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Old 06-25-2008, 09:10 PM
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Default Confused about Threshold vs Volume?

Many newbies go out and buy a detector that has an adjustable, audible threshold and are confused by its function or proper use. The situation gets even more confusing if the detector also has a volume control. Oh wait, you also bought a nice set of earphones and they also have a volume control. What to do, what to do.

Ok, first, lets look at what is this so called threshold and what purpose an adjustable threshold serves. Then we can tackle what to do with the volume control. Setting both properly maximizes your ability to get as much depth as you can from your detector.

Not all detectors sold now a days have an adjustable threshold that you can hear. These latter machines are termed "silent search" detectors. Basically, they are quiet running until you sweep over a target and then they beep at you.

You can think of the threshold as a type of squelch control as found on a CB or short wave radio. Without a squelch control circuit on a radio, the speaker would be putting out a constant stream of hizzing noise when you are not tuned in to a station. This can be annoying. By dialing up on the squelch control, we are basically telling the radio that we don't want to hear a signal until it reaches a certain strength. The hizzing noise coming from the speakers is suppressed because it is low energy background noise and the radio is quiet until you tune in an actual signal. By the way, on some detectors the manufacturer calls the knob that adjusts the threshold a "tuning" knob. Now you know why.

If you dial in even more squelch, you'll start to miss some of the weaker signals from far away stations.

This is exactly how the threshold control works in a metal detector!

With a silent search machine, the "squelch" control is factory set to suppress the small signals and allow signals with a strength above the threshold setting to get through and make a "beep". If you bought a detector with an adjustable threshold (good for you!), you want to set it such that you can barely hear the threshold sound on your earphones. The sound is hard to describe, but it sounds like bees buzzing your ears. By having the threshold at this setting, right on the edge, you are guaranteeing that very weak signals (like from deep targets) will be able to get through the threshold control. This gives a machine with adjustable threshold a slight depth edge over a silent search machine, everything else being equal. This is because a very weak signal may not get through the "squelch" setting of the silent search machine.

You don't want to make the threshold any louder than what you can barely hear. If the threshold hum gets too loud, you'll miss the real small signals again. This time because they are drowned by the louder threshold buzz.

If like some other folks you can't stand that constant low level buzzing sound of the threshold, you can always back it down a hair until it is not audible anymore. Essentially, you'll be running the detector as if it was a silent mode machine. But you pay a price for doing so. First, you'll lose some of that depth advantage you had over a silent search machine. If you back off on the threshold to just make it inaudible and no more, then you haven't lost much depth. Second, you'll lose some potentially valuable information. You see, when you go over a ferrous item, the threshold sound will usually null (will go quiet) momentarily. When this happens, your detector may not detect a valid target for some time after that. This elapsed time is called the recovery time and varies by the detector model used.

When you hear the threshold null out, you know you went over a ferrous item and you need to slow down and or change your sweep direction lest you miss a valuable target buried nearby the ferrous item. With a silent search machine you won't know it's happening.

Ok, now that we understand threshold, lets look at volume. This is again, just like a radio. Your detector may or may not have a volume control (separate from the threshold or tuner). If it doesn't have one, you definitely would want earphones with volume control. If you have volume control in both the detector and the earphones, then set one at max volume and set the other one to actual use conditions.

My MXT does not have a volume control but it has a very powerful (read that loud) audio amp. So, I set the volume control on my earphones to produce as loud a beep as I can without it being painful. I do this by just dropping my digger on the ground and sweeping over it and advancing the volume control until it as loud as I want, without hurting my ears.

Then I set the threshold control until I can barely hear the buzz. As a side note, if the threshold is erratic instead of stable, the Gain is too high for the ground you are sweeping. Back off on the gain until the threshold is stable.

That's it! Hope you found this useful. For information on understanding the sounds your metal detector makes you might want to look at
Understanding the sounds from your detector.

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  #2  
Old 06-25-2008, 09:13 PM
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Very informative... you had me @ the CB comparison... made perfect sense!

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Old 06-25-2008, 09:21 PM
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Originally Posted by sectshun8 View post
Very informative... you had me @ the CB comparison... made perfect sense!
Thanks.

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Old 06-25-2008, 09:39 PM
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excellent description of threshold
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Old 06-25-2008, 09:40 PM
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Can you say that 10 times in a row really fast Rudy . Great post & very informative. I know from alot of reading and alot of experience that if a person really wants to get every last bit of umph detects in all metal mode with a slight threshold hum, but myself I prefer a silent search mode. I just never could get used to detecting with a constant hum although my Tesoro Sidewinder (that I just got) has a threshold based all metal mode. Thanks for the post Rudy. Steve.
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Old 06-30-2008, 03:02 PM
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Now this makes perfect sense as to why some Minelab X70 users suggesting now to tune the threshold so high. Thanks for the explanation.
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Old 06-30-2008, 03:28 PM
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Great post! I have been confused about these features on my Sov GT, but I now understand what the threshold hum represents and how the threshold knob differs from the volume knob.

