[Elecraft] HRD cw copy

[Julian, G4ILO]

Sverre Holm-3 wrote:
> 
> It is interesting to see the responses to my statement on the difficulty 
> of machines copying CW better than humans. Although this is a little 
> off-topic here, I hope we can have a short discussion of it anyway.
> 
> First, the success of negative SNR communications methods such as 
> Olivia,JT65, and PSK31, are evidence that a well-designed computer 
> algorithm should perform better than a human. But it is on codes that 
> have been designed for machine decoding.
> 
> Second, 'better' may mean many things: faster, many QSOs in parallel, or 
> - what I imply - at lower SNR and under difficult conditions with fading 
> and interference. There is no doubt that a computer has much more 
> capacity for speed and parallel decoding than a human.
> 
> The steps that a good algorithm needs to do are something like this:
> - real-time frequency analysis and filtering
> - detect morse signal and lock on to a particular frequency
> - adaptive estimation of datarate and adaptive matched filtering for 
> optimal detection
> - decoding of dashes/dots/spaces into letters
> - decoding into words
> 
> The first steps are signal processing such as filtering, detection and 
> adaptivity. See e.g. 
> http://www.journal.au.edu/ijcim/jan99/ijcim_ar1.html for some ideas on 
> the adaptive estimation. As a side remark, Coherent CW, was a way of 
> avoiding the adaptation to variable rate and ease machine decoding, but 
> it does not seem to be a success.
> 
> I believe that it takes an extraordinary algorithm to lock onto a very 
> weak signal reliably, but even more so to do the last and maybe even the 
> second last step, and that this is where the similarity with speech 
> recognition is largest. As an example, say that my call is a weak 
> DX-call and I'm sending CQ de LA3ZA LA3ZA LA3ZA. On the receiver end you 
> hear DA---, LA3-T, L-3ZA due to fading and interference. This is where a 
> good operator is able to use a priori information on the syntax of a 
> callsign, similarities between morse codes for various letters, and the 
> three partial calls to piece this together to LA3ZA.
> 
> I'm not saying this is not doable, only that it may take more than a 
> month for a good programmer to do this, and maybe much more also.
> 

Weak signal modes like WSPR, JT65, MFSK etc work as well as they do and can
dig below the noise because they use different tones, rather than tone/no
tone as in CW. The timing of the signal elements is also precisely known.

Even with computer sent morse the program does not know the speed at which
it is being sent, so it has to work that out before it can start. The
vagaries of propagation then throw their spanner in the works, as the
decoding algorithm does not know if absence of a tone is a valid signal
element, or QSB. If you then throw in the imprecision of timing caused by
hand sent morse, then you can see the computer algorithm really has a hard
job to do.

Computer morse decoding algorithms in use currently go no further than
assuming the tones are clearly distinguishable from the spaces and that the
timing of the elements are predictable.

To improve the decoding performance would I think require the application of
artificial intelligence to get the computer to reasses what it first thinks
it receives in the light of what makes sense in the context of an amateur
QSO. This is pretty much what we do when we receive code by ear. 

First of all the human brain is probably more adaptive to irregular element
timing - left footed sending - than a computer algorithm. It "learns" the
guy's rhythm and uses that to decode what he is sending, rather than the
rigid symbol lengths of computer generated morse.

Secondly the human brain uses context and knowledge to fill in the gaps and
make sense of what is received. If someone sends "QTH IS" you expect a place
name to follow. If you miss a couple of letters or what you got doesn't look
like a word you use your knowledge to work out what it would be.

It probably would be possible to write a computer program to do that but it
would be an incredibly challenging piece of programming that would need an
extremely keen mind and a great deal of time to accomplish. It would
probably be a PhD level project.

-----
Julian, G4ILO. K2 #392  K3 #222.
http://www.g4ilo.com/ G4ILO's Shack   http://www.ham-directory.com/ Ham
Directory    http://www.g4ilo.com/kcomm.html KComm for Elecraft K2 and K3 
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