An impossible and insane thought ???

[mcgov51]

In 1969 this nation put astronauts on the moon and spoke to them on live TV.
That moon capsule was not tethered to a telephone line back to earth.

Sure there are serious challenges to what is being discussed here but are they
really insurmountable ?

Unrelated but . . . I'm watching "60 Minutes" on TV and they are discussing
how cutting edge, fast, cheap and popular face recognition has become . . .
And much of the progress is being done by STUDENTS. Amazing . . .

We're no longer talking about additional depth . . . but just handling the unwanted
audio, in a better and more effective way . . . in todays MD.

We currently have Iron Disc. and Notching . . . so somebody had a good thought way
back when . . . because it was necessary. The game has changed today and new
issues have emerged ?

Once that is learned . . . more depth will be voluntarily introduced . . . I am talking
inches . . . not yards . . . but that may someday turn into a foot or so . . . who knows?

Here's just an after-thought ? Could a VLF or PI do without any audio ?
Hard to imagine but . . . ?

I am not suggesting we try to make a VLF into a GPR.

[mcgov51]

Altra - I read every word about Jerry Tyndall and his line of Nautilus metal detectors.
Very innovative and very interesting . . . A man with a vision. I enjoyed it very much.

THANKS FOR SHARING THAT WITH US . . . I HAD NO IDEA.

[Overtheedge]

mcgov51 has made this thread too tedious for me.

I started my own party with MDs back in the late 60's with a HB bfo made from parts out of Japanese 5/6 transistor radios. Where I lived, they were the only cheap source of transistors. That MD was crap. Now I'm back and learning all the stuff that wasn't generally known in the 60's.

... but this party's been running now for 12 years or more. Maybe we're dancing a little slower than you think we ought to, but most of us have other parties we have to attend, and some of us are just too damned old to dance any faster. ...

Well said Carl. Just came off the river from panning (8.5 hours round trip on foot). Too much shore-fast ice to work the better ground. I may dance slow at 62, but I have fun and this thread is NOT fun. Yah, now you know my primary interest in MDs. Chasing magnetite stringers that are indicative of pay streaks.

mcgov51, may I suggest you spend a few days/weeks reading the posts on this site. Read "Inside the Metal Detector" and do the experiments. I bought the PC boards from silverdog and am busy populating them. You learn by studying and doing. Quit being such a condescending *** and realize these folks are far more knowledgeable about MDs than you and me. I don't think noise is your problem anyway. I think it is the same problem as mine: lack of operating skill.
eric

[Skippy]

Quote Altra:"Jerry Tyndall, has been doing the stereo all metal/Disc since the 1980's in his line of Nautilus metal detectors".

[p]Yes, I'm fully aware of this, which is why I used the expression "something more than just left-ear disc, right-ear all-metal". Sure, it's better than mono, but it reminds me of the early hi-fi stereo recordings, with guitarist on the left channel only, the drummer on the right only.[/p]

[Davor]

I like the 'scarecrow organ' analogy. Tom Dankowski refers to excessive diddle-doodle as 'like a hillbilly jug band'. Personally, I think the opportunity to present audio information in some form of stereo could be worth doing. Something more sophisticated than Left-ear = Disc, Right-ear = All-metal, but not really exotic like binaural.

But binaural is not exotic at all, and it is not complicated to produce. You need two amplifiers, same as running stereo, but just a couple of simple all pass networks land you to a promised binaural soundspace.

I'm designing (painfully slowly) an audio stage for my future builds that I'll share here, and binaural network is included. It consists of 2 audio modulators. There are one analogue input (all metal signal level) and two binary gating inputs. The idea behind this approach is your brain being the ultimate processing device of the whole system, so let it play with something it prefers. Plus it is also beneficial to use the spatial distinction of signal, as your brain uses this technique to improve S/N, you know the cocktail party effect etc.

I expect this approach to gain some extra dB-s of S/N as it enables disengaging the "antichatter" filtering. Signal will remain focused, while the noise will get scattered, and your brain will know which is which.

[Qiaozhi]

But binaural is not exotic at all, and it is not complicated to produce. You need two amplifiers, same as running stereo, but just a couple of simple all pass networks land you to a promised binaural soundspace.

I think that binaural soundspace would be an interesting project to experiment with. Wireless World magazine had a article on this subject many years ago but I'm unable to find it after some superficial searching amongst my papers. I know someone who built the published circuit, and I was impressed how well it worked.

