FT817 digimodes

We (family) are planning our Summer holiday. We will visit the same location of last year (IOTA EU-136, IOCA CI-096), with one more child onboard. Radio time might be even more limited than July 2008, and/or operator's energies further reduced.

A tired operator is a very lousy CW'er, so I am considering to try digimodes this year. SSB is out of question because it is too noisy and not effective at QRP levels.

I had once built a fully-isolated general-purpose soundcard interface. I also had prepared a cable to interface with FT-817 through the MIC/earphone plugs so that I could use VOX (VOX works only on the MIC plug, not on the rear DATA connector). But a strong hum was coming out together with PSK signal. After a lot of debugging and a bit of soldering I mentally re-counted the purpose of 8 wires in the FT-817 MIC cable: up, down, mic, gnd, ptt, fast, +V... what was the eighth? A brief look at the manual revealed a "mic gnd" line.

According to the schematic diagram, both "gnd" and "mic gnd" lines are grounded without further processing. But apparently they are not grounded right away near the plug.

Once I had connected both to my interface ground line the hum disappeared and I could complete two PSK31 QSOs. ALC down to zero, power to 5W.

Now I need more digimode practice and some macro review.