HDCP (High-bandwidth Digital Content Protection) encrypts protected video between a source and a display so it can’t be copied in transit. It’s separate from EDID: EDID tells the source what the display can do; HDCP decides whether the source is allowed to send protected content at all. When HDCP fails, you typically get a black screen, a downscaled image, or an explicit “HDCP error” — even though the cable and resolution are fine.
Versions
| Version | Used for | Notes |
|---|---|---|
| HDCP 1.4 | 1080p / early 4K | Older, widely supported; broken master key published |
| HDCP 2.2 | 4K protected content | Required for most 4K streaming/Blu-ray; not backward-compatible |
| HDCP 2.3 | 4K+ | Incremental hardening of 2.2; interoperable with 2.2 |
HDCP 2.2 is not backward-compatible with 1.4. A 4K HDCP 2.2 source feeding an HDCP 1.4-only display (or extender) will refuse to show protected content or drop to a lower resolution.
How authentication works
- Authentication — the source verifies the connected sink is a licensed HDCP device by exchanging keys.
- Key exchange — a shared session key is established and the video is encrypted.
- Repeater handling — switchers, extenders, and AVRs act as repeaters: they authenticate downstream, report the topology upstream, and re-encrypt. HDCP allows up to 127 devices and 7 levels of repeater depth; exceeding either fails the whole chain.
- Revocation — sources can carry a System Renewability Message (SRM) that blocks compromised device keys.
Common failure modes
- Black screen / no protected content. A device in the chain doesn’t support the required HDCP version (classically a 4K HDCP 2.2 source through a 1.4-only extender or matrix).
- Downscaled image. The source falls back to 1080p because the 4K path can’t complete HDCP 2.2.
- Intermittent drops. Flaky handshakes after switching inputs or hot-plug events — often HPD or repeater topology issues.
- One display in a distribution fails. In a 1×N splitter, HDCP negotiates to the least capable sink; a single non-compliant display can knock out the group.
How to troubleshoot
- Match versions end-to-end. Every source, matrix, extender, splitter, and display must support the HDCP version the content requires. 4K protected → HDCP 2.2 everywhere.
- Test direct. Connect source straight to display to confirm HDCP works, then reintroduce each device to find the offender — same isolation method as EDID debugging.
- Mind the topology limits. Deep chains of repeaters (cascaded matrices/splitters) can exceed the device/level caps.
- Check for “HDCP off” content. Non-protected sources (many PCs, cameras, signage players) don’t assert HDCP — if only protected sources fail, HDCP is the cause, not the cable.
Rule of thumb
Design HDCP support as a chain requirement, not a per-device checkbox: the weakest link sets the ceiling for the whole path.
Related
Sources
- Digital Content Protection LLC — HDCP Specifications — HDCP on HDMI Specification Rev. 2.3 and HDCP 1.4 (authentication, repeater topology limits, revocation).