Guide

What Is Alpha-Channel NDI Captioning? (And Why It Beats Burn-In)

If you run live events, you have seen captions done badly: text burned into the program feed, or a browser window dragged onto a screen. Alpha-channel NDI captioning is the clean alternative — and it is what SCAPTION outputs.

What is an alpha-channel NDI source?

NDI (Network Device Interface) carries video over a standard network. Analpha-channel NDI source carries transparency alongside the picture, so everything except the caption text is see-through. A video switcher such as vMix, TriCaster, or OBS receives it as an input and composites it over the program feed exactly like a lower-third graphic — the transparent area shows whatever is underneath, and only the caption text appears on screen.

Why it beats burned-in captions

• Nothing is baked into your video. Burned-in captions are permanent; an alpha source keeps the text on its own layer you can restyle, reposition, or remove without re-encoding the show.
• Your switcher stays in control. Position, size, and timing are handled downstream like any other graphic — no separate chroma-key stage, no second machine to manage.
• No browser overlay to babysit. There is no web page to keep open, no tab that can crash mid-event and take the captions with it.

What about latency?

For live captioning, the caption has to arrive while the words are still relevant. SCAPTION delivers translated captions to the venue screen in under two seconds, glass-to-glass, and the operator can correct a name or term in the live transcript before it is shown — without stalling the feed.

No NDI gear? Use chroma-key HDMI

When a venue has no NDI on the network, the same captions can be sent as a chroma-key HDMI signal — a solid key color behind the text — that a hardware switcher keys out. You keep the layered-graphic workflow without the network.