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LF: Re: Jason Tests signalling comparison

To: [email protected]
Subject: LF: Re: Jason Tests signalling comparison
From: "James Moritz" <[email protected]>
Date: Thu, 24 Jan 2002 18:45:02 +0000
In-reply-to: <[email protected] v.uk>
Reply-to: [email protected]
Sender: <[email protected]>
Dear Andy, LF Group,

At 14:54 24/01/2002 +0000, you wrote:
All three of these modes use the same basic technique, looking for the
presence or absence of a signal in a given bandwidth - in the case of JASON
doing this 17 times in parallel.  The bandwidth that matters is not the
total bandwidth of the whole signal, just that occupied by one of the tones,
and that is defined in the decoding software...

So, in principle, if one was looking at a spectrogram of a signal made up of dots of a certain length, with evenly distributed noise, it would not matter from the viewpoint of detecting them whether they were all in a line (QRSS) or distributed over 17 different frequencies (Jason), provided you knew what the 17 frequencies were. The Jason signal is equivalent to 17 separate QRSS stations on different frequencies, using a funny type of code arranged so that only one station is transmitting at any one time. So the ability to detect this kind of signal with a given signal power and noise power spectral density only depends on the length of the dots and not at all on the number of frequencies used. But using a larger number of frequencies enables faster transmission of data because then a larger number of different codes can be made up from a given number of dots. So Jason should work with the same signal and noise levels as QRSS using 12s dots, but at the same time be 5 times quicker - is this correct?

I suppose there are practical restrictions on this - for example, you have to find 4Hz of bandwidth with no narrow band noise in it, like Loran lines, in order to make the scheme work.

A machine can decide on the presence or absence of a signal in a defined
bandwidth a lot better than a human eye can on a waterfall - it most
definitely can, believe it, its true, even if this fact makes you feel
uncomfortable :-((

I can believe that machines are better at detecting a small change in level of signal/noise than a human operator, but with the simple encoding schemes we are using, the human operator has the advantage in that they can quite often "guess" what the data should be when it is corrupted. But then you could argue that error correcting codes of one sort or another achieve similar results for machine decoding.

Cheers, Jim Moritz
73 de M0BMU



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