Recently there has been a discussion about AGWPE and 300bd AX.25 decoding.

I've run various programs for 300bd AX.25 sound card decoding over the years and
it may be of interest to some to hear my observations.

The problem with any HF ASPRS system is trying to get all stations on the same
frequency. In the recent versions of APRS Messenger I've had to compromise on
having no more than two modes so that we could have multiple receivers in
software for each mode to allow signals off frequency by +/- 100 Hz to be
decoded.

Any 300bd AX.25 receiver system working at HF really needs to be capable of
receiving stations off frequency by a similar amount.

In the tests I've run I've always compared the different sound card programs to
the KAM Plus TNC I use in my 30m AX.25 Igate. The receiver input to the sound
card programs has also come from the same receiver for a direct comparison.

The KAM Plus TNC is probably one of the best hardware TNC's for 300bd AX.25 but
it is limited to only receiving signals +/- 25 Hz from the centre frequency.

AGWPE (Pro version to get the 1600/1800 Hz tone set) also has a very narrow
receiver passband and signals more than 25 Hz off frequency will not be decoded.
The decode performance is not as good as the KAM Plus and I find that the AGWPE
set-up to be very critical. Packets can be sent to UI-View via a TCP/IP port but
the format is unique to AGWPE and UI-View. The Pro version with the 1600/1800 Hz
tone set costs $49 to register.

Another alternative is Mix-W written by UT2UZ and UU9JDR. This is a multi-mode
digimode program with 300bd AX-25. The performance is similar to AGWPE with a
very narrow frequency bandwidth (+/- 25 Hz) and it can also send packets to
UI-View over a TCP/IP port in KISS mode. The performance is very similar to
AGWPE and is not as good as the KAM Plus. It also needs registering price $50
for long term use.

So far both sound card programs have used very narrow filtering to restrict the
operating bandwidth.

Probably the best program I have used is TrueTTY written by Sergei Podstrigailo,
UA9OSV. This is a multi-mode digi-mode program that has a 300bd AX.25 option.
Packets can also be sent out to a TCP/IP port in KISS format. This program has a
very fast AFC function and can lock onto an incoming signal very quickly. The
default setting for the AFC is 3000 so that it will lock onto the strongest
signal in the audio bandwidth. If this is set to a value around 50 then the AFC
is kept in a range +/- 50 Hz from the centre frequency and no packets are lost.
The program is very sensitive and it's probably the only sound card program
that's better than the KAM Plus TNC. The only problem is that it isn't freeware
and needs registering for $39. It's free to test for a short time.The website
for TrueTTY is:
http://www.dxsoft.com/en/products/truetty/

The sound card program I'm using at present is AX.25-SCS01-DS-FD.exe written by
Frank Perkins, WB5IPM for use with the APRS-SCS APRS program. The author of the
APRS-SCS program, KB2SCS, recently decided not to support the program any more
but the sound card AX.25 program is still available from the TAPR website:
http://www.tapr.org/~kb2scs/scs01.html
This program uses similar DirectX techniques to my HF APRS SDR program based on
AC5OG's original SDR articles and looks for tones going either side of a centre
frequency of 1700 Hz. This means that stations up to 100 Hz off frequency will
still be decoded.
In practice this works well. It's not quite as good as the KAM Plus in
sensitivity for stations on frequency but it will decode many packets that the
KAM Plus can't. This program can either output packets in KISS mode to a TCP/IP
port or display them as text on a window. It also has a few simple displays of
decoder output and incoming signal. The program is the easiest to get running
with and works reliably for weeks.

If I was to rate the programs for performance it would be TrueTTY first,
AX-25-SCS01 second with AGWPE and Mix-W as third. If you want a freeware program
then it's AX-25-SCS01, no question.

While we are thinking about 300bd AX.25 another hardware option used by many is
the Tracker DSP TNC by SCS. This can receive AX.25 +/- 400 Hz from the centre
frequency and receive packets in parallel. It only costs $333! Their website is:
http://www.scs-ptc.com/shop/products/modems/tracker-dsp-tnc

73,

Chris, G4HYG