[aprssig] New VARA-HF Softmodem A Spectacular Improvement For HF APRS

Stephen H Smith WA8LMF2 at aol.com
Thu Jan 12 12:44:30 EST 2023

On 1/12/2023 11:29 AM, Jess Haas wrote:
> One of the cool things about software modems is that it is possible for 
> igates to receive multiple modes.  I had a 70cm digipeater/igate going for a 
> bit that could receive 9600, 1200 or 300 baud. The idea was that if you 
> couldn't get in but needed to send something you could drop down to a more 
> robust mode.
> -Jess
> On Thu, Jan 12, 2023, 5:15 PM Scott Howard <showard at nd.edu> wrote:
>     On Wed, Jan 11, 2023 at 3:40 PM Stephen H Smith via aprssig
>     <aprssig at lists.tapr.org> wrote:
>     > YES!   Here is the preliminary write-up on the planned test in February.
>     >
>     >     <http://wa8lmf.net/APRS_VARA_MFSK/>
>     Thanks, these are interesting results and I'm looking forward to
>     hearing how they go!
>     After VARA HF was released, fldigi added "OFDM" modes [1] which I
>     believe are based on the same underlying idea as VARA. If it is
>     possible, it could be interesting to see how the fldigi OFDM modes
>     (e.g., OFDM500F) compare to VARA HF as well since fldigi is common and
>     open-source.

I hadn't even noticed that recent releases of FLdigi did this new mode (OFDM).  
I just tried switching my test setup written up in the link above to OFDM500, 
and it seems to work just fine.  It's noticeably faster, taking 16 seconds 
including the RSID. MFSK16 took 27.5 seconds to send the same compressed posit 
and comment.    But does OFDM tolerate weak noisy signals with multipath as well???

You now have me curious.  I'm going to have to gin up ANOTHER test run running 
MFSK vs OFDM.  Setup would be similar to my described setup, but with TWO 
copies of FLdigi running from separate folders instead of FLdigi vs VARA.

I am running my 60M HF igate with a 500-600 Hz IF filter bandwidth (Yaesu 
FT-891 which does have a DSP continuously variable IF), so the OFDM-500 mode is 
the only OFDM mode that would work. I have already validated that the TX RSID 
works consistently to switch between MFSK16 and PSK63.  I could probably depend 
on it to switch to OFDM500 as well.  I.e. igate has automatic  multi-mode 
receive capability.

However, I would want to use two copies of UIview and two copies of FLdigi to 
do this comparison test, so that I can used different SSIDs on the two modes -- 
beaconing alternately so that I can map the received posits separately on 
APRS.fi or findu.com.

I drive up to Traverse City semi-regularly for the Cherryland ARC meetings, so 
I could repeat the test run again next month with the dual FLdigi setup.

Stephen H. Smith    wa8lmf (at) aol.com
Skype:        WA8LMF
EchoLink:  Node #  14400  [Think bottom of the 2-meter band]
Home Page: http://wa8lmf.net

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