What I can't figure out is why the threshold volume level slowly increases as I hunt without me touching any of the controls; enough so that I need to continue adjusting it down every 20 minutes or so. What I also don't understand is why the threshold volume goes from inaudible to painfully loud with maybe a 1 degree turn of the knob. Any insight from the Sov users out there?

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Old 06-30-2008, 04:07 PM
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Originally Posted by z118 View post
Great post! I have been confused about these features on my Sov GT, but I now understand what the threshold hum represents and how the threshold knob differs from the volume knob.
Thanks. That was the intent of my post.

What I can't figure out is why the threshold volume level slowly increases as I hunt without me touching any of the controls; enough so that I need to continue adjusting it down every 20 minutes or so.
It is happening either because of drift in the electronics (changes as it warms up or with time), or as you move around the soil mineralization changes enough that you should re-ground balance. There is a close connection between the threshold volume and the ground balance setting.

What I also don't understand is why the threshold volume goes from inaudible to painfully loud with maybe a 1 degree turn of the knob. Any insight from the Sov users out there?
I'm not a Sov user so I can't answer that one for sure. It may be a case of the electronics being too sensitive to small changes in the setting, or a bad potentiometer (the electrical item the knob is adjusting). Hopefully another Sov user or two will jump in.

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Old 07-02-2008, 03:01 PM
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Originally Posted by z118 View post
Great post! I have been confused about these features on my Sov GT, but I now understand what the threshold hum represents and how the threshold knob differs from the volume knob.

What I can't figure out is why the threshold volume level slowly increases as I hunt without me touching any of the controls; enough so that I need to continue adjusting it down every 20 minutes or so. What I also don't understand is why the threshold volume goes from inaudible to painfully loud with maybe a 1 degree turn of the knob. Any insight from the Sov users out there?
The reason the threshold is getting increased most likely the knobs are being brushed with the headphone cord and causing the knob to turn to a louder position, it is a common occurence with the sovereign series, the knobs are not not set very deep inot the faceplate and they are easily brushed during a hunt
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Old 07-02-2008, 08:55 PM
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Thanks Rudy!

This is the cream of the crop when it comes to informative post. I learn a lot from this information. The forum shines at it's best when I can add to my knowledge base. Don't get me wrong I enjoy sharing finds and pictures but, shared experience is worth it's weight in gold...figuratively and literally. Thanks again.

Keep Swing'in
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Old 07-02-2008, 09:09 PM
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Originally Posted by CyberSage View post
Thanks Rudy!

This is the cream of the crop when it comes to informative post. I learn a lot from this information. The forum shines at it's best when I can add to my knowledge base. Don't get me wrong I enjoy sharing finds and pictures but, shared experience is worth it's weight in gold...figuratively and literally. Thanks again.

Keep Swing'in
Jack
Thanks Jack! It's comments like yours that make the efforts to post this kind of article a satisfying experience.

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Old 07-04-2008, 08:14 PM
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Default good show jack...

i tend to think of threshold as noise-floor but explaining it as squelch is more easy to understand.

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Old 07-15-2008, 04:12 PM
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Thanks Rudy. Your explanation was just in time for me. I am a newbie MDer with an X70. I was wondering what the threshold was all about. I am also an old CBer so I understand squelch. Just curious. Why don't the manufacturers call it squelch? Then it might just be more understood. Please keep the info coming. It's really a blessing for us newbies.
Ray
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Old 07-15-2008, 05:24 PM
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Originally Posted by SoCalToolGuy View post
Thanks Rudy. Your explanation was just in time for me. I am a newbie MDer with an X70. I was wondering what the threshold was all about. I am also an old CBer so I understand squelch. Just curious. Why don't the manufacturers call it squelch? Then it might just be more understood. Please keep the info coming. It's really a blessing for us newbies.
Ray
Hi Ray,

Welcome aboard! Glad you found the article on time.

You asked a good question. I don't know why metal detecting has a jargon of its own. It just does Maybe they do it to confuse the uninitiated.

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Old 07-15-2008, 08:01 PM
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Originally Posted by Rudy View post
Hi Ray,

Welcome aboard! Glad you found the article on time.

You asked a good question. I don't know why metal detecting has a jargon of its own. It just does Maybe they do it to confuse the uninitiated.
i think you are exactly right Rudy!...

perhaps the term "squelch" is in reference to the observation of one "channel".

