Emi. Oh My!

[Digalicious]

Hello.

I didn't see a general detector talk forum, so I hope this forum is ok for this topic.

I own 3 detectors with emi noise reduction, and on all 3, it does little to nothing to reduce emi noise. I've also watched countless videos in which numerous types of smf detectors are very noisy due to emi, then a  noise reduction is done, and as usual, it does diddly squat. They end up doing what we do, and reduce the sensitivity, or use a single frequency.  SMF is so prone to emi because apparently, when the frequencies are combined and "added together", then emi noise from the various frequencies, is also added together and therefore the noise is exponentially increased. 

So, my question is: What exactly is technically occurring when you run a noise reduction? Does it filter out or reduce the sensitivity of the frequencies that are receiving noise? If so, why doesn't it work very well? Is it because if it worked well, then that filtering would significantly diminish the benefits of smf? Also, is it possible that if a particular detectors noise reduction works significantly better than a different detector, then it's because the noise reduction was achieved by reducing the "hidden" fundamental sensitivy? If so, that means you wouldn't see the sensitivity drop, but you would indeed lose sensitivity.

So what's up with all that?

 

[Jeff McClendon]

You already got answers to this question on another forum. Many of the people who answered are members of this forum too.

 

If you use a single frequency or SMF VLF detector that has been "dumbed down" with very low gain for use by beginners or for people who want to avoid smaller targets like for instance the Garrett ACE series, Vanquish models and the Garrett Ace Apex......EMI is not much of an issue.

If you choose to use a single frequency VLF or SMF VLF detector that has not been dumbed down and instead has very high gain, EMI may be much more of an issue if any is present no matter what one does as an end user to try to mitigate some of it. 

The same goes for Pulse Induction detectors using mono coils.........turn up the sensitivity too high in a detecting environment with EMI..........it is unbearable and much more debilitating to the user than the EMI on VLFs.

[Digalicious]

Uh, no. On the other forum, I never got an answer to the question of, "What exactly is technically occurring when you run a noise reduction?". I'm asking on this forum, because this forum seems to have a lot more technically inclined detectorists.

 

[JCR]

Noise reduction is basically a scan of close adjacent frequencies to see if any of them are quieter. The detector selects the one deemed best. You can do the same thing manually & judge for yourself. 

The Legend is the only detector I have that seems to do a good job at it. The new Manticore is suppose to have very good capabilities also. I'm not sure about the new Equinoxes.

[Jeff McClendon]

22 minutes ago, JCR said:

Noise reduction is basically a scan of close adjacent frequencies to see if any of them are quieter. The detector selects the one deemed best. You can do the same thing manually & judge for yourself. 

The Legend is the only detector I have that seems to do a good job at it. The new Manticore is suppose to have very good capabilities also. I'm not sure about the new Equinoxes.

That is the answer and it was given to you on another forum. Minor frequency shifts are all that happens during a noise cancel procedure. 

Since you did ask five questions not one, I tried to deal with the reason why some SMFs are more prone to being heavily effected by EMI than others. Same with hot single frequency detectors that just happen to operate in a heavily EMI trafficked frequency.

We can discuss filtering and algorithms all day along with shielding.........but ultimately high gain SMFs like the Equinox, Legend, Deus 2 and Manticore that are setup to easily hit a wide range of target sizes will be more susceptible to EMI.

 

[kac]

EMI is actually a spike in the signal causing the machine to go out of balance and as it corrects it causes a flutter in the threshold if you have an all metal mode or in severe cases jumpy numbers.

Keeping coil close to the ground, keep coil wire coiled nice and neat help reduce emi.

Many single and selectable frequency machines have frequency offset so you can chose which works best. Dropping gain does also help and so does using a smaller coil.

Not sure how exactly noise canceling works on an SMF. It could be simply frequency offset or it could be averaging the spikes in signal. You can switch to single frequency if it is really bad and pick one that is the quietest for that area.

[Digalicious]

Thank you for all the replies so far.

Is it actually true that one detector can reduce EMI noise significantly better than another? I mean, I would think that all noise reduction works on the same principle, so the effectivness of it should be similar from one detector to another. But, if some reduce the noise much more than others, then would it not mean that the better one is applying more aggressive filtering which means a performance loss? Or, could be outright reducing that "hidden" (for lack of a better term) sensitivity?

That all leads me to another question:

On a high emi site, when you have had to reduce the sensitivity to get rid of the noise, have you ever had to decrease it so much, that you could definitively say, "Wow, I'm really noticing a depth loss"?

[Jeff McClendon]

My apologies Digalicious and anybody else I have been snappy with today on this forum like Walt and Simon. I have a bad case of the grumps today that I am trying to choose not to feed. I am failing....

Anyway Digalicious, here is a portion of my answer to your question over on Friendly.......

"Blocking EMI from being received by the coil is the function of physical EMI shielding. Some detectors have more, some have less. Some have plenty but they are also setup to detect sub tenth of a gram objects=SMFs like the Equinox, Deus 2 and Legend. Some are setup to ignore many of the smaller low conductors=SMFs like BBS/FBS/FBS2, V3i, DFX, Vanquish, Ace Apex, and may seem to handle EMI better even if they have the same level of shielding.

EMI noise reduction is done by choosing either automatically or manually a frequency or frequencies that are least effected by any EMI that is present. Deus 1 users could manually pick 7 slightly different frequencies for 4 or 5 different primary frequencies. Some Tesoros I had could only pick two so if EMI was bad.....SOL.

My former GPX 5000 would go through something like 250 different frequency increments automatically but if the EMI was strong enough it wouldn't help much. The only recourse was to put on an anti-interference DD coil with heavy duty shielding and lose a ton of sensitivity.

Some detectors have no frequency shift/scan capabilities for mitigating multiple detector cross-talk or EMI. Using those detectors where EMI was a problem......hopefully a person had another detector in their vehicle that operated at a different frequency."

 

So some pulse induction detectors go through a ton of frequencies to find the least offending ones. Some VLFs have only one frequency. The Equinox, Legend and Deus 2 have a limited preset number of very small incremental frequency choices on either side of the main frequencies in the SMF weighting. If those tiny shifts don't move far enough away from the offending EMI.......lower the sensitivity or shift to a different SMF program or a single frequency or don't detect in that spot that day.

Loss of sensitivity to deeper, harder to hit or tiny targets will be possible. But, these newer detectors with SMF and multiple selectable single frequency choices do offer the possibility to keep detecting. 
 

[Digalicious]

No problem Jeff.

Due to winter, I haven't been able to hunt for 3 weeks now, and won't be able to for about the next 4 months. It's made me a little snappy.

So, would it be a true statement to say, "The more EMI noise reduction, the less sensitivity"?

[Jeff McClendon]

By EMI noise reduction in this case, I am assuming you mean shielding, filtering, software factors and factory chosen lower gain in general that can reduce overall EMI susceptibility BEFORE doing a noise cancel procedure if the detector being used has one. Sensitivity to smaller, hard to hit targets might be affected depending on what frequency/frequencies are being used.

Just my opinion based on using a lot of different detectors in urban areas.