Now I know a few copies of PSK08 are out there I'll transmit this
evening around 2000z. Frequency will be 137.500 +/- 0.5 Hz, chosen
after careful consulation with this group :-|) Probably won't
listen too much as no one has replied to say they can transmit yet and I
know that three of those who have downloaded the code are listeners
only.
There is another modulation scheme suitable for this band that does
allow constant amplitude, and hence highly efficient class D amplifiers.
That is Continuous Phase Modulation (CPM), of which Minimum Shift Keying
is an extreme example. Listen to the transmission from Rugby on 16kHz
or 22.1 kHz for an excellent example of a 100 Baud version of MSK.
CPM is a very processor and memory intensive mode hence it is only now
becoming popular, but at the speeds used at LF my feeling is that a PC
could cope even if a 56002 EVM does not have the necessary memory
capacity. CPM is theoretically a few dB better than PSK in S/N terms,
and in fact with the right coding is said to be able to get to within
0.7db of the Shannon limit. Take this together with the constant
envelope waveform and it could prove very exciting for very weak signal
LF dxing.
If anyone reading this has a more intimate knowledge of CPM please get
in touch. As far as I 'm concerned its all squiggles in a text book at
the moment, and as for turning these into DSP code....
Andy G4JNT
Now ignore what follows :-|(
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