Without going into technical details, NES and SNES generate jittery sync signal in 240p mode. That is a challenge for digitizers which use it in clock generation, and especially for devices like OSSC of which output clock is derived directly from it. While many displays are able to cope with the resulting signal, some are less tolerant, with symptoms ranging from occasional audio/video issues (sync drops, audio cutting) to total incompatibility (rolling / partially missing picture, no sync).
Before starting new topic about an issue with NES/SNES and OSSC, I suggest making the 2 following simple tests to avoid yet another thread that relates to the same root cause.
1. Try another 240p source with OSSC. If there are no similar problems, it’s an indication of this issue.
2. Try another display. If the problem disappears or has different appearance, it’s also an indication of this issue.
The fix? It is possible to eliminate sync jitter with a console modification. A small mod board is currently in testing with different console revisions – more details are coming soon.
Update 05/2018: Design files for an add-on board and installation instructions for several models are avaialble on Github. Pre-assembled boards are expected to be available sometime in June.
Update 07/2018: Pre-assembled boards are now available on the store page, both as standalone and with installation service.