Wednesday, August 18, 2010

Sound card packet modem

Last night I spent a couple of hours trying a program that had been mentioned by Chris, G4HYG in the Cross Country Wireless Yahoo group called AX25-SCS. It is a sound card modem for 300baud and 1200baud packet which was written by Frank Perkins, WB5IPM for use with a now unsupported APRS program called APRS-SCS (nothing to do with the SCS that makes TNC hardware.) Chris claimed it works better than AGW Packet Engine at decoding HF APRS. It is also freeware. I had never heard of this program before despite quite a bit of time spent searching for packet sound card software. You can get it from the TAPR website, installation is simply a matter of unzipping it into a folder, and there is a .pdf file that tells you almost all you need to know to run it.

One of the things the documentation doesn't tell you is that AX25-SCS uses the default Windows sound card. For testing, I had to make the sound card used by my Elecraft K3 the default. This is not convenient, as it robs my computer of the ability to play sound through the speakers (and risks broadcasting Windows sounds and web audio over the airwaves) so sadly I am unable to make permanent use of it.

The documentation also doesn't advise you to select "Cancel APRS-SCS support" when you first run it. If you don't, nothing will appear in the window until an APRS client connects to it, making you think that the program just isn't working.

For HF use you need to select 300baud from the menu, as 1200baud is the default. You also need to select Enable SCS TX to allow transmission. It would be a bit of an annoyance that you have to do this every time you start the program as the settings are not remembered.

Once the receiver is tuned in - AX25-SCS uses the KAM standard 1600 / 1800Hz tones for HF - and the sound levels adjusted so that packets are being decoded, you can close and then restart the program this time choosing "Enable APRS-SCS support" from the initial dialog. After that, start APRSIS32 and create a KISS TNC RF port at address 127.0.0.1 port 4000. When APRSIS32 connects, the display should appear on the AX25-SCS window as it did before and this time any packets received should be displayed in APRSIS32 as well. Beacons and messages sent from APRSIS32 should result in audio being generated by the sound card modem for transmission.

For transmit, APRS-SCS requires the transceiver to use VOX. This is not a problem with my K3 which supports VOX via the rear audio connections and which allows the delay to be adjusted down to a very short interval, but it would make the program unusable with other transceivers like the FT-817 which don't.

I tried running APRS-SCS and AGWPE Pro in parallel to see which was the better decoder but it was difficult to do a fair trial as APRS-SCS needs a lower level of audio input so the signal level was either too high for one program or too low for the other. As far as I could tell, APRS-SCS appeared more tolerant of off-frequency signals but less sensitive to weak ones than AGWPE Pro. This is also the verdict of Chris, G4HYG.

I would be willing to give APRS-SCS more of a try, but the inability to specify which sound card to use makes that impractical. It's a pity that APRS-SCS is not open source so that someone could fix that and the other minor niggles, because it is a nice program that is quite easy to set up and use with APRSIS32.
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