets. So, they both perform specific functions and have distinct consequences if set too high or too low.

Generally, it is better to lower gain a bit so you can increase sensitivity. It is the sensitivity that gives you the most depth.

There seems to be a lot of info on this stuff out there, it can get confusing.
As Rudy mentioned above there can be several areas where the gain feature can be utilized.
From what I gather gain is more often described as a form of amplification.

Sensitivity is described as not so much amplification but more as an adjustment that controls a threshold level.
Raise that threshold level and only targets that are picked up above that level can be heard, those that fall below that level will be silent.
In effect it acts as though the more sensitivity the more power the unit is using to penetrate deeper but in this context it seems more like a door.
Everything is there to be seen with your detector at its basic limit of depth all the time, but adjusting that sense doesn't actually penetrate deeper, or pick up smaller targets better on high settings, in actuality just increasing the sense opens the door...you can use the already preset max power that is there all the time to lower that threshold and see and hear more things.
Lowering the bar so to speak so more targets can jump over it.

Gain is not threshold based but amplification based.
Again that open door example.
Higher settings the door opens, lower settings the door closes on smaller and deeper targets.

It might seem like they are almost doing the same thing but they use two different paths to achieve what seems to us like similar results.

Combinations of different levels of these two together can go a long way to achieve the best depth, the strongest signals and the most stability depending on different sites and conditions.
Optimum control over what we can find.

Those units with an actual threshold control add a fine tuning ability to the equation.
Those that only have a sensitivity adjustment it could control just the threshold level alone or a combination of threshold level and the gain depending on the design.

Like if you are having trouble holding up your pants.
You can use a belt or use a pair of suspenders...each does the same job in different ways.
Use both and you have the most control.




Most of this info came from this Fisher Lab article from 2009.


http://www.tekneticst2.com/tekfiles/sensitivity and depth.htm



Sensitivity control” A control labeled “sensitivity”. It actually controls either gain or threshold, or a combination of both, depending on the machine. If both, the higher settings vary threshold and the lower settings vary gain.

“[Audio] threshold control” Determines the signal strength level corresponding to the threshold of audibility. A negative threshold setting is used to suppress signals by a fixed amount so that only signals stronger than that amount will be heard. Negative threshold settings are used to silence internal “circuit noise” and electrical interference. Machines which have no threshold control have an internal threshold which allows silent operation, or a control labeled “sensitivity” which actually controls threshold. …..Some models allow positive threshold settings. In most cases the positive range controls the loudness of a minimum detectable signal, a separate internal threshold determining what will or will not be detected.

“Gain control” This makes signals bigger or smaller. High gain settings make signals bigger, and therefore signals which were originally weaker can more easily exceed the audio threshold, and be heard. If the gain setting is too high, electrical interference or internal circuit noise may cause constant audio chatter. ….Lower gain settings reduce the size of signals, so that relatively weak unwanted signals (electrical interference, deep iron fragments, aluminum foil shreds, etc.) can be silenced.

BottleCapKing,

Thanks much for your post #21 and the well-written explanations on Gain, Threshold and Sensitivity !

ToddB64

Digger27,

WOW ! You really "Tackled" my question !

I'll be reading your reply several times to absorb it all.

Great job my friend !

ToddB64

ToddB64 said:

BottleCapKing,

Thanks much for your post #21 and the well-written explanations on Gain, Threshold and Sensitivity !

ToddB64

Click to expand...

He said the same thing I did but I used way more words.
If there was an award for that I win.
If the award was for clarity he wins.

Re Sensitivity, Threshold and Discrimination

Hi All !

With reference to BottleCapKing's post #21 and Digger27's post #22, it seems to me when you combine the two excerpts below that explain and demonstrate the affect on threshold and target audio/visual reporting when increasing/decreasing sensitivity, that this is similar to using discrimination as far as target reporting goes, except disc. gives a much broader range of control of course.

Is there a similar or any other kind of affect between the sensitivity and discrimination controls ?

[BottleCapKing]:
"When you increase sensitivity, you are lowering that threshold. When you decrease Sensitivity, you are raising the threshold."

[Digger27]: See attachment

Thanks in advance for replies !

ToddB64

ToddB64 said:

Is there a similar or any other kind of affect between the sensitivity and discrimination controls ?

Click to expand...

No. Discrimination is basically a filter that the processor uses to reject signals. Now, discriminate with caution, because if you try using tight filtering, you can get signals that are "close" to what you are trying to reject that sound kind of broken and you wouldn't want to dig them.

