[treboR2Robert]

I know there was a problem a while back where certain hdr Blu-ray's did not have any max CCL data. (terminator 2 was one example)

These showed a very dark picture but this was fixed by adding a fallback to 1000 nits for movies without that information embedded.

FLL is to do with individual frame brightness so I would of thought this does not affect HDR10.

Surely only HDR10+ and Dolby Vision would be affected by FLL.

I am no expert though and I am only speculating here.

No - AIUI both are required for decent tone mapping to take place in non-ideal displays. They are both single metadata values for the entire movie and apply to HDR10 content.

Max CLL is the Max Content Light Level - i.e. the brightest pixel in the entire movie.
Max FALL is the Max Fame Average Light Level - i.e. the highest average value of all the pixels across a single frame in the entire movie.

Both of these values are important in allowing a display to decide how to tone map stuff outside its capabilities (as they let the display know the 'worst' case scenario for that movie)

Displays can have very high Max CLL capabilities but much, much lower Max FALL capabilities before they start having to shut down (OLEDs in particular). That's a major reason for this metadata being present in HDR10 sources. HDR range content (i.e. >100nits) is supposed to be restricted to specular highlights etc. - but some content pushes 'normal' content into the >100nit HDR range or has a LOT of bright highlights (think of a bright stellar explosion reflected in shiny windows in a SciFi movie)- and this is where Max FALL becomes important.

If you don't have a display that can display the full range of HDR10 ST.2084 PQ content for the mastering level (another bit of static metadata in a movie) then your display will need to know how to cope with stuff outside its range, knowing the Max CLL and Max FALL informs that decision.

Thanks for the explanation, I sort of get it I think.

So the odroid n2 (s922) running CoreELEC can do this but the s912 devices running CoreELEC can't?

My TV is an LG B7 OLED.

Will the difference be noticeable ? The HDR picture from the s912 does look pretty good on my TV.

Thanks
Rob