How DEMON works ?

[Dr.Sid]

Any idea how DEMON works ? I mean in real life .. technically. What kind of signal processing does it use ?
Noise demodulation says pretty nothing to me.

My analysis:

Each blade is source of noise. The amount of noise received will vary as the blade travels the circle. Each blade will generate noise signal with amplitude peaks, mostly 1 distinct maximum per turn.
You get several similar phase-shifted signals summed together. For each blade one.

You roughly know the frequency, so you can demodulate the noise, by watching amplitude of the noise. Standard 'radio' demodulation, consisting off cutting minus half-wave and filtering for low frequency would do. Then you can do normal frequency analysis in 1-100 Hz range.

But this would only give you blade frequency. Not blade count, not turn frequency.
It would be hard to find which peaks come from the same blade. Especially sub's blades would be made very precisely, so there should be no distinct blade signature.

Only thing I can think of is dopler. Blades on one side will make slightly different sound then blades on the other side, because of relative speed. This would vary heavily with listener aspect, but there should be some difference almost always.

So .. any idea ?

[[dc]Blade]

The lines that you see on the DEMON display shown from left to right first show the prop shaft freq, then the count of how many props it has.
The TPK`s from the the classification manual help you demodulate the freq as the blades have already been counted, guess you can call it intel:up: So from the whole display the only thing that you can get from it is the targets speed, this is then handed to the TMA to give better your contact data, even though it displays to you how many screws the contact has the only purpouse is to realy gain speed knowledge, in real life workings this would be classified im sure:yep:

The old school way would have been the 3 minute rule using a maneuvering chart to plot a course and speed, so you can see TMA is a modern version of this too but minus the speed calculations hence DEMON

I hope this helps.

[Dr.Sid]

I'm asking about how it works, now how it is used. I know that. Thanx anyway.

I'm talking aboud real-life, not DW. I could post is elsewhere, but there I would have to use 3 paragraphs to explain what I'm talking about.

Intel really can tell blade count. But DEMON is used for classification too, not just speed estimate, at least in DW. Maybe in reality it does not give blade count, and it must be input together with TPK.

Somebody who really knows ?

[UglyMowgli]

At Ms we had some month ago a great description of the DEMON in real life but the post was erased after an official request :damn: as the real sound of submarines I had post ("this is not a place (the internet) to have such sounds" write to me the officals).

[Dr.Sid]

Uh .. I know this .. I have studied computer science.
In US, any form of publishing some encyption algorithms is illegal. Some rebels was inspired by formulation 'any form' and created many obscure forms of the algorithms, like T-shirts and dramatic reading recorded on CD, just to show stupidity of this.
I hope CIA has file on me, cause I'm from 'eastern block' and I know the algorithms, I even had to implement them as my homework.
I guess things like this too fall in the same basket. Anyway, if americans must shut up, maybe some russia guys here ? :rotfl:

[Dr.Sid]

After really painful googling I found some thread where British bubbleheads talk about some new sub propeller. They say: impossible to get blade count, hard to get turns.
Maybe blade count really relies on blades differences.

[Dr.Sid]

Lol .. after even harder googling, I tried search here, at subsim .. so .. something is here:

http://www.subsim.com/radioroom/showthread.php?t=88181&highlight=demon+blade

and here:

http://www.fuzzytech.com/e/e_a_kumm.html

All around .. you mostly don't get blade count, since shaft, at least on subs, are build to give no sound. All you get is blade noise, and you are mostly not sure if you get blade frequence or double blade frequency or even triple blade frequency (2nd and 3rd harmonic). So it surely needs good operator to get some data from it.

You only can get blade count if there is distinct shaft line (which usually is not the case). Then you'll see first line of the shaft, next line will be it's 2nd harmonic (double frequency), then 3rd and so on .. they'll be weaker and weaker. Then there will be blades line. 5 bladed propeller has blade frequency 5 times the shaft frequency. That means quite strong blades line will be at the same position as 5th shaft line. Blades frequency will also have some visible harmonics.
So you'll get something like this:

O o , . O . . . . o . . . . , . . .
O o , . O . . . . o . . . . , . . .
O o , . O . . . . o . . . . , . . .

You see strong line is the 1st line, the shaft, and then each 5th line. This show that there are 5 blades. First 5th line is blades frequency, other are blades harmonics.
But it seems with subs you'll get more often something like this:

. , .
. , .
. , .
. , .

Now you know nothing without classification and good intel. That can give you lot's of usefull information. Blade count in the first place. Tentency to show 2nd blade harmonics more loud then first blade harmonic, and many other things.
Then you could say .. yeah .. the strong line is twice blade frequency. There is something weak at the half frequency. That would be 1st blades harmonics. Blades count by classifications is 7. So TPS is strongest line divided by 14 (!). That by TPK gives .. lets say, 7-8 knots. Or .. it is some different sub at quite different speed .. :-D

[UglyMowgli]

normally you can detect the harmonics by looking the signal strenght, the first blade normally give the gretaer signal (firt on the left of the display) and when you see a line on the display (on the right) with a strong signal is the fisrt harmonic of the 1st blade, so you just have to count the number of line on the left of the display to get the number of blades of the screw.

[Dr.Sid]

Yeah .. that gives sense.

Just watch the formulation 'first blade first harmonic'. All blades together forms the 'blades frequency'.
Each of the blades itself turns at same rate as the shaft. So first blade frequency is same as shaft frequency, same as the frequency of all other blades.
But since you can't distinguish blades, all blades together forms new frequency.
3 equally phase-shifted blades will give triple frequency and so on.

So I suggest not to use 'first blade frequency' at all. Let's differentiate between shaft frequency and blades frequency, which is n-times bigger, where n is blade count.
Term 'first blades harmonic' is of course correct.

Of course .. if you have any better terms, bring then on !


Now I need to reevaluate DW completely :rotfl:
Hey .. last time I whined about layers, SCS actually improved it a lot !
But SCS, ping return sound volume bug is more important I guess. :|\\
PS: Jammie, if you by coincidence read this, nice to meet you on that video !
PPS: Btw. English is not my native language, I hope you guys can get at least something I'm trying to 'encode'.