In order to use digital modes on the air, you need two major components. One is an SSB-capable radio. The other is a Windows-equipped computer, either desktop or laptop, with a sound card. (Actually, it is possible to use a Linux box or a Mac, but the range of software available for these is much more limited.) In addition, you need some cabling to wire the two together. The same cable arrangement will work for all of the digital modes and software described on these pages.

To receive, all you need is an audio cable from your receiver to the sound card's line input. If your sound card doesn't have a line input, you may have to use its microphone input instead. The input connector on the sound card will probably be a miniature (1/8") stereo phone plug. You can either use a stereo patch cable with a stereo plug, or a mono cable with a mono plug and a mono-to-stereo adapter (available from Radio Shack). At the radio end, you will want to connect to the radio's headphone or external speaker output, which is probably a mono connector. You may need an adapter if the jack on the radio is not a miniature phone jack, and if the patch cable is a stereo cable, you may also need to use a stereo-to-mono adapter (or you can cut the original plug off the cable and wire your own mono plug). I find it convenient to be able to listen for the signals, and not just rely on the software's display, so I use a Y adapter and plug the computer into one side and a set of headphones into the other.

The next step in receiving is to adjust the sound card input levels. Start up the "Volume Control" Windows program (Programs/Accessories/Entertainment in the Start menu on my computer). Under Options/Properties, select "Adjust volume for Recording". Set it up to display all of the available volume controls. Below each volume control is a box marked "Select". Put a check mark in the box corresponding to the Line-In (or Microphone, if that's the input you used on the sound card), and remove all the other check marks. Set the corresponding volume control to 30-40% up, and ditto for the master recording volume control. You can adjust these later to fine-tune the receive volume, but that should get you started.

To transmit, you will need a similar cable arrangement from the sound card's line output to the radio's audio input. If the sound card doesn't have a line output, you may have to use the speaker output. At the radio end, your radio may have a jack on the rear panel for this purpose, or you may have to connect your cable to a microphone connector and use the front-panel microphone jack.

In order to avoid overdriven, broad signals in digital modes, adjustment of the audio level controls is important. Start with the "Volume Control" program again, select Options/Properties/Playback, and display all of the available controls. Check the "Mute" check box under all of them except for the "Wave" control and the master volume control. Both of these controls should be kept below the 75% setting to avoid overdriving the audio in the sound card. If you have a mic gain control on the radio, you may be able to use it for fine adjustments; again, it should be set fairly low in its range. You will be adjusting these controls again during transmit setup, so you can leave the Volume Control window open on your screen.

Now turn off any processing or compression in the radio, and set your radio's meter to display ALC. Using either a dummy load or your antenna on a band that is not open for the moment, you are ready to start transmitting. Start out with the computer's volume control all the way off, put your software into transmit (for example, by pressing a CQ macro button), press the manual transmit button on the radio, and slowly increase the sound card volume setting. You should respect any power limits on your transmitter; many amateur transmitters are not rated to deliver as much output in "key-down" modes like digital modes as they are in intermittent modes like SSB and CW. Failure to respect this limit can destroy your transmitter's final amplifier stage. You also need to make sure that there is no ALC action, i.e. you can increase the gain until the ALC meter just starts to twitch, then back off the gain so there is no ALC indicated. Failure to do this, or use of compression or audio processing, can result in broad signals which will not win you any friends on the air.

If your sound card's output levels are high, if you are using your radio's microphone input and/or if you have to use the sound card's speaker output, you may find it difficult or impossible to keep the audio level low enough while still having the ability to do fine adjustments. In other words, you may have to set the sound card's volume control too low. In this case, you may need to wire a simple potentiometer or two-resistor attenuator in the transmit audio line. The help files for the software should show how to do this, or there are several Internet sites that describe interfacing in more detail (see below). If you use such an attenuator, you should be careful not to insert so much attenuation that you are forced to turn either the sound card's or the radio's level controls up too high - doing this could introduce IMD, the very thing that the attenuator is intended to prevent.

Another problem you may face is RFI and/or ground loops. Simple audio isolation transformers in the audio lines, possibly supplemented with ferrite core snap-on chokes, may help in these cases.

To control the radio's transmit/receive switching, there are several options: manual switching; VOX (may not work on all radios, depending on which audio input on the radio you are using); computer control (if your radio is wired to the computer for direct computer control, and again possibly depending on which audio input on the radio you are using - this method has the added benefit of allowing your software to read out band and frequency information and put it into your log automatically); a hard-wired interface to a serial port on the computer; and finally, an external circuit that detects the audio signal from your sound card and triggers your radio's PTT (in other words, an external VOX circuit).

If all of this seems too complicated, there are several commercially-available interface boxes. The features available include isolation, T/R switching, switching between your microphone (for SSB) and sound card (for digital modes), CW keying, and even FSK keying in a couple of cases. I have not tried any of these, but I have found the following links:

- Donner's Digital Interface
- the G3VFP OPTO-IF Interface
- the RASCAL interface
- the RIGblaster
- the SignaLink interface

If you prefer to roll your own, W5BBR and WM2U have Web pages devoted to the subject. KK7UQ also has a Web page with detailed information on the design, adjustment and operation of interfaces, and supplies printed circuit boards for a compact full-featured interface. This site is well worth visiting, even if you already have a working setup. Finally, for those who are interested in the external VOX switching solution, WA8LMF and G3VFP both present circuit diagrams so you can make your own (or you can investigate the SignaLink unit, which has a built-in VOX circuit).

If all this worked, you are ready to go on the air. You can use signal reports from other amateurs to help you make any final transmit audio level adjustments, or you can install a level-control monitoring system by KF6VSG called the PSKmeter. This is an excellent way to avoid overdriven PSK31 signals.

Have fun on digital modes!