GPX vs Earlier Models. Sound only

amdmakki · November 25, 2009

As some may know my real professions is Sound Engineering and never detected in my life, but have a huge interest in knowing about it. Would you agree if I say that Sound plays a big role in metal detecting? I believe I have read in this forum in one of Steve Herschbach posts and I quote

â€œIt is not that the GPX goes deeper, it is that you can hear nuggets you would miss with an SD as they could not be discerned as clear signalsâ€

I totally agree with the principle, and have a theory I would like to share with you, which will improve an SDâ€™s performance and I think this is how Minelabâ€™s could have already addressed the issue in their newer models. Bear in mind, I have never detected before but only talking about clearer Sound.

The oldest trick in the book for us sound engineers is to use a Noise Gate. In audio post-processing, noise gating reduces steady noise sources such as rumble from LP records, hiss from audio tape, static from a radio or amplifier, and hum from a power system, without greatly affecting the source sound.

A Noise Gate or gate is an electronic device or software logic that is used to control the volume of an audio signal. In its most simple form, a noise gate allows a signal to pass through only when it is above a set threshold: the gate is 'open'. If the signal falls below the threshold no signal is allowed to pass (or the signal is substantially attenuated): the gate is 'closed'. A noise gate is used when the level of the 'signal' is above the level of the 'noise'. The threshold is set above the level of the 'noise' and so when there is no 'signal' the gate is closed. A noise gate does not remove noise from the signal. When the gate is open both the signal and the noise will pass through, but your ears donâ€™t tell you that.

Youâ€™d probably say we all knew that! But thatâ€™s not it. In a professional Noise Gate, there are more knobs like Attack, Release, Range, Decay & Gain. These will make a huge difference. Letâ€™s have a quick look at what I think they can do in the detecting world. At the same time I have attached an image of how a Noise Gate looks like. (Itâ€™ll be easier to follow if you have it open)

Treshold: Control to set the level at which the gate will open

Detecting world: Finding the sweet spot is crucial. Remember even a 1db change in level can open the gate. Although it actually takes a human ear a 3db change to notice the difference. So now your faint signals arenâ€™t that faint according to how you set the other knobs.

Range: Setting the amount of attenuation when the gate is closed

Detecting world: If you donâ€™t want to totally wipe out the background noise, you can control how much back ground noise you want to hear. Why this is very important? Here is an e.g. Say you set a threshold of -30db, this means anything under that is actually noise, by setting a Range you are asking all the noise under the -30db to go to where you want it to be. So say I tell it to go -3db less in volume. So now you are actually listening to a noise signal which is -33db. What happens is: Say that faint signal comes along with a +1db increase. So now you have 30db noise + 1db signal= -29 db which will cause the gate to open. Your ear couldnâ€™t hear a +1db change before, but now it is actually hearing +4db change. All a human needs is 3db. So with the Range knob you can make it as clear.

Attack: The Attack control sets the time for the gate to change from closed to open, much like a fade-in.

Detecting World: So this means the more attack you set, the slower you have to swing but will be more accurate. The less you Attack, the faster you can swing. This will have a minor effect or sacrifice on how deep or shallow? or how small or large a nugget is? Depends

Release: The Release sets the amount of time for the gate to go from open to fully closed. A fast release abruptly cuts off the sound once it has fallen below the threshold, a slower release smoothly changes from open to closed, much like a slow fade out. If the release time is too short a click can be heard when the gate re-opens.

Hold: The Hold control allows you to define the amount of time the gate will stay open after the signal falls below the threshold.

Detecting world: Your preference really can be used in many ways

Gain: You are not affecting the gate but only increasing the sweet signals. I donâ€™t know if an Enhancer works the same. I have never used one, but if it boosts the whole signal, then that is no use. Just increasing noise.

So I think all Minelab machines would have the same Input Sound as an SD in my opinion, but it all has to do with the manipulation of the sound signal, frequency & time into tricking the ears. This is not all. There are also more additions like Compressors, limiters and Multiband frequency boosters (which could help discriminate if metals do have different sound frequency, I don't know if they do) and that can make it way better.

Thatâ€™s why I also think the GPXâ€™s turned Digital, cuz itâ€™s much easier to add all these features to their best in 1â€™s and 0â€™s. If you wanna do all these features in Analog, youâ€™d need a trolley for batteries. :wacko: but if only a Noise Gate, that can work on a 6- 12v battery seperately.

I donâ€™t know if a laptop causes interference with a SD ( I doubt) but if the theory is right, all Iâ€™d need is a tablet pc as my Control Box and get the clearest sound possible. As the Tablet would have all these in Software Logic, even with a touch screen.

P.S Only my opinion regarding sound, not electronics. Im not an Electronic Engineer.

Edited November 26, 2009 by AMD

grubstake · November 25, 2009

Kind of like a streoe equalizer, Base, treble, boost. fader, and a background nosie limiter. Like they had on old 8 and 4 track car stereo's? remember 4 tracks? most people can remember 8 track tape players, but before that they had 4 track one's Grubstake

And before stereo, there was a thing called a reverb unit, that made you car radio sound cool.

DOC · November 25, 2009

AMD thank you for this post, I love to learn knew things and this was a very exceptional post. It is obvious that you really know your business.

While undoubtedly Minelab employs these techniques, I am sure that other detector manufacturers employ them as well so this does not account for Minelabs outstanding performance when compared to other machines.

Minelab's forte' has always been it's ability to track ground mineralization and using their proprietary technology, whether it be multi-period sensing, or SETA, balance that mineralization out, quiet it down, and maintain the sensitivity of the detector to find gold.

Many other brand detectors try to increase depth and ability to find gold by having a knob that increases sensitivity or gain. However a lot of these lower priced units have no ability to filter out the mineralization, so when they increase the gain they also increase the sensitivity to highly mineralized ground, and the noise that accompanies it. The end result is that the mineralization interference cancels out the benefits of increasing the sensitivity in all but the lowest of mineralized conditions.

I don't know all of the technical details of how Minelab accomplishes this, but I know they keep a running track of the fluctuations in mineralization, and automatically keep the detector balanced.

Ground mineralization has always been the bug-a-boo when it comes to metal detecting, and the ground in Australia is so highly mineralized, that Minelab had to develop technology that would deal with that mineralization. Because they managed to accomplish that, and because it is hard to find ground anywhere on earth that is more mineralized than Australia, the Minelab detectors handle just about any ground you can find.

I think you can actually access Minelabs patents online and you would probably be intriqued to learn some of the technology that they are employing. You probably would be in a much better position to explain all the technical jargon to us in more understandable terms, because you gave an absolutely excellent presentation.

Also remember, Minelab is the largest supplier of land mine detectors to NATO, therefore, many of the major technical breakthroughs that Minelab has invented, have been finanaced because of a very healthy R&D budget. Obviously government contracts can be very lucrative. This allows Minelab the financial where with all to research and develop technologies that other companies can only dream about, and these advancements are passed on down to the consumer machines that we own and enjoy.

Doc

DOC · November 25, 2009