Re: Trying to join the hamradio team
Hi David,
On 09.11.21 at 17:01 wrote David Ranch:
...
> It's been a long time since I've used Thomas Sailer's soundmodem and I
> don't remember if it supported such a configuration but I would be
> surprised if it did.
So let me surprise you: Yes it does.
> Even if it does, it's decode performance is so low
> (especially on 9600bps FSK), why would it really matter?
I know all this and I already started working on the "problem". As you
know for sure, a straight implementation of FSK over FM requires that
the audio channel over the radio is capable of transmitting very low
frequencies, ideally down to only a few hertz. You are lucky if you get
down to 50-100Hz but it can get as worse as 300Hz. The direwolf approach
is a clever one by running a couple of modems in parallel, each with a
decision level which is slightly offset to every other. After a packet
decode all CRCs of all modems are checked and the good ones are taken.
Effectively the approach converts the CRC into a kind of FEC and the
decode rate gets substantially better.
My approach goes beyond the multiple levels by restoring (or equalizing)
the baseline (for a single modem). Consequently I term the algorithm
"baseline restoration algorithm" (BLR). You can think of the BLR as of
an equalizer which makes use of the fact that only two signal levels of
opposite polarity are transmitted. The algorithm, however, is not one of
the usual equalization algorithms but something I have invented after a
discussion with OM Chris OE5DXL. I implemented an initial prove of the
algorithm in GnuRadio and then in soundmodem with encouraging first
results. Of course it will require more testing before I go to the
larger public. Just to wet the appetite: 9600Bd works well over a radio
with a 300Hz lower usable frequency. I found the code base of soundmodem
very easy to work with and since it is unlikely that I will stamp on
someones else feet because it is not used much, I decided to go with it.
> Btw.. if you're looking for alternatives to Direwolf that are actively
> maintained and have good to excellent decode rates, you might want to
> check out John Weisman's soundmodem which is based on Andy Kopanchuk
> UZ7HO's Windows soundmodem:
This might be a good modem, but using a modem on the wine emulator under
linux definitively is not something I want to do. So, thank you, but no.
vy 73 de Roland oe1rsa
Reply to: