Neil asked me to describe the interface I use for digi modes between my sound card and my radio.  The following article and figures are the result.  Perhaps this will inspire someone to build their own interface, so I encourage you to steal ideas from my experience.  I’m too busy or too lazy to spend the time to write a good technical article, so I hope this is at least interesting and helpful.  This is a pretty easy project and doesn’t need much explanation.

It has been well over a year since I became interested in digi modes, and it started because Neil had been talking about some of the modes he was trying out.  At the time I wanted to build something, a kit preferably, and the number of QRP radios in my shack was soaring.  QST ran an article about the VolksRTTY 2, so for a change from QRP rigs and CW, I ordered the partial kit.  It was a good kit, and it got me on RTTY pretty quick.

After playing happily with RTTY for about 2 weeks, I wanted to try to get some of the other digi modes.  I think the expression digi mode means a mode where you use a PC with your radio somehow, but it doesn’t necessarily mean the modulation is strictly digital.  Anyway, downloading the software was easy, but I soon found out that I couldn’t use the VolksRTTY 2 (VKR2) interface for other modes.  At this point, it was clear that I would need to build an interface between my sound card and my radio.

In the course of trying to find a mode that would work with the VKR2, I learned enough about the various modes to know what I wanted from the new interface.  There had been an article in QST about an interface that would do most of what I wanted, so I dug it out and studied it.  (“PTTSound,” Salvador Esteban, EB3NC.  QST, Nov. 1999, pp. 60-61.)  I stole what I wanted from the design and came up with the schematic you see here.  You can tell that this is my working schematic.

All the parts are labeled on the schematic, except for some of the jacks.  The interface was built in a Radio Shack plastic box (see picture).  Most of the components were mounted on a small Radio Shack perf board, and the rest are on the controls and connectors.  The wire colors were added to the schematic as I put things together, so it could be more easily repaired when it was finished.  It helps me to take the attitude that whatever I build won’t work to begin with.  It makes me more careful and causes me to keep some useful notes on the construction.  Some of those notes appear on this schematic.

The cable drawing not pretty, but you can see there are plenty of wires to mess with.  Note that all the audio lines use shielded cable, even though I didn’t draw it in every case.  Have a look at the picture and you can see that one of the primary layout considerations for the box was where to locate connectors to minimize the rat’s nest.Cables

The picture shows part of my radio shack.  The prominent items are my DSW-80, NorCal 40A, Bencher Paddle (parked on top of the DSW), and my digi interface and desk mic.  The interface and desk mic are both built in Radio Shack plastic boxes with metal tops, and the mic plugs into the interface, which in turn connects to the mic jack on my IC-735.

Enough rambling.  Here are the important interface features:
Level control in both the LINE IN and LINE OUT circuits
DC isolation
A battery to pull the PTT line up and drive a XMIT indicator
A COM port interface for the keyline (better than using VOX)
A switch to go from computer output to mic output

Some particulars:
A single DIN jack is for signals to/from the IC-735
The DIN jack was used and wired to match the cable I had made to go with the VKR2
The switch has a center off position (receive only) that is VERY useful
The level control in the LINE OUT circuit is a trimmer on the perf board (set and forget)
The level control in the LINE IN circuit, or receive line, is a front panel control

If you have tried digi modes using soundcard software, you know that it is a pain to have to use Volume Control to adjust audio levels.  What works for one mode is not necessarily right for another.  Furthermore, the settings your radio likes may make your computer audio too loud or soft for listening.  In general, it is nice to be able to leave most controls alone as you switch from mode to mode and from computing to hamming.  This interface takes care of that pretty well.

If you build this or a similar interface, adjusting it the first time takes a little experimentation.  Here is my approach:

1) Start by making sure the interface works with the mic.  If it does, set the mic level on your rig.

2) Next put the interface in receive-only mode (center/OFF position on my front panel) and start up some software like Digipan.  Go through the process of setting the Volume Control on the computer, the audio level on your rig, and the level control (LINE IN) on the interface properly for PSK31 receive.  Try to get the rig audio level and computer Volume Control set properly with the LINE IN control near midrange.  If you do this, the LINE IN control will usually be all you need to tweak as you go from mode to mode, unless you change the rig audio level.  You have 3 requirements: rig audio has to be right for listening, sound level has to be right for the software/soundcard, and there has to still be adjustment available.  With 3 controls, you can achieve all 3.

3) Finally, put the interface in transmit-enabled mode (ON setting on my panel).  Adjust the trimmer for LINE OUT in the interface with the computer Volume Control at a decent listening level.  Don’t change the mic gain control on the rig, because you already set it for the mic, and you don’t want to be messing with it when you change back and for from SSTV to SSB or something.

Now is the time for the caveats.  Your mileage may vary and all of that.  If you have RFI or a ground loop, you may need to add isolation transformers or opto-isolation.  If you have amplified speakers on your computer (or a separate line out jack on your soundcard), you are lucky because you have separate way to control listening volume.  If you have a receive audio output separate from your speaker output on your rig you can have separate control of listening level there.  If you don’t want to mess with this, you can buy a Rig Blaster (TM) and be a very happy camper.  I can still think of things to say here, but this is long enough.  Email questions to any of the guys in the GAQHBC (pronounced like a dry heave sound), and they will be glad to help.

Soapbox:  My favorite digi mode is Hellschreiber.  It beats PSK31, AKA the PeSKy mode, hands down.  Your signal has some personality, and you can see the other guy’s signal.  No more losing somebody as though they fell of the edge of the world.  If the band goes bad or QRM is a problem, you can see what is happening and react before the QSO goes south.  Stream is a very robust mode if you have to look at pretty text.  I’m still trying other things, but so far the best advice I can give you is ... go to Hell.  72 Rob KK4R