Hi David
The goal is to improve FreeDV so that it is at least equal with SSB on a
flat VHF channel.
On HF, the performance of FreeDV is better than SSB mainly because it
can both a) work at low SNR and b) can mitigate the effects of a
frequency selective channel (which reduces speech intelligibility) .
At the moment, FreeDV is a bit behind on a flat channel. This is because
the PAPR is costing some ERP. PAPR on SSB on VHF with a good RF speech
processor is about 4.5dB (or less!). If the ofdm modem is pushed, the
performance doesnt improve of course because the BER starts to suffer
(on a non zero BER scenario IE marginal conditions.
I thought about working on getting the PAPR down on the FreeDV OFDM
modem. The last day I have done some revision and 8dB might be doable,
but 6 to 7dB will only be doable with computational intensity, which the
SM1000 does not have. That still leaves us a few dB in PAPR behind flat
channel SSB. Flat channel SSB is OK down to about -3dB SNR for a good
operator . Actually it is really hard to quantity exactly what the SNR
is because of the time varying power. (IE -3dB wrt to PEP is the usual
statement).
So now I think probably back to making the MSK modem work. I will also
put in a patch for a band limited clipper for the OFDM modem as soon as
I get the build environment... building.
cheers
glen
On 6/07/2020 3:52 pm, David Rowe wrote:
Hi Glen
Happy to comment, but there's quite a lot of material in your post below
and I'm unclear on what you are trying to achieve.
Can you pls tell me what the goal of your proposed work is?
Cheers,
David
On 5/7/20 6:43 pm, glen english wrote:
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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