LF project

[Davor]

How about going for gold?

It just dawned on me that there are no amateur projects that would result in a LF machine. Only VLFs and PIs, and not many really different varieties there either. So instead of copying the existing concepts and making incremental progress, why not going for something completely new. Something really different.

My ideas of such a different rig would include:
- balanced Tx and Rx, differential mode Rx preamp,
- Rx coil with centre tap, shielded or unshielded, preferably differential
- 4 quadrant discrimination with two independent channels
- manual and/or automatic wide range GB
- proportional sine tone
- intrinsic enclosure for beach hunting
- no VDI, fancy dials or any other useless distractions
- optional 2box operation.

Fun part is that I have all these solutions already in my head and in a computer, but not assembled in a single piece of equipment. The lure is that most of the nowadays coins are gradually becoming iron-cored rubbish, so there is a real need for stretching the poor conducting materials discrimination range, as LF does. LF is also substantially quieter band.

Any interest? Comments? Any LF owners to share experiences?

[mikebg]

Davor, the frequencies in LF band generate much stronger GND signal, but you are at the beginning of the road to start thinking
like a radio amateur. As you know, the ham radio operates in several frequency bands depending on conditions. May be you know that FBS technology of MINELAB covers three radio frequency bands: ELF, VLF and enters enough deep in the LF band. The principle of MINELAB is like principle of amateur radio communication: Use the suitable band for this moment!

Each HAM DESIGNER knows that the most important parameter of a radio equipment is the SNR. The first step in the design process is the analysis of block diagram, which provides maximal SNR and then the realisation with circuit diagram. I sense that you have a lot of knowledge, but you still do not think like an amateur. Can you explain how offers of your list improve SNR?

[Davor]

There are a few simultaneous approaches actually. LF is a quieter band for a start. There are some 20dB less natural sources and considerably less industrial sources to begin with. There are some radio transmissions there as well, but all are narrowband and can be easily circumvented.
Next line is a differential coil that accomplishes several tasks simultaneously. It suppresses ground signal and also EMI. Paired with balanced Tx it provides "double balanced" front end with considerably better induction balance. Balanced Tx provides for operation free from the offending 2nd harmonic, hence no falses.These operate in B class sharp, exactly at 0° phase, with considerably less phase noise as well, hence considerably less PM->AM noise. So no C class detuned rattles are allowed. There is a negligible power loss due to transmission and most of the energy is conserved in a high Q tank, so there is no benefit of C class at all.
4 quadrant discrimination facilitates differential coil use, but the other types of coils can be used as well. It is the harshest conditions that can benefit from differential coil and 4 quadrant discrimination.
Ground response synchronous GEB shall provide for tough conditions such as salts.
Direct sine sound conversion shall provide for sense of sound immersion, and also a fine feeling for the ground and it's changes.

Perhaps it sounds a bit marketing-ish and far fetched as there is no such thing in existence that I am talking about, but that's my vision.

[Thomas]

I really like the ideas you listed, I've been trying to learn about the same subjects. I may be crazy but, I have an idea to make a detector with stereo audio, so that I might improve the sense of location and faint signals. You will have to take the lead on this, my knowledge and time is limited but I would really like to give it a try, and I will try to keep up.

[Davor]

Your idea is fine. There is a step forward from a mere stereo, and that would be binaural audio. That is the kind that provides immersion I mentioned previously. The idea is to enable your brains to shift attention into the terrain you are searching. It is also important for such audio to be sourced from pure sinus sources, but surprisingly the headphones quality is of minor importance. So any pair of disposable headphones could do. Pure sound will also lessen auditory fatigue.

I could make some simulation sounds from various mock detectors eventually.

[Thomas]

I came across this and with a few changes? maybe something good. listen to the audio sample and imagine.

http://homepage.ntlworld.com/henry01...soundfield.htm

[Thomas]

Here is something else, I would like to know what Mikebg thinks of this altered for a lower freq.

[mikebg]

Here is something else, I would like to know what Mikebg thinks of this altered for a lower freq.

