I trust that your Gold Scanner Pro is off to Keith Will for service. If so, it will probably come back working 'OK.'
The Gold Scanner Pro is simply an variant of the Scanner XP Pro with the manual Ground balance as the primary difference. Compass, desperate to compete with the then-popular gold nugget detector market, needed to do something, and do something fast. While there were a few models out there that lacked discrimination circuitry and were primarily "nugget hunters", there were also several "multi-purpose" detectors. That was the niche Compass was able to target without a lot of extra R&D. I know because I was there at the time as their Dealer Coordinator & Marketing Rep and traveled the US to introduce the Scanner series.
The Scanner's, and by this I am referring to all of them originally designed by John Earle using the Vari-Filter technology, used this same basic circuitry design.
The subject of "filters" is very confusing because it was quite popular at the time to refer to motion discriminators as either "Four-Filter" or "Two-Filter" designs. This isn't, however, an exact count of the number of filters in a detector and causes a lot of confusion.
Motion discriminators, which is almost all we have made any more, has to look at the ground balanced signal, which is usually slow changing, as well as the target signal. A second channel then handles the discriminate function, but it is limited in time to only the period when the ground balanced signal says there is a desired target. Having the dual channel system we can not only detect the presence of a target, but have the ability to try and discriminate it.
The problem encountered when trying to do that comes about because the discriminate channel has a large and slow-changing varying signal that the target is associated with. So, filters are added after each channel to eliminate the low-frequency (ground) signal and pass the high-frequency (target) signals.
After that stage, we can perform discrimination of the target when dealing with the target signal and not the composite target & ground signal. You could say we have eliminated some ground masking.
It is the nature of the filter's dynamics that are usually toyed with in assigning a name to the type of filtering. As I stated, most manufacturers of that time touted having either a "4-filter" or "2-filter" design, but they never really defined or described just exactly what they were referring to as "filters." That certainly would have been more meaningful.
At this point, let me just quote John Earle, himself, from a write-up he produced for the company and their distributors that explains the filter topic. Here it is:
"... there are over a dozen filters performing various tasks in the least expensive motion detectors and the more sophisticated ones with TID have dozens of filters. Your TV, VCR, etc., all have lots of filters just to handle normal electronic "housekeeping." The ones that have been singled out for sales exploitation are the ones following the two detectors."
"Most technically oriented people would say that there are two filters in the motion detection processing, one for the ground balanced signal and one for the discriminating signal. But, the performance of these filters would be defined in other terms like quality (Q), flatness in the passband, steepness of the sides of the passband, rejection ratio and on and on in usually boring technical terms."
"Simple filters are composed of resistor and capacitor sections. A filter for a specific task may contain one or more "filter sections," each containing a single resistor and capacitor. What has been implied in the exploitation advertising is actually the number of simple filter sections per filter. But that is not the only way to make filters and the number of filter sections does not necessarily produce a definite overall filter response because of the way that they can be configured and altered with things like electronic feedback. Unfortunately, we are stuck with labels so for discussion in the way that is being used, we assume that only simple filter sections are meant. Even though simple filters of 2 sections are not very good filters and even 4 sections are not very exotic either, as filters go. But whether that have 2 or 4 simple sections does not alter the character of the detector and we hope to illustrate the basic difference."
"The basic difference in time response between target signals and ground signals can be thought of in terms of an alternating current frequency. The target signal are higher frequency, typically from 5 to 10 cycles per second (Hz), and the ground, for 'nice and proper' ground might be around 1 to 2 Hz."
"What we want a filter to do is typically reject the low frequency ground signal of 1 or 2 Hz and still pass the 55 to 10 Hz. target signal. In other words, we want a fairly steep side to the first filter in that it should pass the desired signal 100% of the time and reject any signal slightly lower in frequency with total rejection. A 2 filter section filter will not approach this kind of performance. A more sophisticated filter could give this kind of response but what happens is that another characteristic besides just frequency response enters the picture for steep sided filters, and that is the ringing characteristic of this kind of filter to continue pumping out signals after detection has ended. Early versions of motion detectors wanted to utilize the best and steepest filters (more than 4 filter sections) to make sure they got rid of ground signals. But, their ringing nature carries the signal on for a period of time that would indicate target presence over a distance of several feet!"
"An example of a very good filter, with steep sides and exhibiting high Q (remember 'Q' stands for quality of the filter) is a tuning fork or even a resonating pipe. When struck, these devices ring on for many cycles as the sound slowly dies away. They don't even need to be stuck, one can simply whistle or sing on exactly their resonant frequency and after several seconds they will be vibrating so that if the energizing whistle is stopped, then the tuning fork or pipe can be heard to continue on. This is a basic fact of all filters, electronic or even mechanical as illustrated."
"Relative to motion detection, the filter composed of just 2 simple RC filter sections will be adequate to remove moderate "nice and proper" ground signals and being what is called low Q, it will not ring noticeably so that targets are well defined, similar to our X100 in 'Sharp' mode. The disadvantage is that severe ground signals, especially if the ground is nasty and has sudden changes in mineral concentrations (not 'nice and proper'), will get through the filter and add to or subtract from the target signal. The result will be that sweeping in one direction it may decide it is a good target and totally reject it in the others direction."
"So, many targets are lost or simply not seen with the simple '2 section' filter use in each of the motion channels. But, for trashy areas with less mineralization, it is the better choice."
"Adding more filter sections to make a 3 section filter increases the filters ability to get rid of the ground and it is the preferred characteristic in heavier ground iron areas. But, target definition will not be as precise because the higher Q and longer ringing of this type o filter will cause a signal trailing that will cause two close targets to appear to be one, but the chances of them being detected is much better than the simpler 2 section filter scheme. Thus we provide the operator with a choice, depending upon the performance he desires."
"Scanner Filters
The Scanner filter system consists of more than 2 section filters for better ground elimination than other "turn on and go" machines that almost exclusively use the 2 filter section type of simple filter. But, Compass has some additional proprietary electronic processing that eliminates the ringing that normally results from a higher Q filter. Therefore, the Scanner gives better ground elimination while maintaining good close target definition in the motion mode or Trashout mode. Compass refers to this electronically modified filter system as the 'Vari-Filter'."
Well, that's all there was and I tried to type it exactly the way the design engineer did, spelling and punctuation and all.
I might try to simplify it by saying that the Scanner series is basically a 2-filter type of detector in that it has a quick response and recovery, but the added electronics to make it a "vari-filter" helps to eliminate the "ring time" of the conventional 4-filter detector, yet the Scanner series handles the ground signal better than the typical 2-filter type.
Monte
