LF project

[Davor]

As for the phase detector mentioned above, it will have to deal with 4 quadrant stimuli, where same phase response is expected of signals in 1st and 3rd quadrants, and also 2nd and 4th. It goes as follows...

Say I have a silver coin, so it will have a response in a 2nd quadrant (counted from Tx phase), but also in 4th quadrant when a signal from coil comes in counter-phase.
I wish to have a continuously rising tone as a discrimination response from Fe to Ag.
So I have to obtain a continuously rising (or falling) phase detector response for the first 180° but also followed by a repeated same slope for the next 180°. A simple XOR gate as a phase detector is not providing this response, as it's ramp goes reverse in the 2nd 180° cycle. JK latch provides a single slope for the whole 360° span, but it can also work as a harmonic phase detector, and then it does precisely what I need: same ramp for both 180° sub-spans. A perfect 4 quadrant phase response. It will give the same target ID for every position of a target against a coil, and the best of all is that it is a linear response obtained without any analogue (or digital) calculation. All it needs is a hard limiter.

It must be emphasised that just because a signal from coil comes shifted for 180° it is still the same target that must be ID-ed as a same metal, and that's precisely what 4 quadrant detection is about.

What follows are the examples of 3 different phase detectors, and the most interesting is the response in red indicating a 2nd harmonic detection. Green is the XOR, and blue is non-harmonic JK latch.

[Davor]

I'm considering a sequential multifrequency approach. It is not a far stretch beyond the approach so far - frequency-invariant and everything condensed in a single amplifier stream. I expect some serious improvement of GB in all conditions.
There are a few scenarios cooking. I'll come back with more details when I reach some conclusions. So far it seem that multifrequency is quite simple if you only keep it simple.

[Davor]

It appears that a 4 phase QSD can be designed simple enough and it can be switched on for the duration of each frequency. It effectively turns a very front-end into a lock-in amplifier. I made some spice models of the missing CD4052 switches for the purpose, and it seem quite OK.
I might make a simple TR device based on this principle, but with wide range and perhaps automatic GB. It could work fine for minerals and gold.

Whoever wishes to receive fast models of CD4052 and CD4053 may ping me here. I also implemented the break-before-make feature for 4052.

[Tim]

Good thread Davor. This explains what I am achieving with my simple design. Superheterodyne. Harmonic resonance and sympathetic frequencies, all interpereted by the most complex processor of all- the human brain. It runs at 109khz with a DD made from 2x40 winds .25mm enamelled, wound on a 15cm form. The coil was set on a Matchless locator. Windings were pva glued on a flat surface. Shielding is shiny powdered milk bag @ 12mm on air side and molded around windings on down side.
I have been most humoured by the earliest threads here regarding BFO and IB, especially the comments about circuit instability, which I have only experienced with non-pursued oscillators. By all means if this is still the misconception, maybe it's time some of the gurus here revisited older ideas. My matched pair takes about 30 seconds to warm up, rising 100hz only. There is zero operational drift. Although it is not as sensitive as my GMT (YET) I hope to achieve this by experimenting with coils.
I recently revisited many of the common simple circuits online, no ic, and only one transistor based circuit, came within cooee.
I know this is a tech forum, but all the real genius's will tell you KISS. Keep It Simple, Stupid.

I don't want a detector that tells me what it thinks I want to hear, or just goes beep.
I don't want a detector that needs a degree to operate.
Chopping,sampling,mixing,amplifying..coil design....will all suffer if the oscillator circuit(s) is inferior.

[Davor]

True. But there is more to it.
I'm playing with a slightly evolved project stemming from concepts described here, but also including some multi frequency ideas. I'll not disclose any of those here as yet, for obvious reasons.
It will include pitch VDI, but audio processing will still remain a coil to brain interface.

[scrungy_doolittle]

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.

Not necessarily pure sine waves. I've played quit a bit with binaural audio, with the BGEN brain wave generation. Interesting program. Interesting idea with the binaural sound. You could feed the two channels (X and Y) and derive a binaural sound that would allow you to pretty much discriminate in your head... Pure sinewaves, and pure triangle waves would work.
Since you need two channels, and there is only 1 received channel, this idea of using the X and y channels and converting them to audio would be quite nice. For anyone attempting to patent the idea, this thread of posts make this prior art.

[scrungy_doolittle]

Just a few thoughts on choice of audible tones. First off - there are no tones any better than others, so the choice is arbitrary. 1kHz is right in a middle of a telephone band, so it can work just fine. It is close to C6.

There is a question of relative pitch when two or more tones are used. It is obvious that frequency relationship of 1:2 (octave) works fine, but there are perhaps some other choices that may work better. In order to visualise our perception pitch is often mapped in a circular form or in a spiral. If we use C6 as a reference (~1046Hz) it's opposites are F#5 at 740Hz and F#6 at 1480Hz. C to F# is related as 1:1.41
In fact all frequencies that fall between 1:1.33 (G:C) and 1:1.5 (F:C) would be quite distant in pitch, but not too distant in frequency. This may come in handy when designing sound output using resonant transducers such as piezo.

To put things into perspective, attached is a typical pitch class spiral. Octaves are separated vertically, but horizontal separation can work too. It also suggests that a VCO that spans in a narrow range of frequencies is quite effective way of audibly accentuating some feature.

Davor, you need to consider hard of hearing people. Typically as the ear ages, it loses high frequency response unless it is exposed to trauma at a particular range of frequencies. Things like shooting a gun with out muffs, operating heavy equipment without hearing protection, tend to produce notch losses in the 3K/4K range.
lower frequencies are easier to hear. i'd stick to 400-600 hz because that is the band least affected by age and noise.

just food for though.

[Davor]

The downside of low frequencies is that you lose spatial qualities as you go lower. For some it is only a binary sound that works. More options is better.
I'm on my merry way to capitulate and adopt a micro for discrimination processing part. Arduino is on top of my list.

[Tim]

My first AM detector was dual frequency, but from what I can see there seems to be an optimum frequency for a particular circuit/coil combo, so the main advantage would be having an appropriate frequency for another coil, rather than a stand-alone dual frequency detector/coil combo.?? Or a widely variable frequency range for experimentation with a particular coil?
Whatever is the case,I am adamant that re-invention of discarded designs with evolved electronics could result in some eureka moments. It seems that since the advent of PI nearly all the tinkerers interest in vlf and other technologies has all but stopped.
Even the ML boys will grudgingly admit that the GMT will get the crumbs they miss. I have a ML 10x5 coil here, and it's as heavy as either of my complete homebuilds. There's 2 reasons off the bat why this is a good cause.

[bklein]

I have enjoyed playing with binaural audio through the years and I've always wanted to build that CW receiver. I've heard audio from it and just wonder how a metal detector should sound if we had all the electronic tricks figured out. I'm not sure binaural is as important as somehow giving the most character possible to a target audio signal. Is this harder than the traditional focus on ID numbers, depth, and LCD display? What if we initially sacrifice depth for this? Just come up with something you can run across the beach with and quickly discriminate good from bad drop targets...