1200 Baud is a bit slow, and we are looking into something with a
faster data rate.
Exciting times in 'softmodem' development
There are two exciting developments 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. Both lean heavily on sound card specifications and, of course, the transceiver's bandwidth and signal path quality.
Regular audio channel - up to 4800 bps
Without going into too many details have a look at this post concerning the UZ7HO Soundmodem soundcard software TNC...
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 which 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.
Direct to discriminator - 9600 bps and beyond
Traditional high speed modes require direct connection to the modulator and discriminator as you 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 modem ready for you to connect to your SM50 discriminator port and likely get at least 9600 working between you and your friend and perhaps 19200 if the radio and signal path will support it.
The more interesting news is the soft-modem developers are examining this in the sound card TNC regime as well. The Direwolf soundmodem software offers such hope in this document in their bleeding edge development area...
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 may help you and your friend continue to up the throughput of your connection as new releases arrive.
I know normal audio does not work for 9600 baud packet. What are the
The requirements include:
- Direct-to-discriminator/modulator connections as the voltage from the sound card or hardware TNC directly translates to RF frequency and vice versa - aka direct FM.
- Sufficient channel bandwidth, from close to DC to whatever it takes to adequately house the waveform. For an excellent demo and explanation of digitization go here.
- Rock solid transceiver frequency stability... much more so than required for normal voice FM communication. Frequency drift and phase noise have a direct impact on your digital waveform.
- TX/RX circuitry that can take the harsh beating given to them by the constant back and forth of a typical data link.
I have a bunch of Motorola Radius SM50 UHF wideband commercial radio's
and I am wondering if they would work? They have direct discriminator
out on the back.
I am cautiously optimistic these may work.