What you say Klaas is interesting, but I'm still in the dark as to the mechanism by which the missing sync pulse on the Nipkow disk and the missing sync pulse in the "video" signal are processed by the 4046 to create a frame sync.
For curiosity I made a simple signal generator using a PIC and a DAC to meet the NBTV standard as I understand it. It was a simple matter to produce PIC programs to generate signals with and without the missing sync. I found that the missing sync. made no difference to the displayed image, which was from a simple staircase signal with the 2.5ms negative going line sync. pulse.
On a different note, I have not been able to find any info on how to convert, say an .avi or .wmv file to the NBTV standard. I can imagine this is not a trivial task, but I'd like to have a go! Any offers?