Hi Oliver,
It's simply a matter of getting the most data in the least bandwidth. I think
there's room for experimentation with GMSK on HF, and FreeDV will be getting a
GMSK modem that is intended for VHF/UHF, so we can try it then.
Our main problem on HF is very clearly fading. I have a theory that circular
polarization of the receive antenna could help, but can't say when I would get
time to test it.
Thanks
Bruce
Oliver Johns <[email protected]> wrote:
>Dear group,
>
>I have used the search tools in the list archives to try to find the
>discussion about choice of modem. I wonder why FreeDV is using a modem
>with quadrature signals rather than a pure FSK system similar to RTTY
>and MFSK or Thor.
>
>(1) With a pure FSK system (one carrier on continuously, varying
>frequency) I think one can use class C amplifiers. This would be a
>selling point for DV. The need for amplifier linearity is a product of
>the SSB era, but it need not be part of the DV era.
>
>(2) Ionospheric multi-path is notoriously hard on PSK systems since the
>phase gets scrambled more severely than does the amplitude or the
>frequency. This is why PSK31 communication often has a high error rate
>in spite of seemingly good conditions.
>
>Could someone point me to the white paper or email thread that treats
>these issues?
>
>Thanks,
>
>Oliver
>W6ODJ
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