Frequency Shift And Doubts On It

[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...

[UKD2User]

I think it's really important to understand that all D2 search modes, including 'Deus Mono' transmit a set of frequencies into the ground.  The transmit waveform in all cases is a pulse-train with square-edged pulses.  All such pulses contain a set of frequencies ('harmonics') - most strongly at three times the pulse repetition rate and less strongly at five times the pulse repetition rate (the third and fifth harmonics). I attach a snapshot from page 43 of the manual which shows that the maximum frequency used for detection is declared to be 135.7 kHz - which divided by three is 45.23 kHz - acknowledging that there is (at least) a third harmonic component in the signal transmitted.

If you select 45 kHz in Deus Mono mode, you are also transmitting at 135 kHz simultaneously - whatever you do.

How the machine chooses to make sense of the return signal is a trade secret - there are some clues in the manual ('subtractive' vs 'additive' algorithms - presumably adding or subtracting results in different frequency bands).  I think it's a given that the D2 will analyze the return signal using exactly the same type of (DFT) algorithm as used to produce the spectrogram in the video - to give target/response info across different frequencies.  Only XP know how the D2 processes the return signal in 'Deus Mono' mode, but I would guess that it uses a D2 not a D1 algorithm (to give more reliable TID at depth etc that is obtained by examining the response across multiple frequencies).

I don't think that underwater you need to worry about EMI, so SMF is the way to go in all cases.

 

I think that the effectiveness of a 'noise cancel' is limited when the EMI is evenly smeared out across the frequency spectrum (broadband noise vs narrowband noise).  Modern EMI sources like cellphones tend to generate broader spectrum noise, making them less easy to eliminate.  Clever digital signal processing algorithms - making use of correlation between the transmitted and the received signal - can go an amazingly long way to eliminate the effects of even broadband noise.

PS the video also shows the 'noise floor' in that indoor environment!

 

[Sinclair]

40 minutes ago, UKD2User said:

Modern EMI sources like cellphones tend to generate broader spectrum noise, making them less easy to eliminate.

+ EMI sources can potentially emit harmonics, too - making it even more difficult to handle in MF mode.

 

40 minutes ago, UKD2User said:

PS the video also shows the 'noise floor' in that indoor environment!

If you refer to my video - yes, sure. But I'm still surprised, how good this simple setup actually works. The TX frequencies have a very low bandwidth by the way - I turned on smoothing to make them more visible on video. Without smoothing, there are just thin lines.