To COS or not to COS - that is the question...

I took GB7MH off-air most of yesterday and today as I tested some new code designed to help with reception of weak GMSK signals. Whilst it was not a total success, I have learnt quite a bit from the testing which I’ve now been able to replicate at home on the test system.


The problem I’m trying to solve is that reception of GMSK should be able to go to significantly noisier signals than it’s FM equivalent. However, at the moment, GB7MH is running on an FM receiver and uses the squelch to detect a “valid” carrier. This makes no sense for GMSK – I should just check for a readable GMSK waveform and decode. But at the moment, I’m gated by the FM squelch.


The effect is that rather than descend into higher bit-error rates for weak signals, the reception seems to cut out from perfect to nothing. The answer is to leave the squelch wide-open and just detect the GMSK modulation on it’s own merit. Then progressively weaker signals should just degrade to a pre-defined S/N point, where I cut-out.


So, I wound the squelch wide open – and caused all sorts of problems, because noise, especially the kind we had with lots of rain yesterday “looks” like GMSK. The effect was long-lived reception of rubbish. I tried several schemes to work around this (counting valid re-syncs in the GMSK, looking for header/end-of-transmission markers), but none really worked, and a sudden spike of noise kicked off a local transmission using the callsign of the last valid station heard. All very confusing.


The solution lies in not using COS, but using a S/N value from the GMSK demodulator. But this isn’t built into the v1 Satoshi board, and I just can't get the v2 board to play nicely.


So, I'm going to build the G4SZM "S/N to COR" circuit and use this to drive the COS pin. Then I can wind the squelch wide-open and just rely on true GMSK traffic.


That's the theory......


David