Minelab Multi-frequency

[Chase Goldman]

7 hours ago, Dubious said:

If the Nox really uses only two frequencies simultaneously, the deception fooled only consumers like us and protected nothing.  Minelab's competitors knew what was going on as soon as they got an Equinox into their labs.  And Minelab's statements are unmistakable.  "With EQUINOX you can operate across the full spectrum of frequencies simultaneously for maximum results."  https://www.minelab.com/knowledge-base/key-technologies#321586. The illustration under the text makes the meaning even clearer. And Minelab makes the same representation in many other places.

Thanks for making my point.  What you think is "unmistakable" and  clear in meaning,  is anything but that.  In your statement you use the phrase "uses only two frequencies" which itself is ambiguous.  The more precise statement is that Equinox only "transmits" two frequencies at a time, while it may "use" more than two frequencies by monitoring for received harmonics resulting from the two it transmitted.  Also, in the above example, Minelab used a correct but similarly ambiguous term ('operate") perhaps knowing that most people will automatically think "transmit" for that also.  When talking radios, which is basically what a metal detector is, they operate by transmitting and receiving radio signals.  A metal detector is actually more akin to a specialized type of radio - a RADAR which transmits a signal and then discerns information from the reflected radio signal off the target of interest.  However, unlike RADAR,  a metal detector transmits a radio signal and then detects the induced electromagnetic field generated in the target of interest.  If Equinox is transmitting two frequencies simultaneously as evidenced by oscilloscope testing by third party expert detector designers, then harmonic waveforms are also generated that are multiples of the difference between the base frequencies, these multiple harmonics will resonate with the target of interest and create a field comprised of the frequency spectrum of harmonics induced in the target.  The Equinox is very likely detecting (i.e., receiving or "operating") at frequencies across the entire spectrum discussed even if it is only transmitting two simultaneous frequencies.  The amplitude (i.e., energy) in the reflected harmonic waveforms is very small, but detectable nevertheless. 

I am not saying that ML is being fully transparent, but I think they are choosing their terminology and words very carefully to elicit the desired response from consumers while maintaining the essential element of truth necessary to keep from being legally accused of publishing falsehoods.  To be clear, I am not disagreeing with you that ML is "selling" their technology be relying on people to potentially misinterpret ambiguous technical statements as you just have nor am I defending ML's marketing practices.  All I am pointing out is that ML is indeed splitting hairs to their advantage, but I cannot see where what they are saying is technically incorrect or even misleading (if read carefully).  I say that, even knowing now that they are likely only simultaneously transmitting a maximum of two of five available base frequencies they say are available (which all happen to be harmonics of 5 khz).  It actually amazes me how well the public statements hold up to technical scrutiny.  That is good for ML, I guess, but not necessarily good for the consumer.  However, I would be more concerned about it if their product did not actually work well.  Lol.

[Steve Herschbach]

16 hours ago, Dubious said:

Given the many accurate positive things that can be said about the Nox, I really don't understand the "why" of this.

The why of it is simple - you are reading things into the careful gaps Minelab leaves in their advertising. I am not reading it the way you are, and so I am not offended because I do not feel I am being mislead. You are reading something else into it and feel unhappy because you feel you have been mislead somehow. It all boils down to debating perceptions of what you think the advertising means as compared to what is really going on under the hood. Since you don't 100% know what is going on under the hood, you just don't know.

All detectors are transmitting on at least one primary frequency and many harmonic frequencies. This can be determined with a scope and if you count harmonics it is just a matter of picking a number you want to advertise when it comes to transmitted frequencies. You have no idea how many are being processed unless you know the processing algorithms.

This back and forth with Minelab and the others has been going on for years. Garrett picked 96 frequencies for a Garrett Infinium ad as a direct poke at Minelab. How many people thought the machine was really transmitting and processing 96 frequencies based on the ad? It is all about transmitted because a PI detector does not receive and compare frequency information. Meaningless fluff, an industry insider joke.

