HDMI

showstack / protocols / hdmi

Also called High-Definition Multimedia Interface.

Uncompressed video and audio plus control and content protection on one connector, and the default video interface on nearly everything that is not purpose-built for broadcast. Version numbers describe bandwidth, and almost every field problem with it is a negotiation problem rather than a bandwidth one.

Video transportProprietaryHDMI ForumReported

Media Large, continuous, on time and in order

Continuous streams that are large enough to be sized properly, and that must arrive in order and on time.

What kills it. Anything that starves the link — usually a management transfer nobody scheduled — and jitter that outruns the receive buffer.

The four flows, and why the model is worth having →

What it carries

Sample rates: 32 kHz, 44.1 kHz, 48 kHz, 88.2 kHz, 96 kHz, 176.4 kHz, 192 kHz
Audio bit depth: 16-bit, 20-bit, 24-bit
Audio channels: up to 32
Audio encoding: Linear PCM, or a compressed bitstream passed through untouched
Resolution: 10240x4320
Frame rate: up to 120 Hz
Colour sampling: 4:4:4, 4:2:2, 4:2:0
Video bit depth: 8-bit, 10-bit, 12-bit, 16-bit
Video compression: None, or DSC 1.2a visually lossless where the link cannot carry it raw

Audio: Passing a bitstream through means the sink decodes it, so a display that cannot decode what the source sent produces silence rather than an error.

Video: These are separate ceilings, not one mode: HDMI 2.1's 48 Gbit/s does 4K120 or 8K60 uncompressed, and reaches 10K only with DSC. The limit is always the version at both ends and the cable category between them, never the connector.

What goes wrong

A version number belongs to a device, not to a cable. Cables are certified by bandwidth category — Standard, High Speed, Premium, Ultra High Speed — and a cable cannot be "HDMI 2.1".

Locking connectors are not part of the standard. On a show, that matters more than any specification detail on this page.

HDCP is negotiated separately and fails as a black screen. So does an EDID mismatch. So does a hot-plug event. Three different faults, one symptom.

4K60 at 4:4:4 is roughly four times the data rate of 1080p60. A marginal cable or extender that passes the smaller signal will fail on the larger one, intermittently, once things are warm.

Typical use

Source to display, source to switcher, switcher to projector
The signal HDBaseT extenders carry over structured cable
Consumer and prosumer playback into a professional chain

Sources

HDMI Specification — HDMI Forum primary

Understand the mechanism

Why the picture is black

Something wrong, or missing?

Known gaps on this entry: default_ports, implementations. If you can source one of them, that is a ten minute pull request.

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