HDMI versions and bandwidth: what the numbers mean

HDMI version numbers look like they should be simple, and they are not. A device advertised as HDMI 2.1 may support one of that specification's features or most of them, and the licensing rules permit both. Understanding what the number does and does not guarantee is the only way to read a specification sheet usefully.

Bandwidth is the real specification

Every HDMI generation is defined first by how much data it can carry per second. Everything else — resolution, frame rate, colour depth, HDR — is a way of spending that budget.

Resolution, frame rate and colour depth all draw on the same budget. A signal can exceed a connection's capacity by raising any one of them, which is why a set may accept 4K at 60 in one colour format and refuse it in another.

The version-number problem

The specification's features are largely optional. A manufacturer may implement a subset and still describe the port using the version number, provided it states which features are supported. In practice that statement is often buried or absent from retail listings.

This is not a scandal so much as a consequence of how the specification is licensed, but it does mean the version number functions as marketing rather than as a guarantee.

Cables

Cables are certified by bandwidth category rather than by version number, which is the more honest scheme:

Two practical points follow. A cable cannot add a capability the devices lack, so an expensive cable will not turn an 18 Gbps port into a 48 Gbps one. But an under-rated cable genuinely can prevent a capability the devices both have — and it usually fails as intermittent dropouts or a picture that will not hold at the higher setting, rather than as an obvious fault.

For streaming specifically

Streaming is undemanding by these standards. Services deliver at 24, 25, 30 or 60 frames per second, so an 18 Gbps connection carries 4K HDR streaming comfortably. The 48 Gbps generation matters for gaming at high frame rates and for uncompressed 8K, neither of which is a streaming concern today.

The HDMI features that do affect streaming setups are the audio ones — ARC and eARC — and content protection, which is the usual reason a stream produces a black screen while menus display normally.

HDCP and the black screen

HDMI also carries a copy-protection layer, HDCP. Before protected content plays, the devices in the chain authenticate with one another. If any link does not support the required version, that authentication fails.

The distinctive symptom is that everything else works: menus appear, the interface responds, and only protected playback produces a black screen or an error. An added device in the chain — a receiver, a soundbar, a switch — is a common cause, because it becomes another link that must comply.

Terms are defined in the glossary. More broadcast and display coverage is in the TV Technology hub.