Get your 96 frequency detector right here....

Garrett Infinium ad - 96 frequencies!

Carl Moreland weighed in with his frustration on another thread. White's even gave room over in the V3i manual, probably with Carl's input, to comment on the situation. From the White's V3i Owner's Manual, page 6:

"There is much confusion - some of it deliberate - over how many frequencies a detector actually uses, and whether multiple frequencies are truly better than a single frequency. What defines a multi-frequency detector? What do multiple frequencies really do for depth and discrimination?

A multi-frequency detector is defined as one that simultaneously—or, in automated sequence— transmits, receives, and processes more than one frequency. Some detectors have the ability to operate at one of several selectable frequencies, but they still are single frequency detectors because during operation they can only transmit and process a single frequency. The same is true of detectors that have a control to slightly vary their operating frequency to minimize interference; even though they have the ability to operate at many (slightly) different frequencies, they are fundamentally single frequency designs. Currently, all multi-frequency hobby detectors run their multiple frequencies simultaneously as opposed to sequentially; they are all characterized by having multiple processing channels in the receive circuitry. Therefore, a 2-frequency detector will have two processing channels. Spectra V3i has three independent processing channels; it is a true 3-frequency detector.

This all sounds easy, so where is the confusion? It turns out that (currently) all multi-frequency detectors create a transmit signal that is composed of digital waveforms which are designed to produce peak energies at the desired frequencies. As a side-effect, these digital waveforms also produce undesired harmonic frequencies. Lots and lots of harmonic frequencies, 10’s or even 100’s of them. These harmonics have no useful energy and are not part of the signal processing. So while we can claim to transmit many, many frequencies, we cannot claim to process or use them. Therefore, we could easily claim the Spectra V3i transmits 17 frequencies, or 28, or 39, or 55—we could get plain silly with this. And such a claim would be true, technically speaking, but since all those extra frequencies are not actually used, it would be misleading to make such a claim. White’s chooses, instead, to claim the number of frequencies we are actually using and processing. It may not sound as impressive as a 55-frequency detector, but it’s honest and accurate.

Pulse Induction (PI) detectors also utilize a digital (pulse) transmit waveform, so they transmit a tremendous number of harmonic frequencies as well. So is it fair to include pulse induction as a multifrequency technique? Not really, because PI detectors process in the time domain, not the frequency domain. So even though they use broad-band signals, they are not frequency-based detectors at all. Calling them “multi-frequency” is simply another attempt to confuse the consumer.

Once we get beyond the marketing hype, the real question is: What does multi-frequency do for depth and discrimination? The truth is, any time a detector is simultaneously transmitting more than one frequency, the transmit energy must be divided amongst the frequencies. Therefore, a single frequency detector can usually squeeze out slightly more depth than a multi-frequency design at that certain frequency. But this is an advantage only at one frequency, which tends to favor only a narrow range of targets."

 

These are old debates going back to Minelab BBS and it’s 17 transmitted frequency claim. I reference it all in my discussion at Selectable Frequency And Multiple Frequency

Anyway, interesting subject for armchair engineers and armchair attorneys. Hopefully you all get it sorted out while I am out metal detecting! 

[Steve Herschbach]

This is all based on an assumption people make - that more frequencies are better. Are they? Why? Other than advertising, what makes anyone think that?

There is no reason to compare items at close frequencies. Comparing 10 khz and 12 khz is a waste of time and processing power. What you want to do is compare the targets and ground at a few widely differing frequencies. That way you see what happens that is different at say 4 khz and 32 kHz. That difference or lack of difference gives you extra data to work with. And as Minelab points out in the article about Multi-IQ the more interesting question is how few frequencies are needed to get useful information. And that seems to boil down to two or three.

