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The confusion of 'filtering' --- Required Vs Allowable Sweep Speed --- Recovery Time from Rejection.

October 12, 2018 09:20PM avatar
It can get confusing when you try to compare a wide-range of metal detectors. We had our first ground-cancelling Discriminators from Bounty Hunter about '78, the Red Baron, and to make the new circuitry design work in order to reject the ground mineral signal and also reject unwanted trash and pass along a positive detected target signal, those models required a very, very brisk sweep speed.

When Fisher introduced their 1260-X in '82, David Johnson, the design engineer, used the term "Double-Derivative" to describe the different filtering required to handle the ground and rejected & accepted targets. Then in '83 Jack Gifford introduced the Tesoro Inca which was also a slow-motion/quick-response design. Those two prominent model entries really started some discussion to try and categorize the fast-motion and slow-motion design differences, and that was about the time we started to hear the reference to Four-Filter and Two-Filter.

In reality there are far more 'filters' in a detector to handle all the different things they need to do to generate, transmit, receive, process, and Discriminate, plus the 'filtering' that is required to process audio Tone ID and visual Target ID. To keep things less technical and just make use of some general terms so the average folks could kind of classify a model into one of two categories, we had the 4-Filter models that required a very brisk sweep speed, aka fast-motion, and the 2-Filter detectors that were different because they allowed or required a slower-motion sweep speed.

That brought notice to two other differences, and those were the 4-Filter designs had a longer "ring time" after sweeping the target before they could recover and be able to respond to another nearby target. The 2-Filter types didn't have that long ring-time and, instead, produced a quick-response as the coil passed the target. That made the better for working sites with dense targets, both rejected and accepted.

The 4-Filter types were improved upon by the very late '80s and early '90s with models such as White's XLT and the upper-end devices that followed. They could now process signals better and didn't require the brisk sweep, and were then able to be swept at a more controlled moderate sweep speed, but could be used with a faster sweep, if desired.

There were trade offs between the 4-Filter and 2-Filter design. The 4-Filter would lose performance if the coil was swept too slowly so they required a moderate to fast-motion coil presentation. An advantage the 4-Filter design had was that they could generally handle higher mineralized ground easier, better, than the slow-motion 2-Filter design.

The 2-Filter detector model's strengths were that you should use a slower sweep speed, and that reduced fatigue. Also, the required slow-motion & quick-response performance made them very good models to use in trashier environments or confined spaces because they gave a quicker response with a slower sweep and with a shorter coil sweep-length due to debris or dense brush, etc.

The trade-offs were something that some people never realized until someone told them about the dangers of how they were handled.

Example A: You were using a 4-Filter, fast-motion design with a longer ring-time and you tried to sweep very slow because you were tired or just wanted to try and work a confined or high-target area slowly. Without enough sweep speed, however, you had very impaired performance if you tried a too-slow search coil presentation.

Example B: You were using a 2-Filter type detector that is a slow-motion/quick-response circuitry design, but you found yourself limited in time to hunt, or you were trying to hurry and find more than your buddy, or you were facing a large area to search and wanted to just quickly cover it so you got carried away and started using a faster sweep speed than required or allowed. The result would be very impaired performance to possibly no performance at all.

Why? Because the filtering used required a slower sweep speed in order to process the ground mineral signal as well as any target signal ... to include Discriminated targets that you didn't hear ... and if you swept the coil too fast you forced in too much ground signal that the simple 'filtering' couldn't process. You saturated the ground signal processing and the detector would then not be able to also process and respond to any targets.

Matters can get worse, too, if you are hunting in a more mineralized ground environment such as black sand found in many volleyball courts and some tot-lots. Pea gravel or places like a rock-filled parking lot, etc. If we were left to pick from a 4-Filter 'type' detector that allowed a moderate to fast sweep speed and it could better handle bad ground conditions, or a 2-Filter type slow-motion/quick-response type that required a more controlled slower sweep speed that gives us the quick response and recovery for dealing with trashier environments, it could get tricky trying to match the right types off filtering with the mineralized conditions, and still give us the type of search performance we wanted to use..

Then in '87/'88 Compass Electronics design engineer, John Earle, introduced his new Scanner series with what he termed 'Vari-Filter. I was employed there at the time and asked him for a simple description and he said that Vari-Filter was in essence a 3-Filter design that provided the benefits of both the 4-Filter and 2-Filter designs.

The search coil could be worked at a good moderate sweep speed like a 4-Filter for faster site coverage, but it gave a quick-response like a 2-Filter. Additionally, the coil could also be swept at a much slower sweep speed and it would handle trashier conditions better, like a 2-Filter detector. Sadly that was at a time Compass and Teknetics/Bounty Hunter and Tesoro, and who knows which other manufacturers, were struggling and able to be purchased or were on the market for sale. Compass, like Teknetics/Bounty Hunter, soon sold and still continued to die, and that was the end of the Compass Vari-Filter designed models.

