How Backstage keeps time

Last updated 7 August 2026

A media server does not have a clock. Even a single machine running a single show has several, each belonging to different hardware, each free to run at its own rate. Sync is the business of deciding what follows what — and, where nothing can be made to follow, of hiding the difference.

The clocks on one machine

Clock	What runs on it
The GPU output	Ticks at the display's update frequency. Every frame reaching a projector or screen leaves on this.
The audio hardware	The sound card's own clock. Every sample leaving the card leaves on this.
The Global clock	Backstage's software clock, and what media playback — video and audio together — is timed against.
Incoming capture	A capture input arrives on the sender's clock, which belongs to a camera or a device somewhere else entirely.

These are genuinely independent. Nothing makes a graphics card and a sound card agree; two crystals of the same nominal frequency drift apart. The question is never whether they differ, only by how much and what is done about it.

What the Global clock can follow

Left alone it free-runs. It can instead be disciplined to one external source:

An external PTP clock — time distributed over the network.
The incoming frames of a single capture input — the server follows whatever is feeding it.
The ASIO audio hardware — the sound card leads and playback follows it.

An ST2110 installation already has PTP. The standard requires it — every device on an ST2110 network locks to a shared PTP grandmaster, because that is how the video, audio and ancillary streams stay aligned with each other. A server joining such a plant therefore has its Global clock source decided for it: follow the network’s PTP and Backstage is timed with everything else in the building.

One source, not several. The Global clock follows PTP, or a capture input, or the audio hardware — choosing one means the others are no longer leading anything.

Two things the Global clock does not govern

	What it is	What it syncs
TTL / tri-level genlock	A generator feeding a reference signal to the graphics hardware.	The GPU outputs, locking them to a specific frequency. This is a hardware path and does not pass through the Global clock at all.
LTC timecode	Longitudinal timecode arriving from outside, linked to a sequencer.	That sequencer, which then plays on the incoming timecode rather than on the Global clock.

The arrangement that removes the problem

Where everything must agree, the answer is one hardware clock at the centre, distributing TTL or tri-level and PTP, and feeding:

the GPU outputs, through genlock
the audio clock
any external timecode
the Backstage Global clock

When everything is genuinely locked in hardware, Backstage does nothing. It makes no correction, because none is needed. Every adjustment described below exists only to cover clocks that were never brought together.

When they are not locked

Most installations do not have a house clock, and they work. Backstage absorbs the difference in software: it adjusts audio and video tempo to hold everything together, continuously and by tiny amounts, chosen to be as close to unnoticeable to an audience as it can be.

The clock is steered rather than stepped — it is never jumped, even if a reference glitches briefly, because a jump would show as a visible skip. A drifting reference degrades gently instead of breaking the show.

Frame-synchronous is not pixel-synchronous

Two servers can be synchronised to very different degrees, and the words are worth keeping apart because they buy different things.

Term	What it means	What it takes
Frame-synchronous	The servers agree on which frame they are showing. When each output actually paints that frame is its own business, and two outputs can be as much as half a frame apart.	A shared timebase — PTP, or Backstage's software correction.
Pixel-synchronous	The outputs are painting the same pixel at the same instant.	Genlock. TTL or tri-level reference into the graphics hardware of every server.

A GPU does not deliver a frame all at once. It scans it out — pixel by pixel, left to right, line by line, top-left corner to bottom-right — over the whole frame period. Two outputs showing the same frame can be anywhere in that sweep relative to each other.

Genlock aligns the sweep itself. With every server's graphics hardware locked to one reference, they start each frame together and stay in step through it, so the pixel leaving server A and the pixel leaving server B arrive at their displays at the same moment. Without it, the frames match and the sweeps do not.

What half a frame actually looks like

Put a single-frame white flash through a panorama driven by several servers. It is the clearest test there is, because it asks the whole system to do one thing at one moment.

Pixel-synchronous	Half a frame apart
One bright, clean flash across the whole image.	A grey flash — softer, less defined, visibly less intense.

Nothing is dropped. The same light is emitted either way; it is spread across two frame periods in different parts of the image instead of landing in one. The eye integrates that into something dimmer and less distinct, and once you have seen the two side by side the difference is not subtle.

So pixel-synchronous output is a picture-quality decision, not only a stereoscopy requirement. Anything that changes quickly — a flash, a hard cut, fast motion across the join between two servers — is rendered less crisply by a system that is only frame-synchronous. Slow content hides it; the moment the content is sharp in time, the output should be too.

The one case where software is not enough

Where the content depends on the sweep, frame-synchronous is not enough — and no amount of software will make it enough. Stereoscopic playout across two servers is the clearest case: if the eyes are painted half a frame apart, the image separates. That needs pixel-synchronous output, which means genlock into every server, distributed from one central clock.

A single server has no one to be out of step with, so none of this applies to it. Across several servers, genlock is what buys the last of the quality — and for stereoscopic playout it is not optional.

Synchronize Playback Across Multiple Servers (PTP & Observer) — setting up PTP across servers

Enable Genlock — taking a house reference into the graphics hardware

Sync to Timecode (LTC) — running a sequencer on incoming timecode

Set Up DeckLink (Blackmagic Design) Capture and Output — a capture card as the clock source