For example, if you are filtering up to "pull tab" and say, on your machine, pull tabs have a VDI of 30. The machine will fully ignore anything up to a VDI of 30. However, it will also degrade the signals for VDI 31, and to a lesser extent VDI32, and to an even lesser extent VDI 33, etc. This usually goes out to about VDI35 before the signal is no longer attenuated by the filter.

Hi BottleCapKing !

Thanks for your reply, post #27; helps greatly !

ToddB64

Why to Detect in AM, Go Slow & Low and Dig All Target Signals !

Hi All !

I know it's been said hundreds of times "hunt in All Metal mode and dig all target signals, so as NOT to miss something good.", and with the possible exception of experts like Digger27, I believe this is good advice for the rest of us and I'll just add that we should exert a high degree of patience, moving our coil slow and low. This is especially appropriate for metal detecting newbies until they acquire perhaps hundreds of hours and years of detecting experience.

With Dave's permission, I have copied, pasted & colored below an excerpt from one of his essays at http://www.fisherlab.com/hobby/dave-johnson-essays.htm titled: Discrimination mode Sensitivity and Depth to emphasize the statement he makes that offers a supporting reason for why we should follow the above advice, especially during our formative years in metal detecting.

I also made a list of nonferrous metals below Dave's paragraph and included some of the items you could miss using too much discrimination.

DAVE JOHNSON'S ESSAYS

Discrimination mode “Sensitivity” and “Depth”

The effects of discrimination “Discrimination” between different metal objects is done using a different set of signals than the ones used for detection. Since the overall purpose of discrimination is to eliminate response to certain classes of metal objects beginning with metallic iron, magnetic iron minerals in the ground will tend to make nonferrous metal signals look more like ferrous, increasing the probability of their being rejected by the discrimination circuit. Some machines provide data on the amount of iron mineralization, so with experience you can estimate the depth of effective detection and discrimination on that site.

A few nonferrous metals listed here for your reference
• aluminum.....tokens, sporting goods, etc..
• copper.........pre-1982 cents and some '82's.
• lead............civil war bullets.
• nickel..........nickel coins.
• tin...............toy figurines.
• titanium.......jewelry and sporting goods.
• zinc.............zincoln cents.
• brass...........bullet cases and relics.
• gold.............nuggets, coins and jewelry.
• silver...........coins & jewelry, etc..
• platinum.......rings and other jewelry.

The above list gives you an idea of all the good targets you might possibly miss by setting-up your machine with too much discrimination. My suggestion would be to hunt in AM (no disc.) and dig it all until you get too tired and start losing patience, then tune-in some discrimination to relieve yourself of digging the majority of trash. I know this is hard , but just accept that you might miss some good stuff. There's always a new day when you're rested and can go back over those areas again ! (See following paragraph.)

Here's a great post by Digger27 at the address below. It's a long "novel" of his , but really worth your time to read, as it will impact you on the importance of hunting those previously hunted places again and again, using All Metal and Going Slow & Low with the coil. I'm mainly referring to the lower half of the post and the title is shown in the paragraph below.
http://metaldetectingforum.com/showthread.php?t=78080

I Edited this post last night, 11/03/2016 11:09 PM EST, deleted the whole paragraph that was in this spot before where I talked about the above address possibly being incorrect for the post I intended and it was, changed the address number from 86773 to 78080, tested it and it works correctly now, i.e. it takes you directly to Digger27's post titled: Thought I got it all...I was wrong! (warning, a long one) As I indicated in the paragraph above, this post will impact you on the importance of hunting those previously hunted places again and again !

Hope you got some help out of this !

ToddB64

I am an expert?

"Expert...Ex meaning has been, spurt meaning drip under pressure."

That is closer to the truth.

All this time I though my wife used this to wash my clothes. No wonder I can't find anything.

DIGGER27 said:

I am an expert?

"Expert...Ex meaning has been, spurt meaning drip under pressure."

That is closer to the truth.

Click to expand...

Digger27,

You eat too much humble pie.
We want to be like you, by-n-by.
So spread your wings and fly !

ToddB64

ToddB64 said:

My suggestion would be to hunt in AM (no disc.) and dig it all until you get too tired and start losing patience

Click to expand...

There is a difference between hunting in AM mode and hunting with no discrimination. All Metal mode is the same non-motion mode that pinpointing uses. Normal hunting mode (Discrimination mode) requires motion, whether you are actually discriminating anything or not. While technically you are picking up all metals when using no discrimination, the two are not the same.