The 4-Filter models were starting to have less interest and the industry move was more toward the slow-motion/quick-response designs. We didn't have a serious 3-Filter detector entry until about 2000 with White's introduction of the David Johnson designed MXT.

Sorry to ramble but while Randy (Hombre) gave you the basic answers, but there's a lot more to a detector design than just being a 2 or 4 or ??? filter design.

One problem a lot of detectors have had in recent years was their inability to recover quickly to be able to respond to a nearby desired target if they were busy Discriminating/rejecting some unwanted nearby piece of trash. There are some detectors that have a very impressive 'Recovery Time' if all they are doing is giving a positive signal from a series of not-rejected non-ferrous coins. An example would be placing 8 coins on the ground about 3" to 4" apart and slowly sweeping down-the row. Beep, Beep, Beep, Beep, Beep, Beep, Beep, Beep and the recovery seems to be impressive, signaling on each individual coin.

Now, take that very same detector and coil and use about three or four 3" to 4" iron nails and remove every-other coin and replace it with an iron nail. First, however, increase the detector's Discrimination to just reject the iron nail.

Then, sweep down-the-line again and compare how that model's "recovery speed" might not be too impressive when it is also trying to reject (Discriminate or filter-out) the trash and still hit on the next desired target.

A short answer to how to handle these different types of site conditions is to have at least two detectors that complement each other in performance behavior. I only have 1 of the 4-Filter 'type' model in my Detector Outfit and that is my White's XLT. All the rest of my chosen detectors are of the 2-Filter or 3-Filter concepts. The modern designs do a better job in most applications with improved performance, and some of that enhanced ability is due to their digital circuitry designs.

I'll close for now, but refer you to Hombre's newer post that suggests you search a site and find a mid-depth coin-sized target. Something in the 3" to 5" actual depth based upon the ground mineralization where you are, the detector and search coil used. I suggest you locate and mark four or five such targets, then with that detector see how much sweep is required to get a decent hit, as well as what might be a too-slow sweep or a too-fast sweep. Then on those market targets before recovery, do the same with other detectors you own and use.

The better we learn and understand the strengths and limitations each different model design offers us, for the type of ground being searched, the more successful we will be because we will use the best acceptable sweep speed/coil presentation possible.

Now the important consideration ..... where am I going to go detecting tomorrow?


"Your EYES ... the only 100% accurate form of Discrimination!"

Stinkwater Wells Trading Post
Metal Detector Evaluations and Product Reviews
monte@ahrps.org ... or ... monte@stinkwaterwells.com
Detectors: Vanquish 540; CoRe, Relic, Impact & Simplex +; Bandido II µMAX & Silver Sabre µMAX; ORX
Pinpointers: Pulse-Dive .. Headphones: Killer B's 'Hornet' & 'Wasp' ... White's 'Pro Star'
Note: Detectors are listed alphabetically by Brand. Models are chosen based on search site conditions.
*** All working well today to make memories for tomorrow. ***
Subject Author Views Posted

Does the number of filters in a metal detector affect recommended sweep speed ?

Anonymous User 524 October 12, 2018 08:34AM

The confusion of 'filtering' --- Required Vs Allowable Sweep Speed --- Recovery Time from Rejection.

Monte 484 October 12, 2018 09:20PM

Re: The confusion of 'filtering' --- Required Vs Allowable Sweep Speed --- Recovery Time from Rejection.

Anonymous User 356 October 16, 2018 06:45PM

ToddB64, pardon my tardy reply regarding your three models and 'filter' questions..

Monte 352 November 03, 2018 07:49PM

Re: The confusion of 'filtering' --- Required Vs Allowable Sweep Speed --- Recovery Time from Rejection.

Anonymous User 351 October 16, 2018 09:52AM

Let me try to answer your questions, Todd. .. Lengthy again confused smiley

Monte 427 October 18, 2018 08:13AM

Re: The confusion of 'filtering' --- Required Vs Allowable Sweep Speed --- Recovery Time from Rejection.

jmaryt 310 October 15, 2018 09:56PM

Re: The confusion of 'filtering' --- Required Vs Allowable Sweep Speed --- Recovery Time from Rejection.

Anonymous User 338 October 16, 2018 10:55AM

Re: Does the number of filters in a metal detector affect recommended sweep speed ?

Hombre 409 October 12, 2018 12:05PM

Re: Does the number of filters in a metal detector affect recommended sweep speed ?

Anonymous User 387 October 12, 2018 12:53PM

Just a quick tip on sweep speedthumbs up

Hombre 345 October 12, 2018 02:56PM

Re: Just a quick tip on sweep speedthumbs up

Anonymous User 331 October 12, 2018 04:01PM

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