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Date: Fri, 12 Nov 1999 17:15:39
To: rsgb_lf_group@blacksheep.org
From: "Rik Strobbe" <rik.strobbe@fys.kuleuven.ac.be>
Subject: LF: visual-CW
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Visual-CW (or slow-CW or QRSS) has become a rather popular mode on LF. 
It has 2 very important advantages :
1. signal far beyond the audible 'triggerlevel' are detectable, allwing
QSO's that would have been not possible with any other mode
2. is has a very narrow bandwidth, signals only a few Hz apart can easily
be separated
The main disadvantage is the transmission speed as the most elementary QSO
takes already half an hour.

I have been thinking about the possibilities to enhance the transmission
speed without reducing the advantages of visual CW and did some mind (or
paper) experiments :

Assume the following (very basic) QSO :
- CQ ON7YD K
- ON7YD G3XDV K
- G3XDV OOO K
- ON7YD OOO TU K
- GL SK

At a speed of 3 seconds per dot this QSO will take 27 min. 42 sec.
(assuming no 'time-loss' between the transmissions).

Since visual-CW is derived from 'normal' CW we use the same morse code that
was optimized for reception by ear. It has also some 'built-in
intelligence' as the most frequent characters are short, but for visual-CW
where mainly calsigns are exchanged (where the character-distribution is
rather random) is this not so important. An important advantage of morse
code is that we easily can 'decode' it visualy. Most of us would need a
kind of translation-tabels for visual decodation of any other (eg. ASCII)
code, what is not so convenient.

Without abandoning the morse code and the way of visual decoding we are
used to so far, some time can be won by reducing the dash/dot ratio from
3/1 to 2/1.
While this 3/1 ratio is very essential for decoding by ear there is no real
advantage of the 3/1 vs. 2/1 ratio in visual decoding, the signal is
perfectly 'readable', it just looks a bit odd.
But what do we win ? The above QSO, still at 3sec. per dot but with a 2/1
dash/dot ratio would take 22 min. 24 sec, a gain of 5 min. 18 sec. or an
increase in speed of 24%.
The advantage is that only minor changes are involved, but there is only a
limited gain.

Another alternative is replacing the discrimation of dashes and dots from
the time-domain to the frequency domain. This would mean that 'dashes' and
'dots' have the same length (3 seconds) but that they are transmitted on
different frequencies. Since in visual-CW the frequency discrimination is
very high a shift of 5Hz (or even less) is sufficient. Using this technique
we do not need the 'time-gaps' between the dashes and dots, only between
characters we would need a 'time-gap' of 1 dot and a 2 dot 'time-gap'
between words.
Using this system the above QSO will take only 9 min. 27 sec., a gain of 18
min. 15 sec or increase in speed of 293%.
As the frequency is now used to disciminate 'dash' and 'dot' the
transmitter need a sufficient frequency stability. But for a 5Hz shift a
2Hz stability should be sufficient and this is not more difficult to
achieve on 136kHz as a 50Hz stabilty on 80m or a 400Hz stability on 10m.
The next question was how easy (or difficult) our eye will translate the
frequency-shifted signals to 'dashes' and 'dot' (and so to characters).
Therefore I wrote a small program that at randum generated CW characters
and showed them on screen as discribed above. At a speed of 3 sec. per dot
I could easily decode the characters even without using pencil and paper.
Even if there was a sequence of 'O-M-T' or 'H-S-I-E' decoding was possible
al long as there were some other characters arround.

We will need to adapt both our equipement (to FSK) and our mind if we want
to use this technique but the adavantage is a significant increase of the
transmission speed (ca. 300%).

73, Rik  ON7YD