What is HDCP (and why does a film go black through a splitter)?
In one sentence: A secret password check that films and TV apps use to make sure every box between the player and the screen is allowed to show them; if one isn't, the film goes black.
A little more
HDCP (High-bandwidth Digital Content Protection) is copy protection on the cable itself. Before a protected film or TV show plays, the player and the screen, and every box in between, prove to each other that they hold licensed keys, and then the picture is scrambled on the way so it can't simply be recorded off the wire. 4K films usually need the newer HDCP 2.2 or later.
If any box on the path can't take part, such as a cheap splitter, an old receiver or a capture card, the player refuses to send the protected picture. Games and menus usually still appear, but the film app shows a black screen or an HDCP error. Plugging the player straight into the screen, or using an HDCP 2.2 port, almost always fixes it.
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