Here is a link if you need explanation how this design operates:
http://www.kangaus.com/Documentation.../BIQR/biqr.pdf

"The binaural processing supplies the ears with additional information without compromising what
was already there, enhancing the presence.

As you tune through a CW signal on a quiet band (best done with your eyes closed
while sitting in a solid chair), a centered signal enters, but moves to the left background,
undergoes circular motions at the back of your head as you tune through zero
beat, repeats the previous gyrations on the right side, fades to the right background,
and finally drops away in the center. Multiple signals within the receiver passband are
distributed throughout this perceived space.

With training, concentration on one signal allows it to be copied among the many. An SSB signal seems to occupy parts of the
space, left and right, with clarity when properly tuned, leaving others vacant. Static
crashes and white noise appear distributed throughout the entire space without well
defined position. Receiver noise, although present, has no perceived position."

I think this method will operate better in VLF instead in LF radio band.

[Davor]

IMHO allpass networks are far better choice for creation of virtual sound space. Point is that 90° difference is just about right for frequencies around 1kHz and the distance between ears. Perfect binaural sound recording would require a dummy head. In lieu of a dummy head and perfectly aware that the same effect is obtainable by all pass delay networks, we can successfully play with binaural even without I and Q channels. Especially because we know exactly what the sound sources we have. Besides, it all makes sense only in a case where two (or more) independent sources need to be placed in virtual sound space.

Good binaural source would not require much adaptation, it comes naturally.

[bklein]

There are several software packages/apps that allow you to play with this effect - just look for effects for music/guitar/recording software. What if you take this stereo thing all the way - work with some sort of dual RX coil that we haven't tried before...

[Davor]

Stereo is not the same as binaural. You need two channels for reproduction, same as stereo, but the phase/delay/amplitude relationships are a bit different and your brains create a virtual sound space when provided with binaural sound. In such space you are able to "place" coherent sources on their fixed places in such a space, and some interesting effects such as "cocktail party effect" kick in. It is also described as feeling of immersion.

In case of a detector, your mind would place the targets in their proper places, just as if the targets are giving off the sound themselves.

In contrast the stereo sounds odd and separated when listening in headphones.

Just seek the internet for binaural sounds and you'll instantly know the difference.

[mikebg]

PATENTS

There are several software packages/apps that allow you to play with this effect - just look for effects for music/guitar/recording software. What if you take this stereo thing all the way - work with some sort of dual RX coil that we haven't tried before...

There are several patents with twin loops. I will try to find them. Here are two:

[Thomas]

Thanks Mikebg, I never read the PDF file before. I think you're right about the VLF vs. LF. I use to listen to SSB with a retuned AM radio and a BFO I made when I was a kid, it was magical to hear a distant voice in total silence, sort of what gave me the idea of having a detector with spatial awareness.

[Thomas]

Stereo is not the same as binaural. You need two channels for reproduction, same as stereo, but the phase/delay/amplitude relationships are a bit different and your brains create a virtual sound space when provided with binaural sound. In such space you are able to "place" coherent sources on their fixed places in such a space, and some interesting effects such as "cocktail party effect" kick in. It is also described as feeling of immersion.

In case of a detector, your mind would place the targets in their proper places, just as if the targets are giving off the sound themselves.

In contrast the stereo sounds odd and separated when listening in headphones.

Just seek the internet for binaural sounds and you'll instantly know the difference.

YES! this is it, why have mono sound beep beep or flat stereo when Ambisonic or binaural can give finer details to the little things.

[Davor]

Exactly! The majority of detectors (except maybe XP Deus) are flabbergasted removing the small artefacts by the collective name "chatter". While chatter may be a serious problem with a binary sound beep beep kind of metal detector, it is much less so in a proportional audio, but I expect even les so with binaural. So you can hear also a target well buried within the chatters - guess how much more you could find that way!
Also the quality of the chatters becomes more noise-like if you don't overact in suppressing them. They become tweety only in a too slowed down anti-chatter configuration. Just guess how much sensitivity is sacrificed in anti-chatter solutions. I know that - I accelerated my IGSL-s response.