Green tinge : which RGB (YUV?) conversion to apply

I am not sure which color conversion I should try.

=> Can someone give me hint or working tracks for why the background which should be black is a “half” green (I reckon it to be green with 50% alpha : 0,255,0,128 or else in YUV) ?

=> Is it a question of unsigned byte vs byte : why do I obtain black with (255,0,0,128) and red with (255,0,0,255) : why is the red range reached with alpha between 128 and 255 ?
(but green gets whiter at (0,255,0,255) and blue is a little greenish at (0,0,255,128)).

=> is it a question of converting SDI to HDMI or a HDCP issue ?

Thanks to all for any help,
I am a bit dazzled, and quite new to color conversion,
any help will be so great,

Answered on Decklink email list - this is probably a decklink and maybe TV problem.

It may be possible to pre-process pixels to be YUV even when the signal path expects RGB, but that’s a poor idea.