"'How many simultaneous frequencies?' you may ask, wondering if this is a critical parameter. Minelab has been carrying out detailed investigations into this in recent years. Just as you can color in a map with many colors, the minimum number to differentiate between adjacent countries is only 4 – a tough problem for mathematicians to prove, over many years. Similar to the map problem, it’s perhaps not the maximum number of frequencies needed to achieve an optimum result, but the minimum number that is more interesting. When it comes to frequencies in a detector, to cover all target types, how the frequencies are combined AND processed is now more important, with the latest detectors, than how many frequencies, for achieving even better results."

 

My assumption always when dealing with frequencies and what is processed versus what is transmitted goes like this.....

Manufacturer claims multiple frequency operation, simultaneous or near instant sequential, I don't care.

They state a number of frequencies.

I then assume that out of the frequency range mentioned, that at least two or more frequencies are being received and compared at the processing level to justify the claim of multifrequency. Just exactly what frequencies are being processed and exactly how they are being compared.... I do not expect any company to tell me other than in vague terms.

That's it. That way I stay calm while others try and divine the magic. And with that I leave the subject to better minds than mine.

[EL NINO77]

http://www.geotech1.com/forums/showthread.php?24893-Minelab-Equinox-800-Frequency

[Dubious]

Steve, I take your point that other detector manufacturers have also misrepresented their technologies; but I'm still a bit disappointed in Minelab.  These misrepresentations obviously don't mislead people in the know or engineers in the industry who examine the detector but they do fool consumers like yours truly, and I don't think that's fair.

White's explanation is a good one.  We can't count potential harmonics, or every single-frequency detector would be a multi-frequency detector.  The only harmonics that matter are ones the circuitry is designed to enhance rather than suppress and that are powerful enough to energize the coil and produce a working magnetic field; and, that is easily determined (by engineers with the right equipment) by analyzing the transmitted/coupled signals.  Geotech, who apparently is an engineer with the right equipment, determined that for each Equinox mode, only two frequencies are used (i.e., current at those frequencies that energizes the coil in a meaningful sense).  Unless someone can explain how he's wrong, that is the reality.

You are right, Steve, that simultaneous use of five frequencies would not necessarily produce a superior result.  It's Minelab that told us it would--producing more data points to analyze.  Geotech suggested use of three frequencies would produce benefits, but probably not a greater number.  Myself, I really don't know enough to venture a guess.

BTW, for anyone confused, I realize I and others have sometimes used RF jargon in talking about the Nox and its frequencies.  Although there are, no doubt, weak spurious RF emissions, the Nox does not use radar or radio (or propagation of radio waves) to detect.  It energizes its coil with VLF (very low frequency) current; the coil's oscillating magnetic field impinges on targets, producing eddy currents and magnetic fields that the Nox detects.  No VLF transmission in the radio propagation sense is involved in the detection.  (VLF radio transmitters do exist, used for things like long-distance communication with submarines; but that's something else entirely, involving very large antennas and huge amounts of power.)  

[Chase Goldman]

17 hours ago, Dubious said:

BTW, for anyone confused, I realize I and others have sometimes used RF jargon in talking about the Nox and its frequencies.  Although there are, no doubt, weak spurious RF emissions, the Nox does not use radar or radio (or propagation of radio waves) to detect.  It energizes its coil with VLF (very low frequency) current; the coil's oscillating magnetic field impinges on targets, producing eddy currents and magnetic fields that the Nox detects.  No VLF transmission in the radio propagation sense is involved in the detection.  (VLF radio transmitters do exist, used for things like long-distance communication with submarines; but that's something else entirely, involving very large antennas and huge amounts of power.)  

You are correct, Dubious, thanks for making me look this up.  While we are both factually correct in using the radio analogy because (the metal detector meets both the definition of radio and magnetometer) because it is emitting electromagnetic energy in the radio frequency part of the electromagnetic spectrum.  But the electric field energy (the radio part) is being lost in the ground while the accompanying magnetic field is doing all the work, including inducing the current in the target that results in magnetic field detected by the receive portion of the coil (the magnetometer part), as you described.  It is probably more correctly referred to as a radio frequency magnetometer (i.e., what you were describing) than a radio.  Used the radar analogy because while a metal is not a radar, a radar is the sort of the radio equivalent of an RF magnetometer because the transmitter is detecting its reflected transmitted radio signal (electric field) off a distant target so it was the closest familiar RF thing I could think of that detects how its own transmitted electric field signal is modified by a target like a metal detector's modified magnetic field.  I learned (or re-learned) something today which is always a good thing.

