Gary Shows You How To Say Goodbye To Emi!

[Rattlehead]

It’s the same for me in my neck of the woods, 350. EMI isn’t a big problem most of the time. Every now and then I’ll get on a site where it gets a little noisy, but still manageable. Swapping to another base program that uses a different set of frequencies usually helps. The sites I hunt that are near the city are generally very trashy, so I’m usually just coin shooting with a lot of notch, which definitely helps cut down on the chatter.

 

[Digalicious]

Good point about the audio response.

Emi gives a tone and ID, but for the most part, it produces weak signals. Which of course is why reducing the sensitivity cuts out the emi noise, and why the audio response would cause the emi noise to be louder.

Emi typically ID's between ferrous and zinc, so ya, cherry picking for high conductors discriminates out most emi.

 

[Shelton]

Okay next thing EMI cancel feature with Deus II is only a random channel selection. The software does not analyze anything, it just selects a random channel. If the choice is bad, press again, and maybe you'll be lucky this time... Of course with the other devices you have the same problem. Only Manticore working in a different way.  

[UKD2User]

5 hours ago, Shelton said:

Okay next thing EMI cancel feature with Deus II is only a random channel selection. The software does not analyze anything, it just selects a random channel. If the choice is bad, press again, and maybe you'll be lucky this time... Of course with the other devices you have the same problem. Only Manticore working in a different way.  

How do you know this? My impression is that the Manticore just lets you cycle through the available wavebands continuously over a longer period (and combines the results in some way that they do not give detail on).  My impression is that the D2 and the Equinoxes (and probably others like the Legend) are not random, but only sample each possible waveband once during a given 'noise cancel' session.

[Chase Goldman]

7 hours ago, Shelton said:

Okay next thing EMI cancel feature with Deus II is only a random channel selection. The software does not analyze anything, it just selects a random channel. If the choice is bad, press again, and maybe you'll be lucky this time... Of course with the other devices you have the same problem. Only Manticore working in a different way.  

Please show me where it is documented that D2 frequency search is random.  My personal experience has run counter to that assertion as I have never encountered a case where it made the situation worse (which would be the case if it were truly random), and only once or twice was it unable to make things appreciably better.  As far as I am concerned it is the most effective implementation of reduced EMI frequency search and I have owned several Detectors with the feature.  The Nox's are not very effective and do seem to be the most likely to be random (though I doubt even they are).  The Manticore in my experience is better than the Nox's in this regard but not as good as the the D2 but that might be a function of the D2's overall better EMI immunity than the Minelabs. FWIW.

[Shelton]

Check Tom Dankowsky Forum you have a few pages about it. I have D2, Manticore, and Tarsacci, and all devices are suppressed by EMI almost in the same way. Only Nexus and DeepTech analog devices work without any issues.

[Digalicious]

About a year ago, I read a post in which a metal detecting engineer was explaining why SMF is so much more suspectable to EMI than SF. It went something like this:

With SMF, multiple samples (frequencies), get accumulated during the processing. Therefore, the EMI noise also gets accumulated accordingly.

I think the bottom line, and most important aspect of EMI, is that a metal detector does not, and cannot, know if a signal is coming from the air, or coming from a target in the ground. As such, there is no true way of mitigating the EMI, without a performance loss in one way or another. For example:

EMI signals are typically weak, and that of course, is why reducing the gain on the amp, mitigates the EMI, but that means depth loss. Switching to a SF that is outside the range of the EMI is another option, but then the benefits of SMF are gone.

The difference in the channel frequencies is very small. Something on the order of a mere 0.02 khz difference between the channels. That's just too small of a shift to mitigate the various frequencies, harmonics, and random nature of EMI. Further to that, a frequency shift doesn't address the fundamental problem of noise when using SMF. That is, the exponential rise in noise due to SMF accumulating the EMI noise.

[UKD2User]

13 minutes ago, Digalicious said:

About a year ago, I read a post in which a metal detecting engineer was explaining why SMF is so much more suspectable to EMI than SF. It went something like this:

With SMF, multiple samples (frequencies), get accumulated during the processing. Therefore, the EMI noise also gets accumulated accordingly.

I think the bottom line, and most important aspect of EMI, is that a metal detector does not, and cannot, know if a signal is coming from the air, or coming from a target in the ground. As such, there is no true way of mitigating the EMI, without a performance loss in one way or another. For example:

EMI signals are typically weak, and that of course, is why reducing the gain on the amp, mitigates the EMI, but that means depth loss. Switching to a SF that is outside the range of the EMI is another option, but then the benefits of SMF are gone.

The difference in the channel frequencies is very small. Something on the order of a mere 0.02 khz difference between the channels. That's just too small of a shift to mitigate the various frequencies, harmonics, and random nature of EMI. Further to that, a frequency shift doesn't address the fundamental problem of noise when using SMF. That is, the exponential rise in noise due to SMF accumulating the EMI noise. Of course, it all depends on the nature of the noise - its 'spectral power density'.

I think it's more subtle than that - analysing the return signal over a range of frequencies (what we know as SMF) ought (other things being equal) to give an opportunity to reduce the overall effect of noise (EMI and/or mineralization) through clever signal processing e.g. some kind of (auto-) correlation process, on the basis that interference will not affect all frequencies equally at any given time. Of course, a lot depends on the nature of the noise - its 'spectral power density'.

[Digalicious]

24 minutes ago, UKD2User said:

I think it's more subtle than that - analysing the return signal over a range of frequencies (what we know as SMF) ought (other things being equal) to give an opportunity to reduce the overall effect of noise (EMI and/or mineralization) through clever signal processing e.g. some kind of (auto-) correlation process, on the basis that interference will not affect all frequencies equally at any given time. 

I completely understand your point.

Each sample from each frequency should have different EMI signatures. Therefore, a comparative algorithm would be able to discern the EMI noise, from the ground signal and target signal. However, it seems to me that since most of the EMI noise has a "signature" identical to good targets, then the algorithm would often throw out the EMI signals, along with the wanted signals.

 

[UKD2User]

22 hours ago, Digalicious said:

...... it seems to me that since most of the EMI noise has a "signature" identical to good targets, then the algorithm would often throw out the EMI signals, along with the wanted signals.

 

I don't think it's possible to say that most of the EMI mimics the "signature" of a good target - a little will, most won't, I'd argue.  By its nature EMI is unpredictable.  The response signal from a target is very strongly correlated with the (precisely known) transmitted signal - but (almost by definition) uncorrelated with extraneous EMI. 👍