In the meantime, here's some interesting papers on the subject ->
Michael Gerzon

Ambisonics. Part one: general system description
Ambisonics. Part two: Studio techniques
Dummy Head Recording

[Skippy]

Davor, I disagree that binaural is simple, though it is more achievable today thanks to DSP processing. I did some work on binaural as part of a college project in the 1980's, using bucket brigade delay IC's (to obtain the <1 msec delay to the distant ear) and filters (to replicate the effects of head-shape and outer-ear response). The difficulty is caused by every person having slightly different heads/ears, so a 'universal' system would not work for many users. .With the MD application of binaural, I imagine the synthetic nature of the original source could be another problem to creating a good binaural effect. Any tones generated would likely need a range of frequencies, such as white noise filtered to a 1/3 octave wide band in order to create a stereo illusion.
But the fact remains - we have two ears and a brain that's good at interpreting the info from both of them, so something more sophisticated could be achieved with the MD's output. And most of us wear stereo headphones whilst detecting anyway, so that's one problem that doesn't need solving. How effective and worthwhile it would be is a question that I can't answer.

[Tepco]

Not exactly on this theme, but few interesting ideas (crossfeed) can be found here:


http://headwize.com/?page_id=654

[Davor]

Bullseye!
Metal detectors need to indicate a limited set of features, and in my case these are only the two sound sources: iron and coloured metal. These are converted to tones by maximum pitch distance approach. This means a maximum of 1:1.5 frequency distance. Using a balanced mixer to convert all metal level signal to respective tones and XOR discrimination gating retains the phase continuity, in effect producing DSB representation of the ground content and irregularities, aiding in "sense of immersion". Two such channels are partly summed by crosfeed approach using incredibly simple networks, as there are only limited spectra to cross-mix and no DSP or bucket brigade delays are required. This simple approach is effective because you need time/phase shifting of only very limited spectra. That's why I said it is simple.

..............

OK, when reading the above sentences, I realise they are purest form of gobbledygook for the uninitiated. I'll gradually make some explanations to make it digestible.

[Davor]

Binaural is a term associated with stereophonic sound that maintains the amplitude and phase relationships such that a person with healthy sense of hearing can imagine sound sources as originating from distinctive places in his/her surrounding. The listener becomes a centre figure of such soundspace, and all the sounds are originating "outside" the listener's head, from distinctive places one can point his/her finger to. Successful soundspace illusion leads to a "sense of immersion", where a listener can imagine himself/herself to be a part of.

Simple stereo provides separate left/right information, and a whole soundspace is crammed inside one's head. Same goes with mono, only the sound appears as coming "from everywhere".

Binaural is beneficial as coherent and spatially situated sound sources are easily followed in your mind and distinguished even if affected by severe noise. The most famous binaural processing effect is a "cocktail party" effect where one is able to follow a conversation in a crowded party with lots of murmur. The same effect disappears in simple stereo reproduction.

Regardless of the recording method, it is good to know that earphone quality is not affecting the effect significantly, and frugal "walkman" headphones are just fine.

Direct recording method involves artificial head, but also some very simple microphone configurations.

In case of synthesizing binaural tone from artificial sources one can make it simple, or complicate it to heart's desire. The effect is the same.

[Skippy]

A good article you have linked to there, Tepco, as it sums up the problems of getting good binaural, but also shows that a simple method can give some kind of better-than-nothing effect.
@Davor: "and no... bucket brigade delays are required" - I was using BB delay IC's because I was taking conventional stereo recordings and attempting to reproduce a stereo image through headphones. When applied to the metal detector, the ENTIRE sound is generated by the detector. So the delays and filtering needed would be achieved at-source, using software/DSP. Likewise, filtered white-noise would also be DSP generated.

[Davor]

Frankly, there's no need for that. At single frequency phase/frequency/delay dependence is of no concern. You'd need some elaborate scheme to achieve favourable delay for a wide frequency span, but for a single frequency it is just overkill. Simple filters do that just fine.

This better-than-nothing effect kicks the binaural soundspace, so yes, it works.

[Skippy]

Quote:"Same goes with mono, only the sound appears as coming "from everywhere"." - this is interesting, and depends how you feed a mono signal to stereo headphones. If you wire the two earphones identically, (ie. L+ to R+ to + of your amplifier, L- to R- to amplifier ground), the sound appears to come from exactly the centre of your head. If you wire the two earphones in anti-phase, you get a more 'spatial' effect. Likewise, wiring the earphones in series, both in-phase and anti-phase gives different responses.

[Davor]

Yes, it is even more interesting with stereo, as you can play with L+R and L-R combinations to create some kind of binaural effect. Downside is that one of the sources is firmly inside your head, while the other is somewhere outside. I'd say it qualifies as "better-than-nothing" but there are other simple approaches leading to less annoying effects.

[Skippy]

Quote:"You'd need some elaborate scheme to achieve favourable delay for a wide frequency span, but for a single frequency it is just overkill" - I presume you are generating a sine-wave single-tone, then? Because if you are using a square-wave, like nearly all detectors do, you will get different delays for the higher harmonics.
Your detector preference is a single-tone unit, I assume. I personally like using tone-break audio, with iron indication. This is where the stereo illusion/binaural/similar system starts being advantageous, when more than one tone is used. As a random idea, the accepted tone could appear to be in one fixed 'space', the rejected (iron) tone could be 'non-distinct' surround sound.