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The 16kHz transmission is MSK (Minimum Shift
keying). An efficient, constant amplitude, data modulation
format. There was quite a bit of discussion about the mode on
this reflector a few months ago, and I remember stating that the 16kHz
transmission was one of the most "perfect" examples of MSK I have ever
seen off air !
The 'minimum' of MSK refers to the fact that the frequency
shift is the absolute lowest that can be used for FSK modulation, being
exactly half of the data rate. MSK is a mode that must to be
received coherently, like PSK, to take any advantage of its good signalling
properties. The constant amplitude nature would make MSK the ideal
mode for our use on 137k were it not for the difficulty of demodulating
properly. I believe Bill de Carle, VE2IQ has been working on an MSK
implementation for some time now, but a good system for the mode is still some
way off.
Low cost and inefficient, non-coherent, implementations are
widely used at higher frequencies - a quick scan across the VHF spectrum reveals
many MSK transmissions carrying telemetry, data, paging, and all sorts of
signals. A crude demodulation can be done simply using a PLL as an FM
discriminator, but that gives a result no different from narrow shift FSK.
An off the shelf chipset, the CM589, makes for a low cost simple MSK data
modem, but the implementation is geared towards high S/N paths and the chip will
not suit our weak signal purposes. (I've already looked at it
)
Andy G4JNT
Hi all,
During some RX experiments in the VLF spectrum a few minutes ago,
using a
bundle of ferrite rods and lot of wire as antenna. There was..
- a quite strong signal on 16kHz, 100 Hz wide;
- an other signal
(stronger) on 18.3kHz, more than 200 Hz wide;
- the Russian Navy(?) on
18.1kHz, about 100 Hz wide with a typical FSK
pattern (transmitter
switched off after the transmission).
An old list says on 16.0kHz is
"Rugby RTTY 75", another says its "RTTY 200".
But the signal on 16kHz
looks more like a well-shaped PSK signal to me. Or is
it just a very
stable QRM in my neighbourhood..
Any information available on this
reflector ? If the signal on 16.0kHz is
really PSK, where will the planned
CW transmission be on Tuesday afternoon ?
Thanks in advance for any
info,
73's from Wolf DL4YHF.
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