[dewcon4414]

All you electronic gurus........ which im surely not, when it sends ...... lets just say 15khz single into a ring.... does it receive 15khz back or is it converted to something else entirely based on the conductivity/magnetic field of the metal and everything around it?   Meaning ..... does the khz used only LIGHT UP the metal which the length of this time is a known for various metals?   On multi freqs....... does one sustain a longer period that it lights up the metal...... in other words..... 5khz... is transmitted first then followed by 15khz.... giving it a longer period to be seen/read than a single freq might?

[Chase Goldman]

1 hour ago, dewcon4414 said:

All you electronic gurus........ which im surely not, when it sends ...... lets just say 15khz single into a ring.... does it receive 15khz back or is it converted to something else entirely based on the conductivity/magnetic field of the metal and everything around it?   Meaning ..... does the khz used only LIGHT UP the metal which the length of this time is a known for various metals?   On multi freqs....... does one sustain a longer period that it lights up the metal...... in other words..... 5khz... is transmitted first then followed by 15khz.... giving it a longer period to be seen/read than a single freq might?

The induced magnetic field in the target is still 15 khz with a bunch of harmonic junk because the magnetic field waveform gets distorted by the shape, mass, profile, and composition of the target as well as the ground and nearby target effects, the detector is measuring the amplitude (the wave peak) and waveform shape (distortion - through detection of harmonics is one method to do this) of the 15 khz (base frequency) magnetic field using electronics and/or software to make a guess as to the type of target based on those characteristics of the detected magnetic field.  Since there are literally an infinite number of shapes, sizes, and compositions, ground types, and target interferences and only a small subset can be approximated as reference points for comparison/processing in software, you can see where all the target ID uncertainty comes from.  It is quite amazing how well it works, if you think about the enormity of possible scenarios.

[Alluminati]

Just to add some confusions, I was trying to avoid us going round in circles, so I thought I would verify those square waves posted in the other thread, however those square waves posted in El Nino link different then my results. A nice flat square wave like thosev would imply true multifreq something or other because that is how square waves are made, by combining a harmonic series of sinusoidal waves until their peaks emulate a flat top. Conversely Fourier transform can be used to deconstruct a square wave into it's constituent single frequencies. That Carl guy ? said that these run in the freq domain so Fourier very well be used. IMO.

 All detect modes are kinda 7.76 kHz average effect on target excitation I feel when using multifreq.  If I screw with the knobs on this thing I have saw evidence of 6,9 and 25kHz fwiw, but probably best to ignore this for the time being.

With the exception of Field1/2, I don't see much "weighting" at the coil, it could be done on the receive side. Field 2 is interesting, it kinda looks like a squarish wave, like a ghost with the right shoulder constantly moving up and down, while the ghost beside it has it's left shoulder moving up and down.

Beach mode does not have much of a sawtooth, almost sinusoidal looking, but it too has a little bit of the dancing shoulders.

IMO this dancing might imply some mild shifting of frequencies going on. I recall older FBS/BBS machines being steady images.

Actually when you look at the results of a Fourier on a square wave you get a result that looks just like the Equinox scope trace when you superimpose and center them at the crossover point.

I guess regardless of how crude the "square" wave is on the transmit end, this doesn't restrict the harmonic information you can receive from the target and ground. I think that is why Minelab is always talking about subtraction, or channels.

I hope this ads to the confusion and disinformation.  Myself, I have seen enough, it's clearly voo-doo that makes this thing work.

[Alluminati]

Fourier is so powerful, you could even extrapolate a picture from it.

Check this out. https://www.youtube.com/watch?v=ds0cmAV-Yek