First and foremost, an adequate network is required. NDI has a lot of features to allow it to work for monitoring on a “general use” network, but a completely different level of network stability, bandwidth and configuration is required for use as a video transport on par with HDSDI.
Once the NDI input is configured, if you see “black flashes” it means the source is not maintaining the frame it’s “advertising” to the network. The Cinedeck will then fill in any dropped frames with black to maintain the frame rate.
The best test to ascertain if this is happening is to play out video with a timecode burn to the NDI studio monitor and use its recording function to record a short clip. Studio Monitor will maintain the frame rate by duplicating the last good frame wherever there is a missing frame. A video recording of that input will look like this: