>>766321So we think of HDMI as being a one-way thing, but there's actually a low-bandwidth channel that flows in reverse called the "Display Data Channel". This tells the HDMI source what kind of thing it's connected to (TV, monitor, projector, etc.), what resolutions and features it supports (HDMI and DVI are both subsets of a hypothetical HDMI+DVI superstandard), and carries the responses back when two HDMI DRM ("HDCP") devices need to exchange keys.
If DDC is broken, then HDMI DRM can't work, and devices that only speak HDMI may not recognise the device an HDMI device.