Hi John. > 1. I noticed that although most PSK31 signals I receive are showing > the two phase positions as displayed on the scope at 180 degrees, > some are consistently at less angle than that, typically down to > about 135 degrees. When I say consistently I mean it is obviously not > just a momentary shift due to propagation as one can see with DX or > QRMed signals. Is this related to the difference in calibration > between the sound cards?
Yes > 2. In the case of PSK31 how much does mis-calibration really affects > the reception? Does it result in a simple an increase percentage of > mis-decoded characters when propagation/QRM creates unwanted phase > shifts? In case of PSK31 it depends on the implementation of the decoder algorithm. Uncalibrated clock will cause phase shift to be different from 180 grades, which adds some uncertainity to decoded bit value. But uncalibrated clock will also negatively influence decoder ability to bit synchronize. I remember optimizing PocketDigi PSK31 decoder for best SNR. One of my optimizations was decreasing speed of PSK31 decoder clock synchronization. One does not want a single noise spike to have much influence. I was very surprised, when I found that my decoder did not work at all when the sound card clock was 1% off. > 3. Which modes are the most / least sensitive to sound cards > calibration differences? My guest is that in the least category we > should find the psk and ifsk (e.g. Domino) modes and in the most > sensitive category MFSK modes. Is that a correct assesment? Least sensitive will be CW and RTTY, which are asynchronous modes. Narrow spaced multitone modes (MFSK, Olivia, Contestia, Throb, Domino) will have problems if the tone scale is stretched or shrinked against the decoder bins caused by sound card clock error. Also the same applies here as at 2). Ability to bit synchronize if the sound card clock is off depends on the implementation of the decoder algorithm and is usually a compromise between SNR ability, time to synchronize and sensitivity to clock difference between modulator and demodulator. 73, Vojtech OK1IAK
