Fisher Gold & Treasure Detectors | Dave Johnson's Essays

 

                      What are “processes”?

 

There are a number of ways to process target signals in order to extract target presence and target type information.  We call these different ways “processes”.

 

One example of a “process” familiar to many metal detector users is the classic second derivative (“two-filter”) discriminator circuit first made popular during the 1980’s.   Nowadays some metal detectors process the signal in this same way, but in software by “crunching numbers” rather than by using analog computation circuits.

 

Another process familiar to many, is the peak sampled second derivative process, patented in the 1980’s and used on many Bounty Hunter products as well as the Fisher CZ’s.  This process is usually associated visual target ID systems.

 

The metal detecting world was first introduced to the word “processes” with the Fisher F75, which offers the user a selection of several different processes.  For the convenience of the user we give processes names, but we say very little about how the process actually works in software because that’s a trade secret.   In a few cases savvy users have made correct guesses as to what was going on in software, but we don’t confirm that the guess is correct.

 

In the Teknetics T2, there are some process variations folded into the TONES menu but we did not use the word “processes” at the time the T2 was introduced.

 

Some people refer to “processes” as “modes”.  One can think of them that way, but in our more recent user interfaces the word “mode” usually has a different meaning, most often indicating whether all metals or discrimination is selected.

 

Here is a description of several processes which are offered in Fisher and Teknetics products.

 

“default” or “T2” process  Originally developed for the legendary Teknetics T2, we called this the “default” in the F75 because of the processes which are in the F75, this process is the best one under most conditions.  It offers superb target separation in trashy conditions, as well as deep target ID.  It also has response so fast that you can sweep the searchoil very fast over a series of targets and it will “hit” separately on each one almost as fast as a machine gun. 

 

“jewelry process”   This is a variant of the default process which has enhanced sensitivity to small low conductivity targets, and will therefore give the best results when searching for small jewelry.  The resulting increased sensitivity is a disadvantage where there is electrical interference or a lot of noise from buried trash or from ground minerals.

 

“plowed field process”  This process replicates to some extent the characteristics of earlier all-analog “two-filter” discriminators.  It lacks some of the performance enhancements which software-driven signal analysis made it possible to incorporate into the default and jewelry processes.  Lumpy ground and steel bottlecaps tend to negate the benefit of those performance enhancements, making it possible for some users who are familiar with the response characteristics of traditional “two-filter” discriminators to put that familiarity to use, to get slightly better results in lump ground or in bottlecap-infested areas. 

 

“boost process”   This process, introduced in late 2009, is somewhat similar to “default” but uses a different filter system to boost depth capability and to make sounds less “choppy”.  Response is slightly slower, but with a slightly slower sweep target separation is still as good as the default process.   Many customers have said that this has become their preferred search mode, rather than “default”. 

 

“slow process” (on the F70)   This process is fairly similar to “boost process” but with a number of minor detail differences.   It will tend to be a little quieter in heavy trash than the boost process.

 

“cache locating process”   this process, introduced in late 2009, is somewhat similar to “boost process” but with much slower filtering.  Its primary use is for locating large deep targets while sweeping the searchcoil several inches above the ground surface to reduce response from small shallow targets.   A few customers have gotten good results with it for more general use, by sweeping slowly and carefully to preserve target separation.  Another situation where it can be used is where there is several inches of grass or crop stubble such that the searchcoil cannot be brought close to the ground.  In this situation the increased depth capability of the cache locating process helps to reach down to the targets in the ground despite the additional height of the searchcoil above the ground.

 

“F5 process”  The Fisher F5  and “new” Gold Bug, and the Teknetics Delta, Gamma, and Omega use a process first developed for the F5.  The “F5 process” is a variation on the “default” process which emphasizes quieter operation in heavy iron trash.

 

“BH process”   This refers to discrimination and target ID processes which are based on the principle of sampling the peak of the second derivative signal, timed by the zero crossing of the first derivative signal.  Most of the legacy Bounty Hunter products use this process, as do the Fisher CZ’s.  

 

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The so-called “two-filter” (continuous second derivative) process, with variations, has been the industry workhorse analog discriminator since the early 1980’s, from our approx. $60 BH Junior to the approx. $1,000 Troy X5 (no longer in production).  It offers a good combination of simplicity and “feel”.    However it does not support visual target ID, which requires a sampled process.

 

The workhorse visual ID system process used by products the origin of which is traceable to earlier than 2006 is the “BH” process.  Dating from the mid 1980’s, it can be implemented in either circuitry or software and has a long track record of good performance.

 

Our workhorse process family since 2006 has been the T2 (“default”) process and its descendants.  This family of processes combines the advantages of the “two-filter” and “BH” processes and has additional benefits not offered by either of those earlier processes.  To our knowledge none of our competitors has figured out how copy the T2 system.

 

We’re not a company who stands still. Over the last several years we have introduced a whole series of new products on several new hardware platforms, and have constantly advanced the science of target signal processing.  In the future we plan to introduce new processes which, although they may “feel”  like a member of the T2 process family, will operate on different principles and will produce results of a kind which are not attained by any metal detector presently available. 

 

--Dave Johnson

  Chief Designer, FTP-Fisher

 

 

File:  processes FAQ       29 Oct 09   DEJ