"threshold" is a more accurate term to use when a spectrum is the object of consideration. a spectrum is a set of many channels. detectors scan for conductive metal across a spectrum that begins with iron up through silver. each particular class of metal is tecnically a "channel" which is how we descriminate against certain targets.

when we notch out an undesired targets we block it's "channel".

a threshold is what is displayed on our detectors when we are using them if we plot it on a computer. notice that "threshold" sort of imples the edge of a floor, deck or step.

as i mentioned earlier in this thread i perfer the term noise-floor because it is more descriptive. if you plot noise across time you get a sort of ribbon which i call the "noise-floor" because it looks like a floor in a hallway if you plot it graphically on a computer. the point of view of the ribbon is of its leading edge with the observer hovering in space. there are other ways view/display this type of data but this is the one i like. signals look like spikes in the floor of the hallway which is constantly receding in time until it vanishes.

if you set a "threshold" for a signal that tells me you are specifying least-upper-bound for the noise or conversely threshold is a least-lower-bound for signal.

(so threshold is least-upper-bound for noise or least-lower-bound for signal) okay...

the higher the least-upper-bound the more noise you get and vise-versa.

"squelch" and "threshold" are engineering words and "least-upper-boound" and "least-lower-bound" are math terms "noise-floor" is a term we used to throw around in class and everyone seemed to know what was being discussed. i guess "noise-floor" is an engineering term.

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Old 07-15-2008, 09:46 PM
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Originally Posted by zander10 View post
i think you are exactly right Rudy!...

perhaps the term "squelch" is in reference to the observation of one "channel".

"threshold" is a more accurate term to use when a spectrum is the object of consideration. a spectrum is a set of many channels. detectors scan for conductive metal across a spectrum that begins with iron up through silver. each particular class of metal is tecnically a "channel" which is how we descriminate against certain targets.

when we notch out an undesired targets we block it's "channel".

a threshold is what is displayed on our detectors when we are using them if we plot it on a computer. notice that "threshold" sort of imples the edge of a floor, deck or step.

as i mentioned earlier in this thread i perfer the term noise-floor because it is more descriptive. if you plot noise across time you get a sort of ribbon which i call the "noise-floor" because it looks like a floor in a hallway if you plot it graphically on a computer. the point of view of the ribbon is of its leading edge with the observer hovering in space. there are other ways view/display this type of data but this is the one i like. signals look like spikes in the floor of the hallway which is constantly receding in time until it vanishes.

if you set a "threshold" for a signal that tells me you are specifying least-upper-bound for the noise or conversely threshold is a least-lower-bound for signal.

(so threshold is least-upper-bound for noise or least-lower-bound for signal) okay...

the higher the least-upper-bound the more noise you get and vise-versa.

"squelch" and "threshold" are engineering words and "least-upper-boound" and "least-lower-bound" are math terms "noise-floor" is a term we used to throw around in class and everyone seemed to know what was being discussed. i guess "noise-floor" is an engineering term.
You can think of it that way if you wish, however most (all?) single frequency VLF detectors are really detecting the returned signal from the target, at the same single frequency.

For example, if the detector is operating at a transmit frequency of 3 KHz, the received signal is also 3 KHz. The VDI, or Target ID is extracted from the phase shift of the returned signal, which is determined by its conductance and permeability. So, there really isn't a frequency spectrum being looked at.

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Old 07-16-2008, 01:43 AM
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Originally Posted by Rudy View post
You can think of it that way if you wish, however most (all?) single frequency VLF detectors are really detecting the returned signal from the target, at the same single frequency.

For example, if the detector is operating at a transmit frequency of 3 KHz, the received signal is also 3 KHz. The VDI, or Target ID is extracted from the phase shift of the returned signal, which is determined by its conductance and permeability. So, there really isn't a frequency spectrum being looked at.
okay...

... i was thinking target spectrum not radio frequency spectrum. you are correct the 3 KHz signal gets mapped into various target id "bands" by processing the phase shift. i referred to them as "channels" in my discourse. peraps "notches" is what we should say since that is the jargon used with metal detectors.

i was using the term "spectrum" in abstract not particular.

i should have used the word "notch" instead of "channel".

the 3KHz signal gets mapped into the various notches across the spectrum of targets that the machine is sensitive to.

my bad.

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Old 03-19-2010, 01:39 PM
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**BUMP**

Thought I would bring this one up to the top again as Rudy does an exceptional job explaining THRESHOLD in general and especially for us MXT swingers!

Thanks Rudy!

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Old 03-19-2010, 01:58 PM
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Nice! Thanks for the info. Should help a lot of the newbies

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Old 03-21-2010, 06:56 AM
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Thanks for the bump......Very good info.....

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