Pattern KV-C

The display as a pure receiver

HDMI is not one-way: DDC runs in both directions, and EDID is a shared, writable object.

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Area: KVM & VTC
Environments: 4
Products used: 8

How it is built

One-way path
Data path
Path that does not exist

HDMI is not one-way. DDC, CEC, ARC and HEC run the other way unless they are broken.

Used in these environments

Briefing roomA shared room is harder than a single workstation, and the bridge for an external client is a deliberate compromise that must be decided in advance.
Boardroom in a protected companyThe people in the room have decision authority and no security training, and the equipment has to work without either.
Training facilityStudents bring their own devices, and a room designed for teaching is designed to be connected to.
Shared AV equipment in a common meeting roomThe EDID table in the display is overwritten by the fourth computer connected, so the fifth gets a black screen — which is exactly the inconvenience that breeds the workaround.

How it works

Every source sees a diode; none of them sees the display. The diode separates the source’s DDC from the display’s, disconnects ARC, HEC and CEC in both directions, and holds its own captured EDID.

The display is a shared node

A screen is a writable, addressable device on four two-way buses. Calling it “a screen” hides that it is common to every domain that has ever been plugged into it — including over time, and including domains whose users never met.

EDID capture is an installation action

Capture once, at the first installation, and seal it. A capture that any user can trigger is a capture that will be triggered by whoever gets a black screen five minutes before a meeting.

The two exceptions

A touch screen’s USB is a separate cable the diode does not touch — a data path from the shared surface back into a domain, and the most important limitation of this pattern. And audio from any source other than the presenting computer belongs in a separate, fixed chain rather than in a path back through the screen.