Sound-card interfaces … home brew one!

After talking with a recent course graduate about operating using digital modes, etc., the topic of radio-to-computer (sound card) interfaces came up. For those who are unaware, a sound-card interface allows one to safely connect their radio’s audio input and output (and push-to-talk) to their computer, opening up the world of digital modes (such as PSK31, JT65, RTTY, SSTV, and many more).

After a brief discussion of the reason for isolating your computer from your radio (esp. true for desktop computers tethered to base radios) I explained a simple transformer-isolated circuit. It is true that capacitors can be used to DC isolate components, however, I firmly believe that using a transformer is a better practice.

Sound card interface schematic (simplified)

During our conversation, I drew up an interface which I have very successfully used with my KG-UVD1P and FT-857D radios. The above schematic is what I currently use and can be built for roughly $20 (plus another 15 for a USB-to-serial converter).

The basic concept is that the two (600Ω:600Ω audio) transformers DC isolate the radio from the computer while still allowing audio frequencies to pass. Potentiometers then reduce the signal for systems as appropriate for low-level inputs (such as many microphone inputs). The PTT is handled by taking a RTS signal from an available serial port (USB-to-serial in this case) and passes the signal through an optoisolator (LTV817 in this case).

Add in a USB sound-card such as the Creative USB (for $30) and you have effectively made a $65 sound-card interface which would cost well over $100 if purchased from a commercial supplier.

Fast forward to my latest project.

I am currently designing a single-board sound-card interface for amateur use which should come in at roughly the same price (target is <$60), offering a kit-build project to introduce more radio amateurs to digital mode operation.

Sound card interface PCB

The image above shows an early prototype of my PCB .. there will be more changes as I explore different options, however, this is the rough skeleton. The PCB is 85 x 50mm (3 x 2″) at the moment which offers a nice compact foot-print. I will be making conceptual changes as I go along to make the project as user-friendly as possible. I may even integrate a micro-controller to handle mode settings, etc. to further stream-line the whole interface.

… more to follow!