Finally I could get my hands again on a real ZX spectrum (TK90X). The first tests showed that a single sample for each bit produced bad results (video).
A new sample routine was added within the delay loop for each bit. If there is signal a counter is incremented. At the end, if at least 1/4th of the samples were active, then a symbol (bit 1) is recognized and plotted as a black pixel. Otherwise it plots a white pixel .
This sampling routine was tested with the sound sample found in the Wikipedia article about Hellschreiber (video) as well as with some text generated by fldigi (video) and behave very well. It makes possible to use the volume control to change the 'intensity' of the black ink, which may be very useful when testing with live reception signals or recorded from live radio. This was tested using the sample in WB8NUT website.