4 quadrant VLF discrimination

[Davor]

I think I have a solution for an analogue multiplier thing. It just dawned on me, a kind of epiphany, so I developed a sort of a DC accurate analogue multiplier.

A Gilbert cell was not an option because it is not DC accurate. To reference it to the ground I'd need some serious level shifting. The other options are either too exotic or too impractical. So I invented my own. Using only garden variety components of course.

One of these days I'll post it here - a world première. Yet another product of my li'l laboratory

[Tinkerer]

When thinking of man-machine interfaces we often put the machine up front, which is wrong. It may be a good marketing, but nothing else. Interfaces should be mutually orthogonal, complementing in information they provide, and not competing with each other.
You may think of vibration as a perfect candidate for non-motion target response interface. You'll have no problem with fast response there, and it will not compete with your audible motion compensated detection or discrimination indication.

A non-motion machine does not necessarily meant it has to be a slow machine. Non-motion means that the ground signal is not motion filtered.

A motion machine uses a filter that, at a given sweep speed, removes the ground response. If you use the motion machine at the wrong sweep speed, you loose the advantage that the motion filter gives.

With a variable speed motion filter, we can use different sweep speeds.

Or we could add a gravity motion sensor or accelerometer, that controls a state variable filter, automatically optimizing the motion filter with the sweep speed.

Tinkerer

[Davor]

True, but non motion is more continuous in response anyway, and it makes sense to give it a very different interface.

A variable filter for motion response seem an interesting idea, but IMHO it is an overkill. A main benefit of a HPF filter is to compensate 1/f noise. With typical MD response, it's slope is overwhelmed with the approaching target response. In my IGSL build I squeezed the last drop of the time response within the limits of the existing HPF, and as a consequence I enjoy a nice pinpoint-like performance. I concluded that a somewhat slower motion compensated response would be beneficial, provided proper care is taken of 1/f noise.

Sasho of Bulgaria, also present here on this forum have already experimented on slower motion response.

BTW, I really invented a novel analogue multiplier that would fit nicely to my 4-quadrant rig

[Davor]

It seem there is even easier approach to the 4-quadrant: phase inversion. Most of the nowadays IB VLF rigs have some switchers in a front end, and there are only a few different combinations. By observing a polarity of a GEB channel we can drive XORs to invert or not the switching phase of the rest of the channels, and a 2 quadrant rig suddenly becomes a 4 quadrant one. The switcher chip socket can become a "slot" to fit the 4 quadrant upgrade guts for a lot of different existing rigs. Easy as 123.

So instead of inventing all the bits from the beginning, I can invest the effort in a simple upgrade for the benefit of many. Sounds good. Kind of Christmassy.

[Davor]

I think I have yet another one up my sleeve, but most probably someone else must have found that already. It is the ground balance thing.

In traditional VLFs the Geb channel is used to extract sound as it is basically the same thing as "All metal" one. Ground is balanced in a way that a ferrite and general ground response is quieted. It mostly corresponds to perfect discrimination criteria. However, there are some special cases where salts skew the angle of ground response. In extreme case of salty beaches the traditional ground balancing becomes useless for quieting the ground response.

The easy approach could be the one that uses a separate channel to be used for audio extraction steered independently from the other channels in order to quiet ground response, and simultaneously leave the discrimination criteria untouched. It could be made as an add-on for some of the existing rigs as well. My IGSL has a spare channel already, so I could try that soon.