So the development of the ZX8-CCB-SYNC is finished now and it is available at sellmyretro. I wonder that the first 10 devices are sold with just listing them but no further announcement and explanations.
The SYNC version of the ZX8-CCB has a special sync generator on it (a micro controller on the backside) to assure a valid picture using the FAST mode with a ZX81 or using a ZX80. When statements are processed like display of a listing or even longer running programs it results in a black screen for a few hundred milliseconds or even seconds or minutes for a long running program. Modern displays have problems with longer out-of-sync periods and tend to switch to another input or even switch off after 30-60 seconds.
I found out, that most displays have no problem with shorter or longer vertical syncs and tolerate even frame rates between 40-70 Hz without loosing sync as long as the horizontal syncs are present. In the range of 50-60 Hz it may switch from PAL to NTSC mode automatically but that's all.
So the device can not remove flicker at all but reduced to less than a few frames to get in vertical sync again. Newer displays will show a flicker but do not loose sync and do not display a message about that. Technically the controller measures the incoming signal with the exact horizontal frequency (about 0.1 to 0.2%) and repeats with the same frequency. The vertical syncs are measured as well to detect PAL or NTSC resp. 625 or 525 lines. A loss of sync is detected in about 600us or more than 9 lines with no sync. So there is a tolerance while 6 lines is normal for the duration of the vertical sync.
The stripe patterns known from LOAD and SAVE are shown as well (not a problem because syncs are in very shorter periods shown) and the inverted picture is handled as well and give an inverted synced picture this way. Whenever the internal sync generator takes control is shown with a small line in top of screen.
I prepared a video for it as pictures and movies are easier to understand than descriptions ...
https://www.youtube.com/watch?v=1Re-nbXdKeg&t=3s
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