From memory it's something like Device Data Channel / Control Interface.
It's a very simple and I2C protocol for controlling devices (monitors normally but not always). Two of the wires in HDMI are literally just an I2C bus.
You can control things like brightness, switch inputs, turn the screen on/off, etc.
Unfortunately it's ancient, incredibly badly documented, and rarely implemented well by monitors.
A previous Dell monitor I had flickered terribly when you change the brightness (it did work though).
My current very expensive LG monitor has a bug with input switching. It clearly is meant to work because when you tell it to switch inputs it blanks the screen but then just returns to the same input.
I was intended to use that with a USB KVM switch so I didn't have to get a very expensive HDMI KVM switch. Unfortunately it doesn't work... so I got a HDMI KVM switch.
Unfortunately the switch I got ignores DDC/CI brightness commands so now my hardware brightness knob doesn't work. You can't win. :-(
Current plan is to implement a MitM DDC/CI device that inserts sneaky commands. Unfortunately DDC/CI is also the channel used for HDCP. It sends a message every two seconds and there's a required response latency, which means it's a bit tricky to avoid interfering with it.
It's a very simple and I2C protocol for controlling devices (monitors normally but not always). Two of the wires in HDMI are literally just an I2C bus.
You can control things like brightness, switch inputs, turn the screen on/off, etc.
Unfortunately it's ancient, incredibly badly documented, and rarely implemented well by monitors.
A previous Dell monitor I had flickered terribly when you change the brightness (it did work though).
My current very expensive LG monitor has a bug with input switching. It clearly is meant to work because when you tell it to switch inputs it blanks the screen but then just returns to the same input.
I was intended to use that with a USB KVM switch so I didn't have to get a very expensive HDMI KVM switch. Unfortunately it doesn't work... so I got a HDMI KVM switch.
Unfortunately the switch I got ignores DDC/CI brightness commands so now my hardware brightness knob doesn't work. You can't win. :-(
Current plan is to implement a MitM DDC/CI device that inserts sneaky commands. Unfortunately DDC/CI is also the channel used for HDCP. It sends a message every two seconds and there's a required response latency, which means it's a bit tricky to avoid interfering with it.