Tri-level sync

showstack / protocols / tri-level-sync

Also called Tri-level, HD genlock reference.

The high-definition genlock reference, replacing black burst for HD and above. A bipolar pulse whose zero crossing marks the frame start, distributed to every device that has to begin its frame at the same instant.

Timecode and syncPaid specSMPTEReported

Clock and sync Tiny, ruthlessly regular

Almost no bandwidth. Carries no content at all, only agreement about when. Its value is entirely in its regularity.

What kills it. Jitter, an unaware switch delaying the timestamps it was meant to correct, and two devices both convinced they are the master.

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

What goes wrong

Genlock fixes when a frame starts. It does not fix latency: two devices can be perfectly locked and still be several frames apart if one is doing more processing.

The reference rate has to suit the format. Feeding a 25 Hz reference to a device running 29.97 does not part-work, it fails.

Reference distribution is a chain of its own, with its own terminations and its own single point of failure. It is worth drawing.

Black burst is the standard-definition equivalent and is not interchangeable with tri-level for HD formats.

Typical use

Locking cameras, switchers and servers to a common frame start
Any multi-projector or multi-panel surface where a seam is visible
Plants where a shared reference predates or sits alongside PTP

Sources

SMPTE ST 274 and related HD sync reference standards — SMPTE primary

Understand the mechanism

Why the picture is black

Something wrong, or missing?

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

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