[bojan]

OK, I've got it..
It seems that there is variety of those dongles on the market (after all, Maciel mentioned this on his website, but I fully appreciated what he wrote only after I measured my own specimen and when I saw what I saw).
Attached is the screen shot of step pulses from my dongle, there are some artefacts from (built-in) digital filter visible (ringing), so pulses are not clean - but they will be OK after Schmitt trigger.

BTW, the second batch of dongles arrived from ebay this morning (grey ) and they are the same as the first, with blue case.

Interestingly, the waveform depends on OS running - the second shot is from w7 (the first one is from laptop running xp) - most likely because I didn't configure the audio driver properly in w7, so the OS is interfering with SoundStepper). Please note, the second shot is scaled as 0.5V per division (GND is at the bottom of the screen graticule) and in both cases, the audio level was set to 120%.

Anyway, this is all very good - the whole system - telescope, guiding, ASCOM, star charts- all running on a single desktop. And all of that is freeware.
Of course, driver electronics must still be built, but with those Pololu or StepStick PCB's around, this is not a problem...