Digital Modes Need Calibrated Soundcards

I made my very first Olivia QSO tonight. It was a bit daunting, but I took the time to study a little about the standard, already had MixW rigged up with the Olivia DLL, listened around a bit on 20 meters then 40, figured out where to call CQ and just did it.

It was not even two minutes before a ham came back to my Olivia 16/500 call on one of the 40 meter calling frequencies.

The ham was patient with me as he noticed he had to swing his receive calibration value down quite a bit to copy me. He mentioned he had to adjust the ppm value for his sound card receiver down a full 5000 ppm to pull me in. We chatted a while and I decided to put +5000 ppm in my soundcard TX calibrate attribute. It worked. We could now have an Olivia QSO with no adjustments.

Discussing this further, he suggested I run the WWV Calibrate procedure described in the MixW documentation. After the QSO was over, I did this for the RX and TX values using the 5 MHz WWV frequency. I found my RX adjustment was -160 and the TX adjustment +6689. Wow.

The ham mentioned sound card calibration is very important for the multi-tone modes including:

  • Olivia is more tolerant
  • MFSK16 is more sensitive
  • MT63 is very sensitive

Thinking back to some PSK31 QSOs I did notice my transmitting frequency was just a little different than my receive frequency. Using “lock” I was able to avoid the leap-frog up the band thing that happens when two unlocked PSK31 systems try to constantly lock onto each other and pull their RX frequency along.

This could be my Icom-746 drifting in frequency too, but this sound card caliburation is clearly an issue.

By the way, this is the sound card in my US Interface Navigator, not my PC.

So judging by the values, the calibrate procedure for your sound card and software combination may be just the thing to finalize your digital mode exploration.

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