Any metallic element not named Iron is non-ferrous. Any alloy that does not contain iron is also non-ferrous. US coins also do not consist entirely of one metallic element (Nickels are not in fact nickel or even mostly nickel, etc)

Using too much or even light discrimination can bite you in the backside when it comes to deep targets. The deeper the target and the weaker the "response signal", the more the target is going to appear as iron to your detector. Sadly, it is a game of complex vector mathematics where the final VDI you see is the mathematical sum of the ground signal and the target signal. As the strength of the target signal approaches the strength of the ground signal - well, it doesn't look like a great target anymore and will get discriminated out.

Why to Detect in AM, Go Slow & Low and Dig All Target Signals !

BottleCapKing said:

There is a difference between hunting in AM mode and hunting with no discrimination. All Metal mode is the same non-motion mode that pinpointing uses. Normal hunting mode (Discrimination mode) requires motion, whether you are actually discriminating anything or not. While technically you are picking up all metals when using no discrimination, the two are not the same.

Any metallic element not named Iron is non-ferrous. Any alloy that does not contain iron is also non-ferrous. US coins also do not consist entirely of one metallic element (Nickels are not in fact nickel or even mostly nickel, etc)

Using too much or even light discrimination can bite you in the backside when it comes to deep targets. The deeper the target and the weaker the "response signal", the more the target is going to appear as iron to your detector. Sadly, it is a game of complex vector mathematics where the final VDI you see is the mathematical sum of the ground signal and the target signal. As the strength of the target signal approaches the strength of the ground signal - well, it doesn't look like a great target anymore and will get discriminated out.

Click to expand...

Interesting stuff BottleCapKing.

Thanks for clarifying !

ToddB64

Technetics will not reply to my question using their Contact Us form !

BottleCapKing said:

No. Discrimination is basically a filter that the processor uses to reject signals. Now, discriminate with caution, because if you try using tight filtering, you can get signals that are "close" to what you are trying to reject that sound kind of broken and you wouldn't want to dig them.

For example, if you are filtering up to "pull tab" and say, on your machine, pull tabs have a VDI of 30. The machine will fully ignore anything up to a VDI of 30. However, it will also degrade the signals for VDI 31, and to a lesser extent VDI32, and to an even lesser extent VDI 33, etc. This usually goes out to about VDI35 before the signal is no longer attenuated by the filter.

Click to expand...

I own a Teknetics Gamma 6000. So, when I first read BottleCapKing's above post in this thread, specifically the second paragraph, it grabbed my interest and I wanted to know if the manufacturer agreed with what he said.

So, on 11-11-16 3:04 PM EST I got on the Teknetics website and submitted my 1st message below, using their Contact Us form. As you see, follow-ups have also been made.

Paraphrasing BottleCapKing, my message read as follows:

1st. message:

"I have the Gamma 6000.

Let's say for example I'm filtering up to 40 at the edge of the "Foil" segment. I assume the machine is fully ignoring anything up to the VDI of 40. However, will it degrade signals for VDI 41 and to a lesser extent VDI 42 and to an even lesser extent VDI 43, etc. and continue minimally degrading out to about VDI 45 before the signal is no longer attenuated by the filter ?

Thanks for answering this question.

Todd"
______________________________________________________________

2nd message:

When Teknetics still hadn't replied, I sent a follow-up on 11- 26-2016 2:38 AM EST reiterating my first message, again using their Contact Us form.
______________________________________________________________

3rd message:

When Teknetics still hadn't replied, I sent another follow-up on 12-24-2016 2:38 AM EST reiterating my first message again and using their Contact Us form.

4th message (Verbal):

I phoned Teknetics @ 800-413-4131 on 12-27-2016 around 5:30 PM EST(Approx. 3:30 PM MST) and talked to Ruby. She would forward my last follow-up message dated 12-24-2016 to their Engineering Dept. and said I would receive an answer soon....Yeah; as of today, I'm still waiting !

You know, back in my younger life, I worked as an inside Sales Engineer and felt that it was my responsibility to keep a copy of customer contacts requiring answers from our internal departments. I would follow-up, the frequency depending on customer needs, until an answer was received and conveyed back to the customer. Some companies respond promptly, others are habitually late or seem to forget about the customer altogether !

Depending on the issue at hand, I would seek an answer for my customer in person or use the phone, but other times I preferred an answer in writing.

Anyone else having trouble getting answers from Teknetics ?

ToddB64

I learned something.

Makes some sense of this Etrac Im swinging for sure. I think it may be called something different on my Etrac though. But I know the setting that you just described lol.