Every live event eventually gets the same request: can we caption this? The answer depends far less on the captioning itself than on how the text reaches your switcher. Here are the five routes that actually work in a booth, and what each one costs you in flexibility.
1. Alpha-channel NDI: the clean route
If your production network already carries NDI, this is the one to use. The captions arrive as a video source where everything except the text is transparent, and the switcher composites it over the program feed exactly like a lower-third graphic.
- OBS Studio: add a NDI Source (via the DistroAV/obs-ndi plugin), tick the alpha/BGRA format, drag it to the top of your scene. If the transparent areas come in solid black instead of see-through, open the source’s Properties and set Latency Mode to Normal (safe). Low or Lowest latency modes drop the alpha channel entirely (we tested it).
- vMix: Add Input → NDI/Desktop Capture, then use it as an overlay channel rather than a full input, so it keys over whatever is live.
- TriCaster: bring it in as a network source and assign it to a DSK.
Because the text lives on its own layer, you can restyle, reposition or drop it mid-show without re-encoding anything. Nothing is baked into the recording either. We wrote up the mechanics separately in what alpha-channel NDI captioning is.
2. Chroma-key HDMI: when the venue has no NDI
Plenty of rooms run a hardware switcher and no production LAN, often a Blackmagic ATEM cabled with HDMI or SDI. Check whether your model takes an NDI input at all; if it does not, or there is no network for it, the fallback is a full-screen HDMI output with a solid key colour behind the caption text, keyed out downstream on an upstream or downstream keyer.
You lose nothing visually. You do lose a display output on the caption machine, and you have to agree the key colour with whoever built the show file. Do that before doors, not during.
3. A chroma-key URL in a browser: without spending a video output
Since SCAPTION 1.7.42 there is a third output that costs you no display output on the operator’s machine: a web address that draws the same two caption lines over the same key colour as the HDMI output, opened in a browser on any computer.
- Add the language to the link first. The one SCAPTION copies carries the room code and
the key colour but no language, and without one the page opens its own settings panel over
the frame. Fine on a laptop you can click, fatal in a source nobody can touch. Append
&l=es(or whichever language you want on the key). - OBS Studio: bring it in as a browser source and key the background out with a chroma key filter. No NDI plugin, no capture card.
- vMix: the same, as a web browser input with a colour key over it.
- Hardware switcher: open it on any laptop nearby and feed its HDMI into the keyer: the route above, without tying up the machine that is doing the transcription.
You set the look on the page itself (font, size, colour, outline, background box and position, in the console’s own units, starting from the console’s defaults), and from then on it rides in the link. The link SCAPTION copies already carries the key colour the HDMI output uses, so a switcher set up for one keys the other with no retuning.
4. The platform’s own caption track: for online events
Zoom, Teams, Meet and Webex all render captions themselves. In Zoom, which we tested, each viewer’s bar shows one language at a time; a translated one per viewer needs a paid add-on or a higher plan on the host’s account and covers Zoom’s own language list. Teams, Meet and Webex we have not tested, so check yours (and its licence) before a multilingual event depends on it. For a single-language webinar the built-in captions are often enough, and none of them reach the screen in the room.
SCAPTION handles online events by listening to your computer’s own audio, so it works the same way on every platform and nothing needs installing on the call. There is no bot joining the meeting, which also means there is nothing for a platform to block.
5. Attendee phones: the one that scales
The route most AV teams underestimate: put a QR code on the venue screen and let people read along on their own phone, each in whichever language they picked. It costs you no extra screen real estate, no hardware, and no seat-by-seat receivers.
This is what Scaption Access does: up to 12 languages at once, from a catalogue of 58, with no app, no account, and no charge per attendee or per language. In our own load testing a single room held 5,000 concurrent phones; the size we recommend selling is 500–1,000, because that is where delivery stays comfortably inside the latency target rather than merely arriving.
Which one should you pick?
| Situation | Route |
|---|---|
| Production LAN with NDI | Alpha NDI |
| Hardware switcher, no NDI | Chroma-key HDMI |
| No display output to spare, or a switcher with a browser input | Chroma-key URL |
| Single-language webinar | The platform’s own captions |
| Multilingual audience, any venue | Attendee phones, plus one of the above on the big screen |
Most real events end up running two at once: the venue screen carries the floor language,
and the phones carry everything else. And if the venue screen itself is what is missing,
the same page without the key is a caption screen: open scaption.app/screen on any TV
or projector with a browser, type the room code, and it shows the event in one or two
languages at once.
You can try the whole thing free. One hour, no expiry, no card required. Download it for Windows or Mac and point it at a mic.