Packet Radio with a Soundcard
by Charles Brabham N5PVL


It's pretty easy to get an Amateur Packet Radio station going if you have a radio, and a PC with a soundcard. Other than these, all it takes is some free software, a transistor, a resistor, and a diode...

Here's the three-component Serial port PTT interface which allows use of a 16-bit Soundblaster ( or clone ) as an advanced TNC with PC-FlexNet, and the "SM" soundcard driver package by Thomas Sailer HB9JNX. The package contains both 1.2kb and 9.6kb versions. The 9.6kb driver naturally requires more from your computer.

Serial Port PTT schematic.

A Transistor, a Resistor and a Diode...


It cost me just over three dollars to buy the parts, and ten minutes to build the PTT circuit shown above, which allows FlexNet to switch the radio from recieve to transmit mode as needed. FlexNet uses the soundcard driver to do the rest, processing audio signals to and from the radio.

The performance was impressive. I have used a variety of TNC's over the years, ( DRSI, AEA, MFJ, PAC-COM, TAPR, ect. ) and the soundcard did just as good a job as any of them. Actually it did better than any of them, but that was more due to to the software (FlexNet) than the hardware. None of those other TNC's had FlexNet's adaptive TNC parameter system going for them, so naturally there was no way they could perform quite as well.

For those who do not have a free Com port, there are also PTT ( Push-To-Talk ) interface circuits for the parallel port, game port, and even a VOX circuit which uses microphone audio to trigger PTT. Though none of these are expensive or complicated to build, the Serial Port interface is the simplest and least expensive of them all, and is ideal as a way for a new ham to "get his feet wet" with an easy but very useful home-brew project.

If you simply wish to monitor a packet frequency without transmitting, the PTT circuit would of course be unnecessary. Simply plug in your scanner or radio's external speaker line to the MIC connector on the soundcard, and use the same free software described above.

For detailed software/hardware setup info for the soundcard packet modem, see:

Using PC Soundcards as Packet Radio Modems
DOS and Win95 setup info, Schematics for PTT circuits using a Com port, Parallel Port, Game port and even a VOX circuit.

Also, see Thomas Sailer's Web Page, listed below. Look for the section entitled "PC Soundcard Modem Drivers".

PC/FlexNet drivers and applications by Thomas Sailer, HB9JNX
SoundCard drivers, and many more.

Please send comments, suggestions to:
Charles Brabham N5PVL



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