Peter has asked me for a write up for the LF Book, so full details will
appear there. But basically it was a bit of fun to show how something can
be done with a DDS in real time other than just using it as a simple
frequency source.
The mode is more commonly known as Hellschreiber, or at least one of its
varients, and is available for Soundcard and EVM with an SSB transmitter
(with transverter for LF). However, that is a wide band system using up to
1kHz bandwidth. What I did was take the technique a stage further and use
a DDS to generate a narrow band version for use with Spectran. It can go
narrower, in fact today I may try a 5 Hz version. DSP derived versions can
transmit the tones in parallel, leading to proper vertical lines, but need
linear transmitters. This one -at-a-time frequency version is called
Sequential Multi Tone Hellscheiber or SMT Hell. Hellschreiber comes
within the term "Fuzzy Modes" like SLOWCW etc where there is a crossover
between data and human interpretation.
See how it compares with medium speed CW - whatever would be transmitted in
10 Hz bandwidth.
I'll change to a 5 Hz version around Midday today
Incidently, to save tying up a useful computer just generating a message
repeatedly, I've pressed an ancient 10MHz 286 Laptop into service. Who
needs Pentium 400s for Data comms ?
Andy G4JNT
-----Original Message-----
From: john sexton <[email protected]>
To: [email protected] <[email protected]>
Date: 09 June 2000 23:12
Subject: LF: Re: Mystery
Hi Peter,
The secret is to be patient. My first reaction was there was nothing there,
but after waiting a whole page, all was revealed. It reminds me of sky
writing.
I hope that Andy is going to explain to us all, what it is all about, i.e.
is it just fun or is there a serious purpose? Nothing wrong with fun by the
way.
It is interesting how it varies in intensity with time of day. At 17.00
today, it was very clear, but by 22.30 had faded considerably.
John, G4CNN
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