Message-ID: <002e01c17f5d$f490c920$d89a17d2@steve>
From: "Steve Olney" <ollaneg@zeta.org.au>
To: rsgb_lf_group@blacksheep.org
References: <003601c17c07$4c58fc40$587e883e@g3aqc> <3.0.1.16.20011204101542.2bff5384@pb623250.kuleuven.be> <002f01c17db2$4c388df0$0400000a@parissn2>
Subject: LF: Re: Re: Improving extreme weak signals
Date: Sat, 8 Dec 2001 07:29:59 +1100
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G'day Stewart,


>
> display.  If the Tx were capable of transmitting three frequencies,
> and the third assigned to the space function, one could compare the
> energies in each of the three bands, and choose the strongest as the
> most likely sent.  That would give most of your 3 dB advantage, and
> yet be compatible with current practice.
>

This function was available in the earliest versions of VFSKCW (more than a
year ago now) and is called the "toned space" option selectable from a menu.
The option of inserting toned spaces between characters or between words, or
both, was provided.

The only feedback I got on the usefulness of this was from a small group
using VFSKCW when "aircraft bouncing" on 2m.     They found it confusing,
but I suspect that that had a lot to do with trying to cope with the large
amounts of doppler shift on those signals.     Also, using one of your
earlier arguments against FDK/Wanjina, if the transmitter is dissipation
limited then it doesn't get a breather between characters and/or words and
so the power level would have to be lowered.   This would most likely negate
any visual decoding gain.

BTW, one of the intriguing things I find about trying to read things off the
screen is that the analysis can't end with theoretical calculations.     The
theory gets you the electronic S/N behaviour to the screen, but the final
word is determined by our powerful visual processor - the brain.   The
behaviour of that visual processor is probably more defined by fuzzy logic
and decisions made by the weighting of many clues and factors (including
what we want to see:-).    An amusing hearing effect happened to me when I
was involved in trying to pick out slow morse signals in noise over a
weekend.    On the Monday I put on my headphones at work to which I feed
some white-ish noise from a CD recording I had made to block out some of the
loud talkers over the partitions.   I had to forgo this masking technique
for that day as I swear I could hear morse code signals in the noise
(although I could never quite decode them).   This was more distracting than
the chatter.    The next day the effect had disappeared.   I realised that
some of the signals I thought were there but just couldn't quite decode
during my listening weekend were most likely these "phantoms".

I wonder how much "visual processing gain" the brain provides and could it
be trained (like experienced morse code users).     I note that Bill Ashlock
uses a sawtooth wave for some of his tests.    Theory says that you would be
giving away S/N by moving across an FFT bin during the period of the FFT
record, but how much does the visual processing of the brain on a
distinctive signal like that gain ?     BTW, I suggested to Bill that he
might like to try to have equal durations for dots and dashes, but slope one
way for the dots and the other way for the dashes.    He said he would try
it.

73s Steve Olney (VK2ZTO/AXSO - QF56IK : Lat -33 34 07, Long +150 44 40)
=============================================
HomePage URL:
http://www.qsl.net/vk2zto

Containing:-
ULF, ELF, VLF & LF Experimentation
MF 22m Experimentation
InfraSonic Experimentation
Laser Comms DX
Amateur Radio Astronomy
=============================================


> However, if you have the ability to send multiple frequencies, a much
> larger gain is obtained by using a different frequency for each letter
> or digit.  This gains at least 6 dB over the system above.  If the tones
> were spaced 0.1 Hz apart, then your existing 4 Hz screen could show an
> alphabet of up to 40 symbols.  The software could display corresponding
> letters and digits on a scale, but the operator would still be deciding
> which characters were sent.  It would also work with existing software,
> by looking up the received frequency in a character table.  This scheme
> has been described and tested by many hams, under names such as PUA43,
> Piccolo, AFK, and PGP-1.
>
> If the message length and start time are agreed upon in advance, then
> systems like PUA43 offer a huge advantage, because you know what part
> of the message is being sent at any time.  If you get partial copy,
> you know which character(s) were received, and you fill in a piece of
> the puzzle.  After several repeats, if all characters were received
> at least once, the puzzle is solved.  Such a system would also permit
> advanced software to combine the received energy from multiple message
> repeats, forming a composite with better S/N.
>
> The variable length nature of Morse thwarts the above idea, but perhaps
> a similar code using say, four elements per character could be used.
> This code would always start with a "." or "-", but could have a space
> as the second, third or fourth element.  Letters with 4 dots and/or
> dashes would be sent without any following space.  Letters with 3, would
> have a space at the end.  One or two dots would pad out shorter letters.
> There are 14 valid (per above rules) codes left over; these would be
> assigned to word space, digits, and 3 punctuation.  For example if
> "- .-" were assigned to "7", then your call would be sent as
> "--- -. .- .--.---.. " which is two elements shorter than standard Morse,
> but the main idea here is not to be faster.  It is that all characters
> take exactly 8 minutes to send; if you receive "-.--" 24 minutes after
> the start, you know that "Y" is the fourth character.
>
> One could also develop a code which is a compromise between Morse and
> PUA43.  For example, if seven frequencies were used, one could use two
> tones, sent sequentially, to represent one character.  That would be
> twice a fast as the above, more robust (no space), would fit in a 1 Hz
> bandwidth, and could easily be decoded by eye.
>
> Let me know if you think that any of these are promising.
>
> 73,
>
> Stewart KK7KA
>
>
>
>
>