I've been looking at an coherent MSK modem for Codec2. I have not done
any coherent MSK modems since the late 90s.
Using an SSB radio , at something like 1600 bps , there will still be
appreciable PAPR on MSK due to bandwidth constraint. probably about 3dB
PAPR on a 2400Hz wide radio. When bandwidth is constrained, the
constant envelope signal is no longer constant envelope...
The is quite obvious using MSK441, the meteor scatter mode in WSJTX. It
operates at 2000 bps, and struggles a bit on bandwidth with most non-SDR
types . PAPR ends up around 4.5dB . Also the phase response of non flat
radios is usually aweful. A channel equaliser turned on to model and fix
your radio
Now...
OFDM doesnt need a specific radio equaliser, that's built in of course,
the uglyness of most radios can be obscured. .
And if the only complication from the existing OFDM modem of David's is
the need to constrain PAPR, I think that perhaps constraining the PAPR
of David's existing modem , rather than a MSK modem from almost scratch
, might be more overall beneficial to everyone (HF as well) . I am
thinking an incompatible change to the existing modem to reduce PAPR to
something like 4.5dB might be the best of both worlds. yeah it might
cost a 0.5dB but oh well, you probably lose that now with PAPR
constraints. The existing 2FSK or 4FSK modem should also be brought up
to working.
David, what are your candid thoughts ?
glen.
On 6/23/2020 6:16 AM, David Rowe wrote:
Hi Glen,
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