There are two exciting developments in the world of soft modems in play currently: One expands on what can be squeezed out of the normal audio path in an FM transceiver while the other pushes the standard 9600 speed to 19,200 and beyond. The first reminds me of the remarkable advances made in telephone modem speeds eventually exceeding 30k. Both lean heavily on sound card specifications and, of course, the transceiver’s bandwidth and signal path quality.

Soft modem through audio channel – up to 4800 bps

Without going into too many details have a look at this post concerning the UZ7HO soft modem sound-card software TNC…

http://uz7.ho.ua/packetradio.htm

To quote:

With these latest versions, the Soundmodem can now transmit and receive AX.25 packet on: Classic 300, 600 or 1200 Hz baud FSK; 300, 600, 1200 and 2400 baud BPSK; 2400, 3600 and 4800 baud QPSK.

All of this fits into the bandwidth of a normal voice channel of a transceiver. Obviously the required quality of the audio link has to be higher for the QPSK modes, but at least now we have (or soon will have) the option to try.

Soft modem direct to discriminator – 9600 bps and up

Traditional high speed modes require direct connection to the modulator and discriminator as most of us know. The G3RUH modem design is the default approach for both 9600 and 19200. There is one TNC on the market, the SCS Tracker TNC, that, among other amazing feats, can handle both G3RUH speeds.

What this tells us is there is an off the shelf “hard” modem ready for you to connect to your FM transceiver’s discriminator port and likely get at least 9600 working between you and your friends and perhaps 19200 if the radio and signal path will support it.

The exciting news is the soft modem developers are examining this in the sound card TNC regime as well. The Direwolf soft modem software offers such hope in this document in their bleeding edge development area…

https://github.com/wb2osz/direwolf/blob/dev/doc/Going-beyond-9600-baud.pdf

This is sort of vaporware at the moment, but highlights the required and rather stringent specifications for your sound card device.

The active and exciting development of the sound card soft-TNC-modems will help us continue to raise the throughput of our AX.25 connections as new releases arrive.

About John Huggins

John is an electrical engineer working in astronomy and aerospace including antenna/RF design spanning 36 years with experience in antenna design including improvements (US20170201002A1 & USD798847S1) to the J antenna.

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