Frequency Shift And Doubts On It

[phrunt]

This is probably why a lot of people think the noise cancels are all a gimmick, as the small shifts are not enough to mitigate their EMI in that particular area.  In some places a noise cancel works, others it doesn't.  I tend to like doing my noise cancels with the detector in full sensitivity so it has the most hope of "seeing" the EMI around, this carried over from the Minelab GPX detector which I was able to particularly notice a benefit to doing so, in turn I do it with every detector.

I haven't found noise cancels on SMF detectors to be near as effective as they are on single frequency machines, and this video is a good demonstration of frequency shift working very well, this EMI is caused by underground power going to spotlights on a spots field.

Thanks, Carl, for your explanation, it makes good sense and also demonstrates why quite often the scans don't work well, if the EIM is taking up a wide range of that frequency the small shifts applied are not enough.

[Geotech]

Powerline EMI is narrow-band and is usually the easiest to deal with. However, the powerline frequency varies a little bit over time so you might tune out powerline EMI, only to find it comes back an hour later.

Wideband EMI is much harder to deal with. Electric fences and dog "invisible fences" are really bad, as they usually use a brief impulse of energy, not unlike a PI detector.

Then there is the detector itself. A multi-freq design has a wideband front-end which lets in everything, and makes getting rid of it difficult. A (true) single freq design has a narrowband front-end which does a good job of minimizing wideband EMI, so getting rid of it is not as difficult.

[Sinclair]

https://youtube.com/shorts/iD9JgAyx67E?si=PCOYykcfskAKu1iA

Quick test with microphone on a mixing console - it really works quite good!

Have to redo it with an sound interface and proper software. Sounds like a sine-tone 😅

[Digalicious]

4 minutes ago, Geotech said:

 A (true) single freq design has a narrowband front-end which does a good job of minimizing wideband EMI, so getting rid of it is not as difficult.

That "true" part is interesting Carl. If I would hazard a guess, I would guess that you may be suggesting that even the SF modes of SMF detectors aren't true SF. 

In all the EMI I've encountered, a slight frequency shift when using SF on my SMF detector, does nothing. For example, there is little change in the EMI noise when switching between 5 khz and 10 khz, or 10 khz and 15khz. More specifically, in my high EMI sites, 5, 10, and 15 khz is a huge frequency spread, yet the EMI noise is similar in that frequency spread. It isn't until around 20 khz that I get a notable drop in EMI noise, with it being totally gone at 40 khz.

For EMI noise reduction to work effectively, either the source or the receiver has to be narrowband. I've never seen it work effectively when both the source EMI is broadband, and the receiver is broadband (SMF).

 

[Chase Goldman]

1 hour ago, Digalicious said:

That "true" part is interesting Carl. If I would hazard a guess, I would guess that you may be suggesting that even the SF modes of SMF detectors aren't true SF. 

Carl can verify, but I think the SF modes are indeed "true" SF in the sense that a prominent single frequency is transmitted as has been verified by oscilloscope traces on the Nox in SF mode. 

Speculation: The issue probably lies in the input filter on a SF/SMF detector not necessarily being as selective (narrow) as that on dedicated SF machines because it serves a dual purpose.   This is a cost/complexity tradeoff to enable a single set of hardware filters to be used regardless of mode selected with Digital Signal Processing used to clean up whatever the filter let's through.  It works but may not be optimal.  It is not a technological challenge to go the the full mile and provide appropriate switchable filters and more sophisticated signal processing, and is only limited by the ability of the designers to hit management's design budget and assigned unit cost objective for the detector.

[Digalicious]

13 minutes ago, Chase Goldman said:

Speculation: The issue probably lies in the input filter on a SF/SMF detector not necessarily being as selective (narrow) as that on dedicated SF machines because it serves a dual purpose.  

That makes complete sense to me Chase. Thanks!

 

[Geotech]

3 hours ago, Digalicious said:

That "true" part is interesting Carl. If I would hazard a guess, I would guess that you may be suggesting that even the SF modes of SMF detectors aren't true SF.

Yes, that's what I meant. A "true" SF detector has a narrowband TX (namely, a sinusoid) and a narrowband RX. The SF modes on SMF detectors continue with a wideband TX (namely, a square wave) and a wideband RX. In fact, because the SF mode on an SMF is a square wave, you could demodulate its 3rd harmonic and do salt cancellation, making what seems to be an SF mode hunt in salt like an SMF. This is exactly what the X-Terra Pro does (which is "SF" only), as did all the old CZs.

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In all the EMI I've encountered, a slight frequency shift when using SF on my SMF detector, does nothing. For example, there is little change in the EMI noise when switching between 5 khz and 10 khz, or 10 khz and 15khz. More specifically, in my high EMI sites, 5, 10, and 15 khz is a huge frequency spread, yet the EMI noise is similar in that frequency spread. It isn't until around 20 khz that I get a notable drop in EMI noise, with it being totally gone at 40 khz.

5k to 10k or 10k to 15k are not slight changes, they are pretty major. Most EMI gets "aliased" into the baseband and both 5kHz and 10kHz ends up aliasing 60Hz to the same 20Hz offset. As you go up in frequency (20k, 40k) the alias frequency is still 20Hz but there is also a natural reduction in amplitude (something called the "sinc rolloff") so that is what's helping.

A slight change is 10k to 10.05k, and 10.05kHz aliases 60Hz to 30Hz, which is just a little farther out in the baseband filter and may help. This is why noise channel offsets are very small, you are only wanting to move the aliased frequency in small amounts.

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For EMI noise reduction to work effectively, either the source or the receiver has to be narrowband. I've never seen it work effectively when both the source EMI is broadband, and the receiver is broadband (SMF).

I would almost label that a "fact."

[Digalicious]

18 minutes ago, Geotech said:

 

5k to 10k or 10k to 15k are not slight changes, they are pretty major. 

 

I agree. I guess I worded it wrong, because that's what I meant 🙂

For example, when I'm in high EMI and use a SF on my SMF detector, the very slight frequency shift of the EMI noise reduction does little to nothing to mitigate the EMI noise. If I want to stop the EMI noise, I have to manually change my 5k to a massive change of 20k or 40k.

[Sinclair]

Some quick none-scientific testing using a simple dynamic microphone with an audio interface at 192kHz. What you see on the Laptop is the frequency range from 2-50kHz. Funny how some programs use the exact same TX frequencies and intensity, so there's definitely something going on at the RX side.
You can also see, Freq. Shift in MF mode shifts all "masters" simultaneously.

[Skull diver]

First, I want to thank everyone for their technical input and explanations on this topic.

All of this, leads me to a perhaps silly conclusion and probably to state a theory that I come to late.

The widest spectrum of frequency variation obviously resides in the mono program.

As a consequence of this, it results in the most effective of programs in selecting a "clean channel."

Unfortunate point, is that in saline or humid environment, there are not the necessary filters for proper and stable use.

That said, mf modes are thus a balance of frequencies with which to deal with different combinations of terrain and are less effective in noise cancellation due to even the slightest collision of at least one of the waves involved with interference.

I am therefore with a dog biting its own tail in front of me in an attempt to isolate an ideal limiting frequency for dealing with saline underwater theoretically between 27 and 37 kHz...