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Roku has patented a way to show ads over anything you plug into your TV
(arstechnica.com)
This is a most excellent place for technology news and articles.
If I'm understanding what Roku has done, this has nothing to do with HDMI (HDCP) security. Roku is inserting the ads after the signals has left the HDMI subsystem, and before an image is displayed on the screen. They can do this because the Roku is inside the TV.