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I want to setup a TCP/IP link between my friends house 7km away from me on 70cm.

1200 Baud is a bit slow, and we are looking into something with a faster data rate.

I know normal audio does not work for 9600 baud packet. What are the requirements?

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.

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  • $\begingroup$ You might also want to look into D-STAR digital data (DD), it's probably easier to set up and more reliable than 9600 AFSK. Since the proprietary AMBE codec isn't used for data-only D-STAR, the usual concerns about the openness of the protocol don't apply. $\endgroup$
    – Phil Frost - W8II
    Jul 27 '15 at 1:26
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In fact, it appears 9600 baud will work on narrowband FM. Here is a description from Amsat of a 9600 baud packet modem which used a bandwidth of 4800 Hz and its board.

According to the Amsat article, the design is used in many devices including:

  • PacComm Inc: NB-96
  • Kantronics: DE-9600
  • MFJ: MFJ-9600
  • Tasco: TMB-965
  • Symek: TNC2-H

(These were from 1988 when the article was written; here are some slightly more modern instructions.)

Packet is AX.25, which Linux routes natively. Here's an AX.25 howto.

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In addition to what user3486184 said and the direct discriminator on the back, you'll need direct access to the modulator too. A good way to test whether it all works is to fire up a sound card packet program. Once you can make reliable connections with that (that's your Network Layer 2) you can move up the stack to TCP/IP.

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  • $\begingroup$ Does "Flat" audio input count as a good modulator input? $\endgroup$
    – Skyler 440
    Jul 24 '15 at 21:48
  • $\begingroup$ Probably not because there's still roll-off (i.e. band pass filtering of the audio) $\endgroup$
    – Duston
    Jul 29 '15 at 23:17
  • $\begingroup$ As far as I what I understand from what I've read, the problem is not only filtering of the audio, but the FM preemphasis. You mention the direct discriminator which bypasses this, but I thought it's worth pointing out precisely why it matters for those who don't know. $\endgroup$
    – Phil Frost - W8II
    Aug 5 '16 at 11:57
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The following solution has been tried many times and is simple to do. No special connections and very little effort. This has been done by replacing the original antenna from a wireless modem and the reported distances can be modest or huge. Many have used cheap but effective home built antennas or wave guides with horns. Coffee cans and potato chip cans have been used effectively. Some claim distances of more than 20-40 Km and one claimed they would get much more distance, but only with super careful attention to many details. If you have the space a small dish can even be used. You can make the dish too. Be sure to look at the rules where you live. There is likely a legal limit to the ERP (effective radiated power).

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  • $\begingroup$ This does not answer the question asked (which is about 9600 baud data over radio) $\endgroup$
    – Scott Earle ♦
    Jul 22 '15 at 7:52
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    $\begingroup$ This DOES answer the question whick is about a TCP/IP link! "I want to setup a TCP/IP link between my friends house 7km away from me on 70cm." $\endgroup$
    – Keith Martineau
    Jul 22 '15 at 8:02
  • $\begingroup$ You're talking about antennas, when he wants to know how to connect the radio to a TNC without using the audio connections. Given that he didn't mention the radio portion of the link, I am guessing that he has that covered already. $\endgroup$
    – Scott Earle ♦
    Jul 22 '15 at 8:05
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    $\begingroup$ This Nit-picking is why I don't post "Answers". It is destroying the forum. $\endgroup$
    – Optionparty
    Jul 22 '15 at 16:20
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    $\begingroup$ The issue is "70 cm". This answer applies to wifi or bluetooth DXing, which is neat stuff but it's at 2.4 GHz, not 70 cm. It's not nit picking to point out how an answer does not fit the question. On the other hand, suggesting that wifi with coffee cans would also solve the communications problem could be helpful -- but the response should be clear about the frequencies involved. $\endgroup$
    – Martin Ewing AA6E
    Jul 24 '15 at 15:05
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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...

http://www.tapr.org/pipermail/aprssig/2015-April/044345.html

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 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.

http://www.p4dragon.com/en/Modems.html#widget4

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...

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 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 requirements?

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.

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